Northern Prairie Wildlife Research Center
Existing habitats in the Platte River system support a multitude of bird species. As of January 1, 1990, records had been confirmed for 409 bird species in the central Platte area (Appendix B). The total avian list includes nearly 50 percent of all the bird species that have been confirmed in the North American continent including Alaska and Canada (American Birding Association 1986). The avifauna also includes at least 208 species that have been confirmed nesting at least once in the area. Among major taxonomic groups, the wood warblers (Parulidae) are represented by the most species (n=41; 10 percent), followed by 40 species of shorebirds (Charadriidae, Recurvirostridae, Scolopacidae) (10 percent), 35 species of waterfowl (8.5 percent), and 32 species of emberizid finches (Emberizidae) (8.3 percent).
Of all the avian assemblages frequenting the Platte River valley, it is the migration of sandhill cranes during February through April that draws tremendous attention to the river. Nearly 500,000 sandhill cranes stage along the central Platte River and along the North Platte River below Lake McConaughy. It is the largest known concentration of cranes in the world and has been the subject of much research and conservation concern for 25 years. In spite of stable crane numbers, the conservation concern springs from the observed shrinkage of river channel habitat and wet meadows and the concomitant shift and concentration of sandhill cranes to the east where remaining, but unprotected wide channels and wet meadows still occur.
Construction of Kingsley Dam and Lake McConaughy in Nebraska, and the development of other reservoirs on the North Platte River, have been the major factors responsible for alteration of the flow regime in the Platte since the 1930s and consequently for the development of the riparian forest on the floodplain. Following regulation of the Platte River, the native migratory birds which were adapted to unvegetated sandy river channel, standing water sloughs, marshes, and wetland meadows, were forced into remaining areas of suitable habitat. Changes in the distribution of sandhill crane populations on the Platte River since the 1930s exemplify how a species is forced into alternative riverine habitat (Faanes and LeValley 1993). The reach of the Platte River between Cozad and Lexington served as one of the largest roost site complexes for sandhill cranes in the 1930s and 1940s and was frequently used by whooping cranes (Frith 1974). As a result of forest encroachment and wet meadow destruction, cranes completely abandoned this stretch of the river. They gradually shifted their distribution from the road between Cozad and Lexington to more open areas to the east. The river channel near Grand Island is now one of the remaining roost sites for sandhill cranes on the Platte River. This area was sparsely used 20 years ago when the more western channel provided suitable roosting habitat (Faanes and LeValley 1993).
The principal migration staging areas for sandhill cranes includes reaches between 1) the J-2 river return near Lexington, downstream to near Elm Creek, 2) the Nebraska Highway 10 bridge (Minden) and the Shelton bridge, and 3) the Wood River bridge and the Nebraska Highway 34 bridge near Grand Island. The North Platte River staging area is between the City of North Platte and Sutherland, Nebraska. A small staging area (10,000 birds) exists on the North Platte River in the Clear Creek Marsh Wildlife Area near Lewellen.
Frith and Faanes (1982) found that the greatest densities of roosting sandhill cranes per river-mile in the three principal areas on the central Platte River occur between Nebraska Highway 34 and the Wood River bridge. In 1979, about 40,000 sandhill cranes occupied the Lexington to Elm Creek reach (Frith and Faanes 1982). In 1991, about 20,000 sandhill cranes used the same area (USFWS, unpubl. data). Sidle et al. (1993) examined crane distribution using an aerial infrared scanner and drew strong correlations between crane roosts and channel width and proximity to wet meadows. Most cranes were in wide channels close to wet meadows, a set of habitat conditions that continues to decline.
Pair bonding and food consumption are the principal activities for sandhill cranes during the staging period along the Platte River. Most pair bonding takes place in wet meadows (Tacha 1988) and foraging in wet meadows is important for later reproduction. Analysis of the foraging requirements of sandhill cranes in the central Platte River underscore the importance of diversified agricultural production. These data are also applicable to migrating and wintering waterfowl.
The diet of sandhill cranes varies among habitats (Reinecke and Krapu 1986). Cranes obtain the bulk of their total diet from cornfields where they primarily consume waste corn. Cranes foraging in grasslands consumed earthworms, snails, crickets, grasshoppers, sowbugs, spiders, and adult and larval beetles (USFWS 1981). Cornfields provide cranes with large quantities of readily digested, high-energy food items. Cranes foraging in cornfields gained energy at 10-20 times their rate of energy expenditure (USFWS 1981).
USFWS (1981) estimated the food and energy requirements of the crane population. First, the existence energy requirements of individual cranes were calculated with an equation from Kendeigh et al. (1977):
where M = existence metabolism in Kcal/bird/day and W = body weight in grams. Crane body weights were estimated from data collected in the Platte River valley because only a few juveniles were examined, their weight in Nebraska could not be determined, juvenile males and female lesser sandhill cranes and Canadian sandhill cranes were assigned mean body weights of 3,313, 2,807, 4,353, and 3,640 g, respectively, based upon data from collections in Nebraska. Adult male lesser sandhill cranes weighed about 3,051 and females, 2,927 g on 1 March. Males gained 25.0 and females, 13.6 g body weight/day while in Nebraska. Adult male Canadian sandhill cranes weighed 3,972 and females, 3,728 g. Males gained 22.1 and females, 13.1 g/day. The energy increments required for weight gains (9.38 kcal/g) and activity (+10 percent) were taken from Kendeigh et al. (1977). Finally, the energy requirements of individual cranes were converted to food requirements assuming that each gram of corn provided 4.0 kcal of metabolizable energy (Sibbald 1979).
The daily corn requirements of the population were calculated by USFWS (1981) from the individual energy requirements, and estimates of population size and composition. Daily population size was assumed to increase slowly from 1 March to a peak on 31 March according to the equation:
where maximum population can be varied as desired, and date increases from 1 to 31. The daily population then decreases rapidly from 1 to 15 April according to the equation:
where date increases from 1 to 15. Based on the crane census data, 69 percent, 13 percent, and 18 percent of the birds were assigned to staging areas at Elm Creek, Gibbon, and Grand Island, respectively. Measurements of the cranes collected during feeding ecology research suggest that approximately 50 percent, 95 percent, and 100 percent of the populations in staging areas 1, 2, and 3 are lesser sandhill cranes. Juveniles make up 11.6 percent of the population in the model (Buller 1979), and juvenile and adult sex ratios are 50:50 and 55:45 males:females (cf. Lewis 1974).
The predicted daily corn requirements of individual cranes varied from a low of 0.08 kg/bird for immature female lesser sandhill cranes to a high of 0.15 kg/bird for adult males of the Canadian race. The daily corn requirements for hypothetical maximum populations of 350,000 and 450,000 cranes increased from 0.3 metric tons (1 metric ton of corn = 103 kg = 40 bushels) on 1 March to 44.6 and 57.3 metric tons, respectively, on 31 March. The cumulative requirements for populations of 350,000 and 450,000 cranes were 1023 and 1315 metric tons of corn, including 711 and 914, 131 and 169, and 181 and 232 metric tons for staging areas 1, 2, and 3.
The availability of waste corn in the Platte River valley was sampled on randomly-selected cornfields at the time of harvest, before arrival of the cranes, and after departure of the cranes (Table 2). On each field, all the corn on ears was collected from three random 436 square foot quadrats and all the individual kernels from three quadrats in the larger plots. The mean density of waste corn on the fields after fall harvest was nearly 7 bushels/acre. Interviews with farm owners and managers indicated that yields on the same fields averaged 101 bushels/acre for a 6-7 percent rate of loss during harvest operations. The acreage of harvested corn in the central Platte area was taken from the analysis of the habitat characteristics of staging areas (USFWS 1981).
Significant changes in the availability of total waste corn occurred between the fall harvest and the arrival of cranes and between the arrival and departure of cranes. Nearly half of the available corn was removed during winter when 26 of the 30 sampled fields were grazed by livestock. Changes in the availability of corn during winter were probably due to livestock feeding because the loss was primarily of corn ears; kernel corn losses were not significant (P>0.05). Conversely, changes in availability of corn during the crane staging period were kernel corn losses; there was no significant decrease in ear corn at that time.
Waste corn availability at the time of crane arrival was 205 kg/ha, and at the time of departure 128 kg/ha. With 25 percent of the fields under fall tillage, the total weight of waste corn present at crane arrival of between 6,529 and 8,424 metric tons, and at crane departure, between 4,077 and 5,260 metric tons. The loss of corn during the staging period was, therefore, between 2,452 and 3,164 metric tons. Using the estimates of population food requirements (1,023-1,315 metric tons), the cranes accounted for nearly half of the total corn use. Among the other factors affecting corn availability would be continued livestock feeding, and use by ducks, geese, and other wildlife. The data and analyses indicate the cranes used only 10-20 percent of the corn available at arrival, and left behind a quantity equal to 3-5 times their total requirement.
