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Food habit studies stressing the dietary value of invertebrates to waterfowl during the breeding season (e.g., Bartonek 1968, Bartonek 1972, Bartonek and Hickey 1969, Dirschl 1969, Swanson and Bartonek 1970, Swanson et al. 1977) provided the impetus for much of the past research on aquatic invertebrates in prairie pothole wetlands. Migratory waterfowl are of considerable economic value and are the subjects of international treaties. Additionally, the PPR is a critical breeding area for many North American species. As a consequence, much of our information on wetland invertebrates in the PPR has been directed towards developing a better understanding of waterfowl ecology and management. Invertebrate species lists and distribution studies also have significantly contributed to our knowledge of prairie wetland invertebrates but the work has been patchy and is largely incomplete. Overall, basic ecology has provided the smallest contribution to our current knowledge of prairie wetland invertebrates, but development of more holistic perspectives of the critical role that invertebrates play in wetland ecology and function (Murkin and Wrubleski 1988) will likely stimulate such work. Aside from their obvious role in the feeding ecology of waterfowl and other birds, invertebrates provide critical food chain support for a wide variety of other organisms and play significant roles in nutrient cycling and overall wetland productivity (Murkin and Batt 1987). Further, invertebrates are sensitive to agricultural chemicals that accumulate in wetlands (Grue et al. 1989) and there is a growing interest in using them as indicators of wetland and landscape condition in the PPR (Adamus 1996) and elsewhere in the United States.

Invertebrate and Habitat Diversity

Invertebrate Responses to Hydroperiod

Seasonal drawdown, as well as the longer term wet/dry cycle, impacts the hydroperiod of prairie wetlands and has special significance to the ecology of aquatic invertebrates. During extreme drought, nearly all wetlands are dry, including many permanent habitats. Under such conditions invertebrate faunas undergo severe spatial reduction, and they are mostly comprised of specialists that can tolerate hypersaline waters or those that have such short life cycles that reproduction can be completed within an extremely short time. Wetlands can shift hydrologic function during severe drought (Winter and Rosenberry 1995), and viable, but normally dormant, eggs in semipermanent wetlands can hatch producing an invertebrate community that more closely resembles those that characterize temporary or seasonal marshes (Euliss and Mushet, unpublished data), especially when salt concentrations decrease due to deflation (LaBaugh et al. 1996) or dilution from overland flow.

As wetlands refill following drought, wetland invertebrate communities develop along predictable temporal guidelines. As pointed out by Wiggins et al. (1980), the early colonizing invertebrate community is structurally simple and is comprised mostly of r-selected detritivorous invertebrates. Later the invertebrate community becomes more complex as predators and other major functional groups of invertebrates become established as water permanence persists and the habitat becomes more complex in vegetative structure. Reintroduction of invertebrates that cannot tolerate hydroperiods of less than a year (e.g., the amphipods Hyalella and Gammarus) likely occurs from dispersal on mammals and birds (Segeraträle 1954, Peck 1975, Rosine 1956, Daborn 1976, Swanson 1984). Accompanying with this successional pattern are changes in the spatial distribution and abundance of invertebrates as water permanence persists. For example, fairy shrimp are very abundant during the initial reflooding of semipermanent wetlands but their numbers drop considerably as the hydroperiod lengthens (Euliss and Mushet, unpublished data). Like mosquitoes, fairy shrimp are vulnerable to predation by carnivorous insects in structurally complex communities (Pennak 1989). Increased invertebrate diversity associated with increased water permanence was noted by Driver (1977), who used chironomid diversity to separate wetland classes of prairie potholes in Canada and by Euliss et al. (unpublished data) who used recalcitrant remains of invertebrates to identify, classify, and delineate wetlands in the PPR.

Invertebrate Responses to Salinity

Nonflying invertebrates possess a different suite of adaptations that have allowed them to exploit highly productive saline prairie wetlands. Adaptations may include eggs and cysts, waterproof secretions, burrowing into substrates, and physiological adaptations (Scudder 1987). Swanson et al. (1988) reports that Lymnaea stagnalis was the dominant gastropod in permanent or semipermanent wetlands having specific conductances of <5,000 µS cm-1. However, as salt concentrations exceeded 5,000 µS cm^-1, Lymnaea elodes replaced Lymnaea stagnalis but was unable to persist at higher concentrations (>10,000 µS cm^-1). Similarly, anostracans like Branchinecta lindahli are common invertebrates in seasonal and semipermanent wetlands in the initial stages of the wet/dry cycle when salt concentrations are low (Euliss and Mushet, unpublished data), but they cannot tolerate extremely high salt concentrations that develop in the drought stage of the wet/dry cycle. At salt concentrations >35,000 µS cm^-1, the anostracan fauna may shift entirely to the brine shrimp, Artemia salina (Swanson et al. 1988). Amphipods are common in semipermanent and permanent wetlands of intermediate salinity; most species are found in waters of low or medium carbonate content (Pennak 1989). Insects also show this trend, with most taxa giving way to brine flies (Ephydridae) and certain water boatmen at higher salt concentrations. Increasing salt typically results in lower diversity, although the productivity of the few specialists capable of tolerating the osmotic stress may be high (Euliss et al. 1991).

Invertebrate Responses to Vegetation

Habitat structure provided by hydrophytes changes between years and within seasons as plant communities respond to hydrology, climate, and human alterations. Different invertebrate communities are often associated with different plant species or plant communities (e.g., Voigts 1976, McCrady et al. 1986, Wrubleski 1987, Olson et al. 1995). Macrophytes increase habitat structural complexity, providing additional food and living space within the water column for species that would otherwise not be present (e.g., Berg 1949, 1950, Krull 1970, Gilinsky 1984, Bergey et al. 1992). These plants also function as sites for oviposition (Sawchyn and Gillott 1974a,b, 1975), emergence (Sawchyn and Gillott 1974a,b), respiration (Batzer and Sjogren 1986), attachment (Campbell et al. 1982), and pupation (Butcher 1930). By increasing structural habitat complexity, these plants also modify predator-prey interactions (e.g., Rabe and Gibson 1984, Gilinsky 1984, Batzer and Resh 1991).

Areas with aquatic macrophytes have been reported to support higher numbers of invertebrates than bare areas (Gerking 1957, 1962, Krull 1970). However, Olson et al. (1995) found that nektonic invertebrates were more numerous in open water areas with dense filamentous algae, but biomass was greater in Typha stands. Wrubleski (1991) observed no difference in insect emergence between areas with and without submersed macrophytes. Voigts (1976) reported that invertebrate groups respond differently to changes in macrophyte communities, but in general, maximum numbers of aquatic invertebrates occurred where beds of submersed vegetation were interspersed with emergent vegetation. As the aquatic macrophyte communities change as a result of natural or anthropogenic alterations, so do their associated aquatic invertebrate communities (Driver 1977, Wrubleski 1991, Hanson and Butler 1994).

Aside from increasing the amount of habitat available to aquatic invertebrates, aquatic macrophytes also contribute to marked changes in the physical and chemical environment. These changes, in turn, may modify invertebrate responses to vegetation and the habitat they provide. Macrophytes restrict water circulation and contribute to gradients in light, temperature, and dissolved oxygen in very shallow standing waters (e.g., Kollman and Wali 1976, Carpenter and Lodge 1986, Rose and Crumpton 1996). Anoxic conditions can prevail within stands of emergent vegetation (Suthers and Gee 1986, Rose and Crumpton 1996) or beneath beds of submersed macrophytes (Kollman and Wali 1976), and this can impact invertebrate abundance, movement, and behavior (e.g., Murkin and Kadlec 1986a, Murkin et al. 1992).

Invertebrate Responses to Weather

Many wetland invertebrates avoid the risk of freezing by migrating to habitats that do not freeze completely. Most Hemiptera and Coleoptera overwinter as adults, and many migrate from shallow habitats to deeper ponds and lakes (Danks 1978). However, Danell (1981) did recover a live corixid and an adult beetle from the frozen ice and the upper part of the frozen bottom sediments of a shallow lake in northern Sweden. Water striders (Gerris spp.) overwinter as adults in terrestrial vegetation adjacent to ponds, frequently in aggregations (Nummelin and Vespalainen 1982). Some mosquitoes also overwinter as adults (Culiseta, Culex, and Anopheles spp.) in rodent burrows, hollow trees, and unheated buildings (Wood et al. 1979).

In wetlands that do not freeze solid, some invertebrates will move to deeper water to avoid freezing. Several studies have reported invertebrate movements to deeper water habitats in the fall (Wodsedalek 1912, Eggleton 1931, Moon 1935, 1940, Gibbs 1979, Davies and Everett 1977, Boag 1981). However, those invertebrates that migrate to areas that do not freeze may experience potentially harmful anoxic conditions and increased levels of hydrogen sulfide and other toxic dissolved substances (Daborn and Clifford 1974, Danks 1971a). Invertebrate adaptations to these conditions include anaerobic metabolism (Reddy and Davies 1993), reduced activity and feeding (Davies and Gates 1991), and movement to microhabitats offering better conditions (Brittain and Nagell 1981).