The following summary from USFWS (1981) is instructive in evaluating the influences of future changes in agricultural practices on sandhill crane food resources. If woody vegetation encroachment eliminates crane use of staging area 2 (Kearney-Elm Creek) and the cranes were to shift to staging area 1 (Mormon-Shoemaker Islands) the population of staging area 1 could increase to over 400,000 birds. The population would not be limited by available food resources, however, assuming that cranes are dependent on agricultural food resources, that cranes can use 50 percent of the waste corn on each field, that fall tillage continues at the current level of 0-25 percent, and that native grassland and wet meadow habitat remain available.
Availability of food would change drastically, however, if fall tillage increases or corn production decreases. If fall tillage approaches or exceeds 50 percent, crane food resources would be seriously affected. If this occurs, changes in crane distribution, movements, and perhaps physical condition would follow. These data underscore the beneficial impact on cranes of the current practice of grazing livestock on harvested cornfields during winter because it discourages fall tillage and maintains the supply of waste corn.
Elimination of fall plowing would produce other environmental benefits such as reduced soil erosion and a reduction in the amount of nitrates leached into the ground water. Negotiations on future federal farm legislation could include provisions to encourage landowners not to fall plow their corn fields. Incentive programs similar to set-aside payments, or tax incentive programs to discourage fall plowing also could be encouraged.
The mean distance from roost sites traveled by radio-marked sandhill cranes was 1.7 miles (USFWS 1981). Population survey data (C.A. Faanes, unpubl. data) show that sandhill cranes may forage up to 7 miles from the river channel, especially later in the spring staging season. To enhance foraging opportunities for cranes and waterfowl, we recommend that efforts to discourage fall plowing of corn fields be focused on lands up to 2 miles from the channel, both north and south of the river. In the North Platte River staging area, special consideration should be given to lands up to 3 miles south of the channel; there is virtually no cropland north of the channel.
The importance of wet meadows to sandhill cranes has been underscored by every scientist studying sandhill cranes before and since the USFWS Platte River ecology study in the late 1970s (USFWS 1981). Wet meadows provide a unique invertebrate food source for sandhill cranes and a place for pair bonding. However, wet meadows have been the most vulnerable of Platte River habitats to conversion to cropland and sand and gravel pits. Most remaining meadows may disappear or become less functional within 20 years given the current rate of loss and disturbance.
The National Biological Service (Sparling and Krapu 1994) recently concluded:
Our findings indicate that staging behavior of sandhill cranes is strongly influenced by the massive anthropogenic alterations that have taken place in the Platte River valley. To date, habitat loss has caused a major redistribution of cranes (Krapu et al. 1982), but sufficient remaining roosting and foraging habitat continues to attract and support most of the midcontinent population for several weeks each spring. So far, cranes have successfully avoided the energetic consequences and associated displacement that would have resulted from massive habitat loss had an abundant supply of high energy waste corn and adequate sources of protein in native grassland not been available. The status of this important Sandhill Crane staging area remains precarious, however, because of continuing degradation and loss of open river channels and native pastures (Sidle et al. 1989).
Davis and Vohs (1993a, b) recommended that meadows be managed to facilitate availability of earthworms and scarab beetles for sandhill cranes: (1) maintain diversity of wet meadow, transitional, and lowland grassland habitats in native grasslands throughout the Platte River valley; and (2) provide favorable water table depths (15.7-31.4 inches) in spring that concentrate earthworms and scarab beetles, the predominant prey items of cranes using native grasslands, in the top 8 inches of soil.
Those who have studied sandhill cranes in the Platte River system have consistently called for the immediate protection of remaining wet meadows, grasslands, and river channel habitat. For example, Tacha (1988) called for protection of wet meadows because of their importance to sandhill crane breeding behavior. Folk and Tacha (1990) also emphasized the importance of protecting wet meadow habitats along the North Platte River through purchase or easement. Tacha et al. (1992) stated that the "maintenance of essential habitats is the primary need for all populations of Sandhill Cranes. Wetland conservation is particularly important in the ranges of nonmigratory populations, and in staging and wintering areas of migratory populations....wet-meadow areas associated with the North Platte and Platte River valleys in Nebraska where 80 percent of mid-continent cranes stage in spring, should be primary foci of habitat conservation efforts."
Among the six priorities for sandhill crane management, Tacha and Nesbitt (1994) stated: "The USFWS and the Nebraska Game and Parks Commission should take the lead in forming a coalition of private, state, and federal agencies dedicated to delineating specific priorities for, and funding acquisition/protection and management of, crane habitat in central Nebraska." They added:
Conservation and management of essential habitat complexes in the North Platte and Platte River valleys, Nebraska are priority habitat management needs. Existing optimum complexes in central Nebraska require aggressive protection via acquisition or perpetual easements obtained by appropriate private, state, or federal agencies. Adequate North Platte and Platte River instream flows must be maintained for cranes because river flows affect distribution and abundance of roost sites, and (probably) availability and quality of adjacent wet meadow habitats. However, conservation and management of habitat complexes should be the priority focus of crane habitat management activities in central Nebraska.
Wet meadow habitat is the most common element missing from potentially optimum crane habitat complexes in central Nebraska. Wet meadows are important for pair formation activities and foraging. Temporary or semipermanent palustrine wetlands in shallow depressions and/or old oxbows within pastures adjacent (and probably hydrologically linked) to active river channels provide the best wet meadows. Reversing drainage, rewatering with increased in-stream flows or other means during the critical spring period, and/or reversion from row crops will restore wet meadows. Restoration of selected wet meadows to add the single missing component of otherwise optimum habitat complexes would be cost-effective.
Other Migratory Birds in Migration and Winter
Migrant waterfowl make extensive use of the Platte River and adjacent Rainwater Basin wetlands, especially during spring migration. Population estimates during migration range from 5 to 9 million individuals in spring (USFWS 1981, Currier et al. 1985). Most of the migration population consists of snow goose (more than one million birds), Canada geese (500,000 birds), greater white-fronted geese (300,000 birds), mallard and northern pintail. Nearly 100 percent of the central Flyway population of greater white-fronted goose occupies this area in spring migration in a manner similar to sandhill cranes.
A link exists between Platte River wetlands and those in the Rainwater Basin. The link is most evident in years when Rainwater Basin wetlands are dry and waterfowl congregate in spectacular numbers along the Platte River and forage in adjacent crop fields. During periods of prolonged cold weather, which serve to keep basin wetlands frozen well into the migration period, waterfowl shift their use patterns to the open waters of the Platte River.
Extensive use is made of Platte River wetlands during fall migration, especially by mallard and Canada geese, although not in the numbers recorded in spring migration. Weekly surveys of waterfowl on the Platte River in winter (USFWS, Kearney, Nebraska, unpubl. data) provide estimates of 40,000 mallards and 10,000 Canada geese through the winter months. Waterfowl make extensive use of wet meadows adjacent to the Platte River as resting and foraging areas during fall migration. The wet meadows are especially important habitat in fall when river flows are low.
An important factor regulating the use of the Platte River by fall migrant waterfowl may be the intensive pressure from hunters. During peak migration in 1987, we recorded a mean of 5.5 waterfowl hunting blinds in daily use in each of the 10 bridge segments in the reach between Grand Island and Lexington, Nebraska. Waterfowl hunting was greatest on weekends. Less waterfowl hunting would probably attract and maintain larger numbers of migrating ducks and geese on the Platte River throughout the fall.
Platte River habitats are particularly important in spring migration to several shorebird species, including the white-rumped sandpiper and long-billed dowitcher. Nearly 100 percent of the central Flyway population of both shorebird species stage each spring at the Cheyenne Bottoms Wildlife Management Area in central Kansas, about 200 air miles south of the central Platte River. During low water conditions at Cheyenne Bottoms (1989), shorebirds seemingly used the Platte River as an alternative migration staging site (G. Castro, pers. comm). Western sandpipers and semipalmated plovers make extensive use of the Platte River in fall migration.
Lingle and Krapu (1986) described several aspects of the winter ecology of bald eagles on the Platte River. Currently about 250 bald eagles occupy the central Platte River during the winter. The central Platte River area provides important habitat for several raptors, especially red-tailed hawks and northern harriers. Perhaps the most abundant fall migrant raptors in the area are red-tailed hawks, Swainson's hawk, and American kestrels.
Data on the abundance of other migratory birds in Platte River habitats are difficult to obtain because no active surveys have been conducted. Empirical evidence suggests that grassland and cropland areas adjacent to the river are used by large numbers of lapland longspurs and other emberizid finches, especially in fall migration. Christmas Bird Counts, although not scientifically valid in many aspects, provide some indication of the early winter occurrence and abundance of several passerine birds common to the area. Over 90 bird species have been recorded on the Grand Island, Nebraska, Christmas Bird Count, conducted from 1980 through 1989. The total number of individuals on the Grand Island count is heavily influenced by mallards. Largest numbers among passerine birds are usually represented by migrant and wintering species such as American crows, horned larks, American tree sparrows, black-capped chickadees, and dark-eyed juncos.
The breeding avifauna is dominated by 17 species of emberizid finches (8.8 percent), 15 species of waterfowl (8.2 percent), 12 species of flycatchers (Tyrannidae) (5.8 percent), and 11 species each (5.3 percent) of hawks (Accipitridae), quail (Phasianidae) and blackbirds (Icteridae) (Faanes and Lingle, unpubl. data). Johnsgard (1979) discusses the biogeographic affinities of nesting birds in the Nebraska portion of the Platte River system. The bulk of the nesting birds are of Pandemic distribution. We believe that pandemism is related to the geographic location of the Platte River area at essentially the center of the continent. The existence of forested habitats along the major river systems has been beneficial in allowing several bird species with eastern or western affinities to invade the region.
Robbins et al. (1986) recently summarized the first 15 years of continental data on trends in breeding bird populations indicated by the Breeding Bird Survey (BBS). The Breeding Bird Survey provides a random sampling of bird populations through various geographic strata in North America.
The Platte River ecosystem is represented in the BBS analysis by five major strata. The strata (Table 3), from east to west, include the Dissected Till Plain, the High Plains Border, the High Plains, the unglaciated Missouri Plateau, and the Southern Rocky Mountains (in part). Perhaps the most striking population decline among breeding bird species was demonstrated by the loggerhead shrike. Robbins et al. (1986) attributed the decline in loggerhead shrike to the consumption of contaminated prey items and the loss of preferred nesting habitat in hedgerows.
A subsequent analysis of BBS data (Droege and Sauer 1989) described continent-wide population declines of several breeding birds. Among the 222 bird species, the 22-year mean annual decline was greatest for the northern pintail (-6.9 percent). At least twenty-nine breeding bird species (13.9 percent of total) are experiencing statistically significant population declines (Table 4). Twenty-three of the 29 species are experiencing highly significant declines (P<0.01). By habitat, the declining species include ten edge species (34.5 percent), eight grassland species (27.6 percent), seven woodland species (24.1 percent), three urban/residential species (10.3 percent), and one wetland species (3.4 percent).
Faanes and Lingle (unpubl. data) observed that among Platte River habitats, the greatest species richness occurred in lowland forest (55 species) and native prairie (51 species). The lowest species richness was in wheat (18 species) and in corn (3 species). Those habitat types comprised over 36 percent of the total available habitat in the area studied by Faanes and Lingle (unpubl. data). The mean density of all breeding birds in the central Platte River valley was 38 pairs per square mile. The projected breeding bird population was 3,100,000 pairs.
Distribution and extent of habitats, and probably climatic variability, influenced the observed population densities. Particularly important was the amount of available cropland and the existence of large areas of native grasslands in the Sandhills physiographic region (Faanes and Lingle, unpubl. data). Native grasslands were nearly 50 percent of the area studied by Faanes and Lingle (unpubl. data), but breeding bird densities were relatively low.
Fifteen bird families comprised 95 percent of the Platte River breeding bird population studied by Faanes and Lingle (unpubl. data) (Table 5). The blackbirds and emberizids were 51 percent of the total. Five additional families (Passeridae, Tyrannidae, Apodidae, Picidae and Sturnidae) were an additional 26 percent of the total. Thirty-five species were about 95 percent of the total breeding bird population in the central Platte River valley studied by Faanes and Lingle (unpubl. data) (Table 6). Among these, the grasshopper sparrow, western meadowlark, and house sparrow comprised over 26 percent of the estimated breeding population in the central Platte River valley (Faanes and Lingle, unpubl. data), each represented by over 240,000 breeding pairs. The common grackle and red-winged blackbird represented over 13 percent of the central Platte River valley breeding bird population, each contributing over 190,000 breeding pairs. The remaining 30 species were represented by populations of less than 150,000 pairs. The density of upland sandpipers nesting on wet meadows in the Platte River valley is the highest recorded in the prairie region of North America (Faanes, unpubl. data).
The Platte River system is occupied by 15 threatened and endangered species of plants and animals listed under the Endangered Species Act of 1973, as amended (ESA)(Table 7). Nine species each occur in Colorado and Nebraska (Table 8); six species in Wyoming. Another 12 candidate species for federal listing may occur along the Platte River (Table 9). Natural Heritage Programs in the three states have identified an additional 208 species of plants with "special concern status" in their respective states (Appendix F). Below we provide a brief overview of the federally listed endangered and threatened species in the Platte River system.
Black-footed ferrets formerly occurred throughout the Great Plains from Canada south to Texas and Arizona (Jones et al. 1985). The historic range included virtually all of the grassland portion of Colorado and Wyoming. In Nebraska, the species occurred throughout the grassland area east to about Lancaster County. The historical range is also nearly identical to the ranges of the black-tailed prairie dog, white-tailed prairie dog, and Gunnison's prairie dog, the prey of the ferret (Anderson et al. 1986). The black-tailed prairie dog is a candidate for listing on the endangered species list. Since the turn of this century, prairie dog populations have declined as much as 98 percent.
Prairie dogs were formerly abundant in the grassland biome of North America. Merriam (1902) estimated areas occupied by prairie dogs in the late 1800s totaled about 280,000,000 ha. Anderson et al. (1986) estimated that by 1910, prairie dogs occupied 40,000,000 ha. The development of effective rodenticides in the early 1900s coupled with expanded agricultural production, caused prairie dogs to disappear rapidly from much of their historic range. Various estimates suggest that a reduction of 90-95 percent of the historically occupied prairie dog range occurred from 1900 to the present (Anderson et al. 1986, Choate et al. 1982, Flath and Clark 1986). The grassland biome has been largely eliminated and most remaining prairie is heavily fragmented (Samson and Knopf 1994). Among the nearly 130 United States counties or Canadian provinces where black-footed ferrets have been found since 1880, only ten had ferrets by the 1960s (Anderson et al. 1986).
At present, black-footed ferrets may still occur in isolated colonies in eastern Colorado and Wyoming and western Nebraska. The last specimen from Nebraska was obtained along the central Platte near Overton, Dawson County in 1949. Currently there is little suitable habitat for prairie dogs in Nebraska (only ten complexes of greater than 1000 acres); Nebraska state law requires the extermination of prairie dogs. Surveys to determine the presence of black-footed ferrets must be conducted prior to initiation of any federal action that may harm the species or its habitat. All such surveys to date have been unsuccessful in locating black-footed ferrets. The last Wyoming record near the North Platte River was an individual ferret found dead in a stock tank near the Miracle Mile in 1972.
The eagle was listed as an endangered species on March 11, 1967 (32 Federal Register 4001). One cause of population decline was impaired reproduction caused by accumulation of pesticides ingested from contaminated prey. The populations and the number of occupied nesting territories have increased throughout much of the United States over the past two decades. As a result, the Service is considering reclassifying the eagle from endangered to threatened throughout its range in the lower 48 states (excepting Arizona, New Mexico, southeastern California, western Texas, and the western panhandle of Oklahoma).
In Nebraska, bald eagles once bred regularly (1870s and 1880s) along the Missouri River and along Indian Creek in Gage County, on the eastern edge of the state (Johnsgard 1980, Rapp et al. 1958). Bald eagles also may have bred along the Platte and North Platte rivers before settlement (Currier et al. 1985). Townsend (1839) recorded a nest on the North Platte River near Ash Hollow on May 25, 1834. A single bald eagle hatched in a nest constructed along the Platte River near Omaha, Nebraska, in 1991.
Wintering concentrations of this species in the midcontinent region are associated primarily with river systems. Studies of wintering eagles and their habitat use over the past 20 years have documented the Platte River as a major wintering area for eagles (Lingle and Krapu 1986, Stalmaster and Associates 1990, U.S. Bureau of Reclamation 1981; Vian 1971). During midwinter surveys (1982-1994), an average of 335 birds (43 percent of the statewide wintering population) were found in the Platte River basin in Nebraska. The 30-mile stretch of the Platte River between Darr and Elm Creek on average supports more eagles than any other area of Nebraska. In 1981, six communal night roosts on the Platte River between Darr and Elm Creek averaged 60 eagles per night. Currently, there are at least eight communal roosts along the Platte River between Brady and Grand Island, Nebraska.
Wintering bald eagles are found in the largest numbers near Lewellen and Overton along the Platte and North Platte rivers (USFWS 1981). In March, ice-cover on the Platte begins to decline with warming spring temperatures. Migrant bald eagles begin to arrive on the river during the spring thaw, and then disburse according to the distribution of open-water areas on the river.