Those invertebrates that do not migrate must posses a means of tolerating freezing conditions. This is accomplished physiologically through freezing resistance (avoiding freezing) or freezing tolerance (Block 1991, Duman et al. 1991). Many benthic invertebrates are able, while encased in ice, to resist freezing by means of supercooling and the production of various antifreeze agents. Daborn (1971) and Sawchyn and Gillott (1975) both describe how coenagrionid damselflies were collected encased in ice, but were not frozen. Freeze-tolerant invertebrates are those that can survive extracellular ice formation within their bodies. Chironomids are a well-known example of this group (Danks 1971b).

Snow insulates and protects wetland invertebrates from the severe temperatures experienced above the ice (Danks 1971b). Emergent vegetation is important in holding this snow. Therefore removal of emergent vegetation would result in lower temperatures and possibly greater invertebrate mortality (Dineen 1953). Sawchyn and Gillott (1974a) reported that adequate snow cover was necessary to prevent egg mortality in three species of Lestes damselflies. Sawchyn and Gillott (1975) suggested that overwintering mortality of coenagrionid damselfly nymphs observed by Daborn (1969, 1971) may have been due to lethal ice temperatures caused by an absence of snow cover. Further research is needed to determine how important overwintering conditions are in structuring PPR wetland invertebrate communities and the role that vegetation plays in mitigating temperature extremes.

Wind and Rain. Weather can be an important factor modifying wetland invertebrate activity and behavior. Inclement weather can reduce emergence of adult chironomids and other insects (Swanson and Sargeant 1972, Wrubleski and Ross 1989), and can force flying insects to seek shelter in stands of emergent vegetation (King and Wrubleski 1998). Rasmussen (1983) reports that windy, rainy weather conditions during the emergence period resulted in a reduction in mating and the subsequent production of chironomid larvae in a prairie pond. Reductions in invertebrate abundances or activity will impact the foraging behavior and survival of waterfowl, particularly the youngest ducklings, which are dependant upon flying insects as an important food resource (Chura 1961, Sugden 1973, Roy 1995, Cox et al. 1998).

Invertebrate Responses to Anthropogenic Disturbances

Recolonization and Dispersal Mechanisms

Flightless life stages of insects and noninsectan invertebrates face even greater challenges to recolonize previously dry wetlands. Common recolonization mechanisms include eggs and cysts resistant to drying and freezing, diapause, aestivation, waterproof secretions, epiphragms (snails), burrowing, and even the use of invertebrate and vertebrate wildlife. Wiggins et al. (1980) outlined a temporal sequence, strongly influenced by climatic conditions, in which various taxa of invertebrates invade newly flooded habitats, using a variety of recolonization mechanisms. In general, the diversity of invertebrates is low because relatively few taxa possess the necessary physiological or behavioral adaptations that allow them to exploit the rich food resources available in prairie wetlands. Such systems favor ecological generalists that are early colonizers and exploit resources unavailable to other taxa lacking the necessary adaptations. The temporal sequence involves early detritivorous invertebrates with mostly r-selected characteristics and later support predatory invertebrates that recolonize habitats that persist for a sufficient length of time.

Passive dispersal mechanisms are important they include dispersal by wind (Pennak 1989), by being carried in the digestive tracts of birds (Proctor 1964, Proctor et al. 1967, Swanson 1984), and by clinging to more mobile fauna. Ostracods and clams have been observed clinging to migrating Hemiptera and Coleoptera (Fryer 1974) and amphipods can be carried in the feathers of waterfowl (Segerströle 1954, Rosine 1956, Swanson 1984). Peck (1975) observed Hyalella azteca and Gammarus lacustris on the fur of muskrat (Ondatra zibethicus) and beaver (Castor canadensis). Although not documented, epizoochory is a common means of seed dispersal on feathers of waterfowl and it is conceivable that invertebrate propagules are transported in that fashion as well. Cladocera ephippia float on the waters surface and sometimes form extensive mats. Some ephippia have elaborate appendages that may facilitate adhesion to feathers as has been described for epizoochory on the feathers of waterfowl (Vivian-Smith and Stiles 1994). However, wetland drainage increases the distance between wetlands and may disrupt transporting mechanisms or delay introductions.