During the USFWS Ecology Study (USFWS 1981), 11 bald eagle nocturnal roost sites were identified. Although these nocturnal roosts were scattered throughout the stretch of the river from Lewellen to Kearney, three major roosts occurred in the immediate vicinity of Overton. Along the Platte, nocturnal roosts consist primarily of large cottonwoods which offer protection from the elements. The same nocturnal roosts are apparently used year after year, as was the case with 11 roosts monitored during the winters of 1978-79 and 1979-80. Roost site tenacity, however, diminishes as ice on the river thaws in early spring and migrating eagles begin to arrive on the Platte (Lingle and Krapu 1986).
Survival of individual eagles, particularly those in their first year of life, probably depends heavily on conditions they encounter during the wintering period. The physiological condition of adults at the beginning of each breeding season, an important factor influencing reproductive success, also is affected by how well their energy demands are met in wintering areas. Thus, the survival and recovery of nesting populations depend on eagles having suitable wintering areas (USFWS 1983).
In Nebraska, eagles spend 4-5 months on their wintering grounds during a given year (Vian 1971; Lingle and Krapu 1986). The majority of wintering eagles are found near open water where they feed primarily on fish and waterfowl, usually taking those which are dead, crippled, or otherwise vulnerable (Lingle and Krapu 1986; Stalmaster and Associates 1990; USFWS 1983). In addition, eagles are known to feed on carrion, small mammals, and game birds (Lingle and Krapu 1986; Lish 1975; U.S. Bureau of Reclamation 1981).
During the primary wintering period of December to March, suitable roosting and foraging habitat is important to eagles (Lingle and Krapu 1986; Stalmaster and Associates 1990). Eagles occur during spring and fall migrations as well (Johnsgard 1980). Suitable roosting habitat along the Platte River has increased over the past 50 years as the riparian forest matured. The riparian forest has replaced open channel and sandbar habitats.
Both available foraging habitat and the availability of prey are affected by the operation of water development projects. For example, reservoir releases maintain open water foraging habitat (free of ice cover) in some river reaches where eagles forage. Flow conditions also influence species composition of the forage fish community, and age and size structure of forage fish. A single low flow event can have lasting effects on the forage fish community. Low summer flows and elevated water temperatures have resulted in fish kills during 6 of the last 9 years in the central Platte River.
Temporal shifts in diet follow changes in prey availability. When fish become difficult to obtain, for example during ice cover or high winter flows, eagles depend more heavily on waterfowl as a food source.
Two subspecies of peregrine falcons, anatum and tundrius, are migrants in Nebraska. Although the anatum subspecies is endangered and the tundrius subspecies is considered threatened in Nebraska, it is virtually impossible to distinguish the two in the wild. There is only one nesting record from Nebraska, but peregrine falcons are an occasional migrant in the state. Four peregrine falcons were reported along the Platte River during the Platte River Ecology Study, two of which were observed at Mormon Island Crane Meadows (USFWS 1981). In all the sightings, peregrine falcons were observed in open grasslands and hayfields, and in sparsely vegetated river channels. During migration through Nebraska, peregrine falcons feed primarily on medium-sized birds, including waterfowl.
Least Tern and Piping Plover
The interior population of the least tern was listed as an endangered species on May 28, 1985 (50 Federal Register 21792). There are currently nearly 7,000 terns widely scattered across the interior of the United States. Along the Platte River, terns nest on sandbars and at sand and gravel pits from the Missouri River to North Platte, Nebraska. The 876 terns censused along Nebraska rivers in 1991 represented 13 percent of all censused terns in 1991, and the 585 terns observed along the Platte River system in 1991 represented 8 percent of the total population (Sidle et al. 1991a; Kirsch and Sidle In Review). In 1992 and 1993, there were 680 and 676 terns, respectively, censused along the Platte and North Platte Rivers in Nebraska. An analysis of tern population data for 1986-1991 for the entire range indicated an overall positive trend (Kirsch and Sidle In Review). The strong positive trend for the entire population was influenced by a strong positive trend on the lower Mississippi River where more than half of the terns nest.
Least tern populations and subspecies in North America nest in areas with similar substrate. Beaches, sand pits, sandbars, islands, and peninsulas are the principal breeding habitats. Over 90 percent nest on riverine sandbars. Unconsolidated substrate consists of small stones, gravel, sand, debris, and shells. A mixture of coarse sand, shells, and other fragments offer cryptic qualities, stability in wind, and water permeability. Vegetative cover is usually less than 20 percent at the time of nesting. Tern colonies in denser vegetation may be a response to habitat loss or a function of strong site tenacity. Eventually, vegetated areas are abandoned.
Terns feed on small fish in the river and at sand pit ponds (Wilson 1991). Most terns nest at sand pits along the Platte River from North Platte to the Loup River confluence (Sidle and Kirsch 1993) because suitable sandbar habitat is uncommon due to cumulative instream flow withdrawals for various purposes.
The plover was listed as a threatened species on December 11, 1985 (50 Federal Register 50733). During 1991, a total of 398 plovers were censused in Nebraska (Sidle et al. 1991a). This total represents 11 percent of the Great Plains population (Canada and United States) and 19 percent of the U.S. Great Plains population (Haig and Plissner 1992). The 206 plovers censused on the Platte River system in 1991 represent 10 percent of the U.S. Great Plains population. In 1992 and 1993, 273 and 291 plovers were censused on the Platte and North Platte Rivers in Nebraska. The Northern Great Plains plover population is declining by 7 percent annually (Ryan et al. 1993). The Piping Plover Recovery Team recommended that the Service reclassify the Northern Great Plains population as endangered.
Plovers feed on invertebrates near the water's edge and on moist river substrate (Corn and Armbruster 1993). They nest on sparsely vegetated sandbars, sand and gravel pits, reservoir shorelines, and alkali wetlands. Nesting habitats on the Platte, Niobrara, and Missouri Rivers are typically dry sandbars located midstream in wide, open channel beds and with less than 25 percent vegetative cover (Faanes 1983; Schwalbach 1988; Ziewitz et al. 1992). These conditions, similar to those of the tern, provide the essential requirements of wide, horizontal visibility; protection from terrestrial predators; isolation from human disturbance; and sufficient protection from rises in river levels.
Channelization, irrigation, and construction of reservoirs and pools have contributed to the elimination of much of the tern and plover sandbar nesting habitat in the Missouri River system. In the Platte River basin, continuing water depletions reduce the width and/or depth of water surrounding nest sites, and this may increase predation and human disturbance. Increased depletions permit vegetation encroachment into nesting areas. Extreme depletions dewater river reaches sufficiently to kill small fishes, the tern's principal food source.
Comparisons of sandbar area, channel width, mean elevation, and maximum elevation of nest sites versus systematic sample sites in the lower and central Platte River study areas indicate that terns and plovers select wide channels with a large area of dry, sparsely vegetated sand (Ziewitz et. al. 1992). By these two measures alone, habitat availability was considerably greater on the lower Platte River than on the central Platte River. These differences between the nest and systematic sample sites and between the lower and central Platte River suggest that habitat availability is limited in the river channel along the central Platte River. A greater number of terns and plovers nest on sandpits rather than on the river along the central Platte River (Lingle 1988, 1989; Sidle and Kirsch 1993); a further indication that riverine habitat is in short supply.
The lower Platte River (Loup River confluence to Missouri River) still experiences periodic high flows which scour vegetation from the channel and pile sand into suitable sandbars for tern and plover nesting (Sidle et al. 1992). Such flows continue to occur because most of the watershed in tributaries of the lower Platte River (Loup, Elkhorn, and Salt Creek Rivers) has not been dammed or diverted.
Recovery plans for the plover and tern call for the maintenance of the distribution and range of both species, protection of essential habitat, and the restoration of nesting habitat (USFWS 1988, 1990). Essential habitat along the Platte River refers to suitable sandbars in the river channel with appropriate flows. Given the degraded habitat conditions for these birds in the central Platte River, channel habitat restoration and adequate instream flows are necessary. Because little nesting remains on the central Platte owing to cumulative water withdrawals, proposed flow depletions must, therefore, be evaluated for their effect on terns and plovers.
Historic observations that started in 1804 along the Platte River reported suitable habitat within the channel, and specimens were collected in 1856 near Columbus. Least terns were found nesting in a colony on sandbars in the South Platte River near North Platte, Nebraska (Tout 1947). Observations made from 1926 through 1929 showed populations at this site were: 1926 - 34 adults (17 nests), 1928 - 36 adults (18 nests), 1929 - 50 adults (22 nests). Terns also were present in 1930, but a storm killed some adults and reduced the colony to about half its former size. No data are available for 1930-40.
The next recorded observation was in 1941 when birds were found nesting on sandbars in the river near Columbus (Shoemaker 1941). Ten and possibly more nests suggest that the colony size was at a minimum 20 adults. A single nest, and then young, was found at Merritt's Beach swimming lake, northwest of Plattsmouth in 1943 (Heineman 1944). Six pair of least terns were found nesting on a sandbar in the South Platte River in 1948 two miles east of Brule, Keith County (Benckeser 1948). Nesting was recorded again on the South Platte River in the vicinity of North Platte in 1949 (Audubon Field Notes 3:244).
The longest field observation of least terns on the Platte River was made during 17 years south of Lexington (Wycoff 1960). The nesting area was a low sandbar not over 75 feet wide and about 200 feet long. Some of the higher populations recorded at this site were: 1949 - 35; 1950 - 20; 1953 - 24; and 1954 - 25. Numbers declined as Wycoff (1960) notes the increasing growth of vegetation in the channel.
During more recent surveys of the Platte River, Downing (1980) estimated a population of 150 least terns in 1975 based on 80 birds observed. Following Downing's initial inventory in 1975, partial surveys were conducted along two different reaches of the river in 1979 and 1981. Surveys of interior least terns on the Platte River have been conducted annually since 1982 by the Nebraska Game and Parks Commission (NGPC). Most nesting along the central Platte occurs at sand pits (Table 10). There is almost no nesting in the river channel because river channel habitat has almost disappeared (Ziewitz et al. 1992).
The former status of the Eskimo curlew in Nebraska has been described by Swenk (1915). The birds frequented tallgrass prairies throughout the state, but their preferred habitat was the wet meadow near the Platte River (Bent 1929, Buechler and Bar 1920, Swenk 1915). Curlews seemed to be particularly attracted to areas that had recently been burned by prairie fires. Their primary diet while along the Platte consisted of young grasshoppers and grasshopper eggs. As the prairie was plowed and tilled, however, curlews began to feed on white grubs and cutworms in wheat fields and corn fields, as well as on grasshoppers in pastures and hay meadows (Swenk 1915).
Thousands of Eskimo curlews congregated along the Platte at the time of settlement, feeding and storing energy to sustain them on their migration and to supply the energy to initiate breeding. Fred Stolley, a farmer near Grand Island recalled flocks of 500-1,000 curlews congregating in newly-plowed corn fields during the 1860s (Currier et al. 1985). The curlews were relatively tame around humans and extremely gregarious. They were almost always seen in great numbers. Such masses of birds were extremely vulnerable to hunters who shot them by the wagonload to sell to eastern markets. Many wagonloads spoiled, however, before making it to market, and piles of rotting birds were dumped on the prairie so that hunters could refill their wagons with fresh curlews (Swenk 1915). By 1890, such slaughters had significantly reduced the population. Although Eskimo curlew received protection from hunting under the Migratory Bird Treaty Act of 1916, its population has failed to recover. The loss of native grasslands in the central plains of the United States and in South America may have significantly reduced the food source for migrating curlews and most likely explains their failure to rebuild their population (Faanes and Senner 1991).
A single Eskimo curlew was seen 16 April 1987, in a wet meadow at the Mormon Island Crane Meadows near Grand Island, Nebraska (Faanes 1990). This was the first Nebraska observation in 60 years. The total world's population of Eskimo curlew is estimated at 100-150 individuals.
The whooping crane was listed as endangered on March 11, 1967 (32 Federal Register 4001). The historic breeding range of the whooping crane extended from central Illinois, northwestward through northern Iowa, western Minnesota, northeastern North Dakota, southern Manitoba and Saskatchewan, and the general vicinity of Edmonton, Alberta, to the present nesting area in Wood Buffalo National Park in the Northwest Territories, Canada (USFWS 1986). Winter distribution occurred primarily along the Gulf of Mexico from Louisiana to northeastern Mexico. The last remaining individual of a nonmigratory population in southwestern Louisiana was captured in 1950.
Although whooping cranes probably were never very abundant, population estimates of 1,300 to 1,500 were made by Allen (1952). Banks (1978) used two independent techniques of population estimation to derive estimates of 500 to 700 whooping cranes in 1870. By 1941, the numbers of the last remaining population had declined to a low of 22 individuals. Today, the sole remaining natural breeding population nests at Wood Buffalo in Canada, and winters on and near Aransas National Wildlife Refuge (Aransas) on the Texas coast. It migrates through Nebraska twice each year. The population gradually increased from the 1940's to the mid-1980's. In 1987 the population of the Wood Buffalo-Aransas flock reached 134 birds and has fluctuated near that level (132 to 146) during the past 6 years. Intensive efforts to reestablish other breeding populations in the wild have not succeeded.
More sightings of whooping cranes are reported from Nebraska than from any other state or province in the migration corridor (Lingle et al. 1991). Spring sightings are more common along the central Platte River than any other habitat area. In recognition of its importance to the whooping crane, a 3-mile wide, 56-mile long reach from Lexington to Denman, Nebraska, is designated as critical habitat (50 Code of Federal Regulations 17.95). Factors considered in making the critical habitat determination for the Platte River are:
Platte River bottoms provide a dependable source of food, water, and other nutritional or physiological needs of the whooping crane during spring and fall migrations. Insects, crayfish, frogs, small fish, and other small animals, as well as some aquatic vegetation in the river and adjacent wet meadows, and cereal crops in nearby croplands appear to be major items taken during the migration period.
Under specific flow regimes, the Platte River generally provides whooping cranes with the required open expanse for nightly roosting. The availability of shallow, submerged sand and gravel bars in rivers and lakes appears to be one of the major factors determining whooping crane use of these habitats as roosting sites. Whooping cranes observed during migration are most often found within short flight distances of these wetland areas.
The Platte River provides needed isolation. Whooping cranes do not readily tolerate human disturbances. A human on foot at a distance of over 1/4 mile can quickly put a whooping crane to flight.
During the spring migration, wet meadows along the Platte River provide whooping cranes the opportunity to obtain essential food for survival and reproduction. Resting and foraging during spring migration ensure that the birds arrive in a healthy condition on the breeding grounds at Wood Buffalo. Healthy birds are essential for successful reproduction and, ultimately, the survival and recovery of the species. The health and survival of whooping cranes are dependent upon the condition and abundance of their habitat. Like other migratory birds with delayed sexual maturity and lifelong pair bonds, whooping cranes adhere to ancestral breeding areas, migratory routes, and wintering grounds, leaving little possibility of pioneering into new regions. The importance of maintaining traditional habitats such as the Platte River is amplified by the impacts from ongoing conversion of pothole and prairie to hay and grain production, which has made nearly all of the whooping crane's original migrational range unsuitable for use by the species (USFWS 1986).
Whooping cranes roost in bodies of water. An evaluation of 10 known whooping crane riverine roosting sites identified the following characteristics (USFWS 1981):
Measurements at 30 whooping crane roost sites on the Platte River from 1983 to 1993 have supplemented this information (Faanes et al. 1991, 1992a). These data indicate that the widest channels are preferred. Use of channels wider than 900 feet is disproportionately high in relation to their low availability. The wetted width of the channel at these sites averaged 92 percent of the channel width (median of 95 percent), and water depth ranged from shallowly submerged sandbars to deeper channels, 1.5 to 3.5 feet deep. Such information systematically collected at roost sites over an 11-year period, has been used to model the quality of roost habitat as a function of Platte River discharge.
Maintenance of roost habitat on the Platte River is dependent on maintaining open channels and suitable flow (Faanes and Bowman 1992, Ziewitz 1992). Altered flow regime and sediment transport processes have resulted in woodland expansion and narrowing river channels (Williams 1978; Lyons and Randall 1988). The decrease of sandbars and open water, and encroachment of forestland into the former channels, coincides with decreased streamflows. In the 120-mile river segment from Kingsley Dam downstream to the J-2 Hydropower Return near Lexington, Nebraska, spring streamflows have been reduced by 85 percent. This segment is now heavily forested and rarely used by whooping cranes (one confirmed use occurred in the past 50 years).
In the 90-mile river reach from the J-2 Hydropower Return downstream to Chapman, Nebraska, peak flows have been reduced by about 70 percent since 1930 (USFWS 1981; Williams 1978). Most of this reach is forested, but some short segments still have wide channels, and other segments have been mechanically cleared and maintained free of vegetation. These areas are the primary suitable whooping crane habitat remaining along the Platte River.
Wet meadows once occurred extensively along the Platte River; however, substantial reductions have occurred during the past century (Currier et al. 1985; Sidle et al. 1989; U. S. Department of the Interior (Interior) 1990). Ground water levels, which maintain wet meadows, are hydrologically linked with river stage. The reduction of springtime pulse flows, owing to cumulative water storage and diversion, has facilitated land leveling, ground water drainage, and conversion of wet meadows to row crop agriculture. Remaining wet meadows compose somewhat less than 5 percent of the Platte River valley near the river. Whooping cranes require isolation, further reducing the value of small, fragmented wet meadow parcels as whooping crane foraging habitat.
Greenback Cutthroat Trout
The greenback cutthroat trout (Salmo clarki stomias) was listed as threatened in 1978 (43 Federal Register 16343-16345). It was reclassified to "threatened" in 1978 to facilitate recovery efforts. The original distribution of this trout included the headwaters of the South Platte and Arkansas River systems in central Colorado. Populations were restricted to the mountains and foothills because the warmer, turbid waters in lower river reaches did not provide suitable habitat. Greenback cutthroat trout populations were rapidly reduced because of competition and hybridization with other trout species, and loss or degradation of habitat from mining, logging, grazing, and irrigation projects. Unhybridized populations of this trout were generally believed to be extinct by 1930.
Much of the protected habitat for this species occurs on public lands contained in various National Forests and National Parks. Waters on private land continue to be threatened by habitat degradation from logging, mining, grazing, and water development projects. The greenback is the only trout endemic to both the headwaters of the South Platte and Arkansas River drainages. Although once abundant, their numbers declined in the late 1800's due to overharvest and the introduction of exotic trout species (USFWS 1993).
Because greenbacks hybridize with other species of trout, populations can range phenotypically from "essentially pure" to obvious hybrids. The Colorado Division of Wildlife adopted a rating system developed by Binns (1977) as a means of rating population purity. Each population is assigned a letter ranging from A (pure) to F (obvious hybrids). Only Type A populations are considered for recovery purposes (USFWS 1993).
As a result of recovery efforts, captive broodstocks were established, nonnative fish were removed from suitable habitat, greenbacks were reintroduced, stable populations developed, and catch-and-release fisheries initiated (USFWS 1993).
Habitat requirements of the greenback appear little different from other species of trout (Bulkley 1959; Nelson 1972). This species requires coldwater fisheries habitat with adequate trout spawning substrates and temperatures for adequate reproduction (USFWS 1993).
The recovery plan (USFWS 1993) calls for the restoration of this species' non-threatened status within its native range by establishing at least 20 stable populations within Colorado, 15 of which should occur in the South Platte River drainage. Once the stable populations goal is accomplished, delisting of the species will be sought.
The pallid sturgeon was listed as an endangered species on September 6, 1990 (55 Federal Register 4001). Although the species' range is large, catch records are extremely rare, with no captures of subadults in recent years. The last reported observation of possible spawning was in 1974. The species may be close to extinction (USFWS 1994a).
The range of the pallid sturgeon encompassed the Missouri River; the lower reaches of the Platte, Kansas, and Yellowstone Rivers; and the Mississippi River below the confluence with the Missouri River. Destroyed and altered habitats are believed to be the primary cause of adverse effects on reproduction, growth, and survival of the pallid sturgeon. Recovery of the pallid sturgeon is unlikely to be successful without restoring the critical portions of morphology, hydrology, temperature regimes, and sediment/organic matter transport of the rivers that provide the life requisites for pallid sturgeon (USFWS 1994a).
Sediment and discharge are the raw material and driving force, respectively, for habitat development in large floodplain rivers such as the lower Platte, Missouri, and Mississippi Rivers. The process of erosion transports organic matter and large woody debris from the floodplain, as well as sediment in the form of rock, gravel, sand, silt, and clay to large floodplain rivers from tributaries. Before the Missouri River was channelized and impounded, it annually eroded 4.8 acres/mile of its floodplain. River impoundments have eliminated 80 percent of this material (Slizeski et al. 1982) since the early 1950's. The lack of sediment upset the natural channel equilibrium and was replaced by a variety of nonequilibrium processes such as hydraulic sorting and bed paving. In the lower Missouri River basin, channel degradation has occurred from Gavins Point Dam, the lowest main stem dam, downstream to near the mouth of the Platte River. Degradation causes loss of connection with backwater habitat areas of the floodplain.
The Platte and Yellowstone Rivers are among the largest tributaries in the Missouri River basin. The Platte River is the only tributary below Gavins Point Dam that originates in the Rocky Mountains and delivers runoff from mountain snowmelt to the lower basin. Because of its importance to the lower Missouri River basin, the Platte River figures prominently in the recovery plan for the species.
Of 17 occurrences of pallid sturgeon reported in the lower Missouri River basin (below Gavins Point Dam) in Nebraska since 1980, 8 are from the Missouri River near the Platte River confluence or from the Platte River itself. Thus, 47 percent of the observations in Nebraska are from an area representing about 10 percent of the range. Recent occurrences of pallid sturgeon in the Platte River are reported as far upstream as the Elkhorn River (Platte River mile 32).
High spring flows may be particularly important for pallid sturgeons using the Platte River. Eight of the nine captures of pallid sturgeon in the Platte or Missouri Rivers near the mouth of the Platte River, occurred during May and June; the ninth occurrence was in April. In addition, eight of nine occurrences correspond with years when May-June flows in the lower Platte River were above normal for the recent period (Louisville gauge, 1970-1993). Only one sighting occurred in years when flow was below normal, suggesting that flow depletions may adversely affect pallid sturgeon use of the Platte River.
Spring flows of the central Platte River have greatly declined since the early 1900's (Williams 1978; Eschner et al. 1983). Since the 1930's, the diminution of flows in the upper basin alone (above the Loup River) accounts for a 40 percent decrease in May and June flows in the lower Platte River. Faanes (1992a) estimated that other projects proposed in the upper Platte River basin would decrease the remaining flow in the central Platte River by 75 percent. Owing to the precarious state of pallid sturgeon populations throughout its range and to cumulative and ongoing water development throughout the Platte River basin, proposed flow depletions must, therefore, be evaluated for their effect on pallid sturgeon.
The Wyoming toad (Bufo hemiophrys baxteri) was listed as endangered on February 16, 1984 (49 Federal Register 1992-1994). The Wyoming toad was first described in 1946 (Porter 1968). The only wild population occurs at Mortenson Lake, southwest of Laramie, Wyoming. This habitat was purchased by USFWS in 1992 and is now the Mortenson Lake National Wildlife Refuge. In 1992, a reintroduction effort was conducted to establish a second Wyoming toad population at Lake George on the Hutton National Wildlife Refuge. Toads were observed frequently in 1993 at Hutton but only two individuals could be located in 1994. It is unknown how many toads remain at Hutton and Mortenson Lake.
Due to a significant decline in the wild population over the last two years, a portion of the toad population was captured and placed into a captive breeding program. In 1995, about 75 toadlets were produced at the Wyoming Game and Fish Department's Sybille Research Center.
Causes for endangerment of this animal are not well known. Widespread spraying of insecticides to control mosquitoes, changes in agricultural practices, increased predation, disease, and climatic changes have been suggested as causes of the decline in Wyoming toad numbers, but no certain cause has been identified. The herbicide Atrazine is known to reduce populations of other Bufo species (Beebee 1973), and can be introduced into watersheds at sufficient levels to kill Bufo eggs or tadpoles.
It is now suspected that a bacterial disease, "redleg", is responsible for the decline of the toad. The population has experienced a significant decline in recent years due to this stress-related disease. Redleg is present in the toads and triggered by stressors such as the onset of cold weather and other unknown stressors.
American Burying Beetle
The beetle was listed as an endangered species on July 13, 1989 (54 Federal Register 29,655). The beetle has been recorded historically from at least 150 counties in 35 states (including the District of Columbia) in the eastern and central United States (Peck and Kaulbars 1987), as well as along the southern fringes of Ontario, Quebec, and Nova Scotia in Canada (Peck and Anderson 1985, Schweitzer and Master 1987, Wells et al. 1983). Its historical range can roughly be described as most of temperate eastern North America, from Nova Scotia as far west as North Platte, Nebraska. The northernmost record is from the upper peninsula of Michigan and the southern terminus of its range is Kingsville, Texas.
Peck and Kaulbars (1987) broadly characterized the distributions for 32 species of nearctic carrion beetles. These authors placed the beetle in the category "Eastern deciduous forest region." Since 1970, the beetle has been documented from Rhode Island, Oklahoma, Nebraska, Arkansas, Missouri, and Kentucky (USFWS 1991a), and there was a single 1972 record from Ontario (Perkins 1983). Existing populations are known to occur in Rhode Island, Oklahoma, Arkansas, and Nebraska.
Historical records for the beetle in Nebraska indicate the species occurred along watercourses where riparian deciduous or scrub forests were predominant (Jameson and Ratcliffe 1989). Recent collections in Nebraska (1970-present) were in Custer, Lincoln, and Cherry Counties. M. Peyton (personal communication) has developed the most up to date information of recent collections in the upper Platte River system, Nebraska. Two beetles were collected during July 1988 and 1993 in Lincoln County within 2 miles of the South Platte River, indicating an extant beetle population in the Platte Valley. The two collections were made within 1/2 mile of Fremont Slough (a wetland complex), and all recent collections in Nebraska have been in the vicinity of wetlands.
The prevailing theory regarding the species' decline involves habitat fragmentation (USFWS 1991a). Schweitzer and Master (1987) stated that the availability of significant humus and top soil suitable for burying carrion is an essential habitat component. Davis (1980) used museum specimen collections to detail the decline in numbers of the American burying beetle. Anderson (1982) found a pattern of increasing localization in capture records.
It is possible that water development may have been a factor contributing to the decline of the beetle in Nebraska. Water storage and diversions substantially reduced high flows in the river, which typically occurred during spring. This in turn reduced the frequency and duration of soil saturation caused by a periodically high water table. As a result, low-lying prairies and wet meadows in and near the river became drier and were converted to cropland. The continuing loss and fragmentation of grassland habitat may have an adverse effect on the beetle. Owing to the cumulative and ongoing water withdrawals, proposed flow depletions must, therefore, be evaluated for their effect on the beetle.
Pawnee Montane Skipper
The butterfly, Hesperia leonardus montana, was listed as a threatened species on October 28, 1987 (52 Federal Register 36,176-36,180). This species occurs in the South Platte River drainage system in four counties (Teller, Park, Jefferson, and Douglas) along the Front Range of the Rocky Mountains. The known range is about 23 miles long and 5 miles wide (Keenan et al. 1986). The portion of range supporting apparently suitable habitat for the species covers about 38 square miles (Earth Resources and Technology 1986). Lands supporting skipper habitat are owned or managed by the Pike National Forest, U.S. Bureau of Land Management, Denver Water Department, County of Jefferson, and numerous private individuals.
Skipper habitat is in mountainous areas characterized by canyons with steep slopes and narrow valleys (Keenan et al. 1986). Skippers occur primarily in dry, open ponderosa pine woodlands on outcroppings of Pikes Peak granite where soils are thin, unstable, and susceptible to water erosion. Two essential components of skipper habitat are blue grama grass which provides the principal food for the larval stage of this butterfly, and prairie gayfeather which is the principal nectar plant.
Construction of an existing dam and reservoir, and road, housing, and other development has destroyed, modified and curtailed much of the skipper's habitat and range. Use of insecticides to control infestations of two other insects occurs in the principal range of this butterfly. Although not presently known to be used within the principal range of the species, application of chemicals may result in the loss of individual butterflies or segments of the population.
North Park Phacelia
Phacelia formulosa was listed as endangered on October 1, 1982 (47 Federal Register 38,540-38,543) because its two small populations are jeopardized by motorcycle activity, cattle trampling, and the potential for coal, oil, and gas development. This plant occurs on sandy bluffs in approximately 20 populations in the North Park region of Jackson County, Colorado. It has been found to occur only on raw exposures of the Coalmont formation which is overlain in some areas by alluvium and Pierre shale with exposures appearing along watercourses (North Platte and Michigan rivers, and reported along the Canadian River).
One population contained 22 plants in 1979, but had increased to about 200 individuals in 1981. Another population comprises five sites in eight miles along the North Platte River and consists of about 2,500 individuals. Judging by the number of individuals, the steep-sided ravines in the North Park area provide the most "productive" habitat for this species. Plant genera associates of the North Park phacelia include Mentzelia, Chrysothamus, Oryzopsis, Arenaria, Eriogonum, and Rosa. The vegetation is very sparse on the exposed Coalmont formation and the North Park phacelia is the most noticeable plant during the fall.
Prairie Fringed Orchids
The eastern prairie fringed orchid (Platanthera leucophaea) and western prairie fringed orchid (P. praeclara) were listed as threatened under the ESA on October 30, 1989 (54 Federal Register 39,857-39,863). The historic range of P. leucophaea extended in a band from New Brunswick west to Nebraska and south to Arkansas and Oklahoma. P. praeclara occurred primarily west of the Mississippi River from Manitoba south through Nebraska to Oklahoma.
It occurs in wet prairie habitats and was distributed historically throughout much of the western Central Lowlands and eastern Great Plains physiographic provinces of the central United States and Interior Plains in extreme south-central Canada (Brownell 1984). Comparison of the historic and extant ranges shows that the species has apparently been extirpated from South Dakota, with significant reductions in counties of occurrence in Missouri, Iowa, southeastern Kansas, and eastern Nebraska.
Historic (observed prior to 1970 and/or confirmed destroyed), extant (observed since 1970), and unverified reports exist for more than 160 sites in 102 counties in 8 states and 1 Canadian province. Extant populations of the fringed orchid occur at 60 sites in 37 counties in 7 states: Iowa (12 counties), Kansas (7 counties), Minnesota (8 counties), Missouri (2 counties), Nebraska (4 counties), North Dakota (2 counties), Oklahoma (2 counties), and in the Canadian Province of Manitoba (USFWS 1991b).
The fringed orchid has declined significantly throughout its historical range largely due to habitat loss and degradation (Freemen and Brooks 1989). Conversion of prairies for row crops, fire suppression, haying, and land development are factors which contributed to the species' decline. Hay mowing annually removes seed capsules and plant biomass before natural seed dispersal can occur, preventing seedling recruitment (Bowles 1983, Bowles and Duxbury 1986). Pollination of the fringed orchids is accomplished only by hawkmoths (Sphingidae). Long-term population survival requires that hawkmoth populations be maintained. Five Kansas populations are known or assumed to have been destroyed due to conversion of prairies for agriculture; three of these since 1969. Fire suppression was a factor in the establishment of shrubby vegetation at two sites in the Nebraska Sandhills where the fringed orchid formerly occurred. Shading by the shrubbery may have contributed to the elimination of these populations.
Annual mowing of prairies for hay is a common practice in Kansas, Nebraska, and South Dakota. This practice, which typically occurs prior to the maturation of the fringed orchid's fruits, may have contributed to the decline of the species. Stream channelization and draining of seasonally wet prairies in the Nebraska and South Dakota Sandhills probably adversely impacted the species by altering the hydrologic regime. In most instances, channelization and draining was done to permit reliable access to wet prairies for hay. Other agricultural practices, such as grazing and herbicide use, also may have impacted the species.
The fringed orchid occurs on wet-mesic subirrigated prairies and sedge meadows along the floodplain of the Platte River. The only known population is on Mormon Island Crane Meadows, in Hall County, Nebraska. Peak flows have been greatly diminished during the past century, facilitating conversion of most low-lying areas near the river from grassland to intensive agriculture. Consequently, little habitat remains that is suitable for the fringed orchid. Interior (1990) estimated that 112,791 acres of wet meadow had been lost along the North Platte and Platte Rivers in Nebraska between 1938 and 1982. A rotational grazing and burning program is probably necessary to ensure the long-term viability of P. praeclara. Flow regimes conducive to maintaining biological productivity in wet meadows, coupled with protection of hawkmoth populations, may be necessary components of P. praeclara conservation in the Platte River system.
The destruction of fringed orchid habitat for cropland is the single most irreversible threat to the species (USFWS 1991b). It is likely that this threat varies from site to site, depending on local climate, ground water hydrology, and soil characteristics.
Penstemon haydenii was listed as endangered on October 1, 1987 (52 Federal Register 32,926-32,929). Blowout penstemon is known from small populations in Cherry, Hooker, Garden, Box Butte, Morrill and Sheridan Counties, Nebraska. Historically this species also occurred in Thomas County (Clausen et al. 1989). The number of plants known to be extant in 1986 was about 2,100 individuals. About 40-45 percent of the populations are located on private and state lands, and 55-60 percent occur on USFWS lands.
Successful control of unstable sand dunes has resulted in restriction of the required blowout habitats of P. haydenii. The blowouts where the species grow are conical or irregularly-shaped craters that are scooped out of sand by the swirling action of westerly winds. Because of successful dune stabilization programs that protect rangeland in the Sandhills, blowout penstemon does not have adequate amounts of additional habitat to invade. The decrease in extent of blowouts also has made dispersal to the fewer remaining natural blowouts more difficult.
Blowout penstemon comprises about nine disjunct populations. The small population size makes the species vulnerable to localized environmental changes. Blowout penstemon occupies a successional niche in the development and eventual revegetation of blowout habitats. As the vegetational cover in these areas increases, this species undergoes local extirpation. Blowout penstemon is not only rare but does not appear vigorous at the known localities; possibly because the blowouts have reached a stage of revegetation that exceeds the optimum habitat conditions for the species and the number of new blowouts is decreasing.
Ute Ladies' Tresses
The Ute orchid was listed as a threatened species on January 17, 1992 (57 Federal Register 2,053). It is endemic to moist soils in mesic or wet meadows near springs, lakes, or perennial streams. The species occurs primarily in areas where the vegetation is relatively open and not overly dense, overgrown, or overgrazed (Coyner 1989, 1990; Jennings 1989, 1990). Populations of the Ute orchid occur in relatively low-elevation riparian meadows in three general areas of the interior western United States: the Wasatch Front and west desert of Utah, the Uinta Basin in Utah, and the Front Range of Colorado. The known populations in eastern Colorado are located in mesic riparian meadows in relict tallgrass prairie areas near Boulder Creek in the city of Boulder and along the St. Vrain Creek near the city of Longmont, both within Boulder County; in the riparian woodland understory along Clear Creek, just west of the city of Golden (Jennings 1989); and within the city of Wheat Ridge open space, Jefferson County. In August 1993, 13 Ute orchids were found on a wetland area on the northwestern edge of the City on the Poudre River drainage. No other populations of the species are currently known from Colorado, though historic collections were made from either Weld or Morgan County (the historical record is not clear) in the Platte River valley in 1856 and at Camp Harding in El Paso County in 1896 (Jennings 1989, 1990). There is also a newly discovered population in Goshen County, Wyoming. The largest populations occur in Boulder County, Colorado, and along rivers and streams in the Uinta Basin in Utah.
Due to the small size of most populations and the erratic population fluctuations noted within monitored populations, it is not known whether existing populations are demographically stable over the long term. The highly variable demographic structure from year to year of the species' largest known population may make it more vulnerable to extinction in years of low population numbers. The species' low population numbers and restricted habitat make it vulnerable to natural or human-caused disturbances. Extant populations in eastern Utah and Colorado are typically very small and potentially vulnerable to habitat changes that could eliminate entire populations. Projects that affect the hydrology and vegetation of the species' riparian ecosystem may have a negative impact on the species.
The Ute orchid is believed to be extirpated from most of its historical range due to alterations of stream hydrography and hydrology. Channelized and depleted streams are no longer capable of creating the semiopen habitats (newly or recently abandoned stream channels) or maintaining the hydrologic conditions that sustain damp rooting zones throughout the growing season. USFWS believes that recovery of the species will involve management of stream habitats to retain, recreate, or mimic natural hydrography and hydrology and related vegetation dynamics. Projects that alter natural hydrography and hydrology may affect the Ute orchid where it is known to exist.
Three families, Muridae (voles), Sciuridae (squirrels), and Vespertilionidae (bats), constitute most of the mammalian fauna along the river valleys of the Platte River system (Appendix C). The black-footed ferret is the only mammal in the Platte River drainage on the federal list of endangered species.
Distribution patterns of mammals are related to environmental conditions. Unlike birds with a pandemic distribution, there are few mammals in our area that are Pandemic. Fully 25 percent of the mammalian fauna on the Platte River reach their western range limit in eastern or central Nebraska. Biotic factors that influence distribution patterns are especially evident among small mammal species, primarily with eastern or southern origin. For example, the distribution of fox squirrels shifted westward in recent years, probably in response to the increased area of forested vegetation along the Platte River. The distribution of other mammals in the Platte River valley has been affected by riparian forest expansion (Freeman and Benedict 1993).
Distribution patterns among large mammals have been highly influenced by habitat loss, hunting, and poisoning that accompanied the spread of human development. At least seven large mammals including the gray wolf, black bear, grizzly bear, wolverine, bison, bighorn sheep, and elk have been extirpated from Nebraska in the last 150 years (Clausen et al. 1989). Today those species are confined to scattered refugia in heavily forested areas or in the Rocky Mountains.
The Nebraska Natural Heritage Program (Clausen et al. 1989) identified 25 mammal species with concern status in Nebraska. Three of the listed species, fringed myotis, western big-eared bat, and Merriam shrew, occur in the Nebraska portion of the Platte River drainage.
The fish fauna of the Platte River system consists of over 100 species (Appendix D). The minnows and sunfishes constitute over almost 50 percent of all fish species. Distribution data (Lee et al. 1980) indicate that the bulk of the fish species occur in the eastern third of Nebraska. Tabor (1993) found that Great Plains fishes, primarily cyprinids, were experiencing unabated declines throughout their range. Fish diversity is declining along the central Platte due to frequent low summer flows and habitat loss (B. Goldowitz, personal communication). Dinan (1992) discusses the adverse effects of high water temperatures on fishes of the central Platte during low summer flows. Clausen et al. (1989) stated that fishes have been affected by habitat loss more than any other group of vertebrate species in Nebraska. Cited as causative agents were: 1) reduced stream flows from irrigation and urban water projects; 2) increased turbidity from agricultural runoff; 3) pollution from agricultural and urban development; and 4) stream channelization and reservoir construction (Clausen et al 1989). Nebraska law lists six fish species as threatened. Included in the total are four species that occur in the Platte River system: the lake sturgeon, northern redbelly dace, finscale dace and pearl dace. The lake sturgeon and sturgeon chub are candidates for federal listing under the ESA. The pallid sturgeon is already listed.
Distribution patterns among the fish species in the Platte River drainage indicate that most species are of eastern or southern origin. Fish species diversity in the upper reaches of the North Platte and South Platte rivers is relatively depauperate, reflecting the lower biological productivity of higher elevation waters.
Forty species of reptiles and amphibians constitute the herptofauna of the Platte River system (Appendix E). Snakes (Colubridae, Viperidae) are about 40 percent of all species. The Wyoming toad, a relict population of the Great Plains toad, is the only federally listed amphibian in the Platte River basin.
Clausen et al. (1989) stated that 60 species of reptiles and amphibians occur in Nebraska. Among those, 30 species are listed as being of special concern in that state, primarily because of their peripheral distribution status in Nebraska. Seven reptiles in the Nebraska portion of the Platte River system were included in the list of species of concern (Clausen et al. 1989) including the yellow mud turtle, sagebrush lizard, short-horned lizard, common kingsnake, smooth green snake, plains black-headed snake, and western ribbon snake.
The yellow mud turtle is probable in the Platte River drainage because of its occurrence in the Sandhills and in the Republican River drainage (Clausen et al. 1989). This species is declining because of wetland loss in the Sandhills and is being considered as a candidate for federal listing (Clausen et al. 1989).
The sagebrush lizard was collected near Broadwater, Morrill County, Nebraska, in 1978 and 1986 (Clausen et al. 1989). In Wyoming, this species is associated with sagebrush with rock outcroppings (Baxter and Stone 1985). Ranges of the copperhead and the timber rattlesnake historically extended westward to the Platte River in extreme eastern Nebraska (Lynch 1985). Those populations have probably been eliminated by habitat loss and human persecution.
The depauperate herptofauna in the Platte River system reflects the area's geographic location. The absence of structurally complex forested habitats and the harsh climate preclude invasion of many southern and eastern forms into the area.
Taxonomic complexities and the sheer number of individuals and groups severely limits our ability to quantify the invertebrate fauna. Because invertebrates are not "large and showy," this group has not received the depth of attention that most of the remainder of the fauna has received. This is not to discount the importance of invertebrates to the biodiversity of the area. In most instances, invertebrates form the basis of most food chains. For example, the Central Platte Natural Resources District (NRD) applied for an instream water right on the Platte River to protect the invertebrate food base of the piping plover, a threatened bird (Sidle and Murray 1993). Loss of invertebrate species can be a causative factor in the collapse of microecosystems, leading to greater environmental problems at higher trophic levels.
Clausen et al. (1989) listed 35 invertebrates (mostly Lepidopterans) as species of concern in Nebraska. The regal fritillary is a candidate species for listing. Many mussel species have a limited distribution along the Platte River (Freeman and Perkins 1992). However, many mussels are being found within canals and reservoirs of power and irrigation projects along the central and upper sections of the Platte River. Such locations usually have a steady supply of water. Lingle (1992) quantified significant environmental parameters which control distribution and abundance of mussel fauna along the south channel of the Platte River in Hall County. She found that water river discharge stability and thus water depth were the most significant habitat parameters determining naid abundance at any sample site.
Natural Heritage programs in Colorado, Nebraska, and Wyoming, identified 208 plant species of special concern in their states (Appendix F). Plant species are included as being of special concern based on The Nature Conservancy's (TNC) global heritage ranking, as well as statewide rankings of rarity. The list contains the names of all taxa known to occur in low numbers or to occupy a specialized niche that increases their vulnerability. For conformity, we prepared Appendix F based on plant taxa known to occur in counties bordering the North Platte, South Platte, and Platte rivers. These species are not restricted to the riparian zone immediately adjacent to the river channel. The Natural Heritate Program data point out the large number of special concern plant species in the Platte River drainage.
Natural Heritage data bases are important sources of information on plant diversity for several reasons. Most importantly, data bases include information on species at the limits of their ranges. These types of data are of particular importance in evaluating the biological diversity of a region. Although one plant species may be widespread in the desert southwest, it may be extremely rare in a given county. This species may never reach endangered status because of its large range. Conservation of the species at the limit of its range is important in maintaining maximum diversity of an area, and also in preserving a potentially important segment of its gene pool.