Northern Prairie Wildlife Research Center
In order that the alternative of no action can be considered as a management option, we discussed its consequences as long-term effects on the habitat and bird communities. We focused on grass-land, the primary natural ecosystem of North Dakota. It would be straightforward to examine wetlands, the other major natural habitat in the state, in a similar vein. If others repeat our exercise for another state, they may wish to consider different ecosystems.
North Dakota's grasslands were never left idle for long, even before settlement by Europeans. They were grazed heavily, but intermittently, by huge herds of bison, which left the landscape in a mosaic of habitats ranging from grazed down to the soil to un-grazed. Grasslands were also subjected to fires, some set natural-ly by lightning, others set intentionally by native Americans for a variety of purposes. Furthermore, varying climatic regimes, geo-logical formations, and topographic features added diversity to the landscape. It is with this perspective, that disturbances were of regular occurrence and in fact help maintain the grassland biome, that management of prairies should be viewed.
Settlement by Europeans destroyed the majority of natural grasslands in North Dakota. Cultivation was the most direct and immediate agent, and a large part of the state has had its prairie turned upside down. Other impacts have been less direct, but equally destructive. Among these are intentional or accidental introductions of Eurasian plant species, such as Kentucky bluegrass (Poa pratensis) and leafy spurge (Euphorbia esula), which have invaded native grasslands and disrupted the original plant communities. Efforts to reduce weedy plants by herbicides have had further detrimental effects on native species, especially forbs. Grazing by free-ranging bison has been replaced by grazing by cattle, often confined within small pastures and stocked for the entire growing season at rates that lead to severe overgrazing, with attendant soil erosion and changes in plant composition from "decreasers" to "increasers." Settlers also suppressed fires, which allowed the intrusion of woody vegetation, especially in moister parts of the state. These impacts have differentially affected privately owned and publicly owned lands.
Most lands managed by the US Fish and Wildlife Service and other public natural resource management agencies are relatively small islands within a mosaic of privately-owned land. Although these lands largely have seen the same land-use practices as private lands, the proportion of each use is different. Much less publicly owned wildlife land is cultivated annually and much more is left idle for extended periods of time for a variety of reasons including lack of resources, local public concerns, and characteristics of the tract.
The consequences of idling grassland and suppressing fire may be summarized in three scenarios of succession, depending on the prevailing precipitation regime. In the more mesic areas, especially in eastern North Dakota, the grassland is ultimately transformed to woodland, dominated by small trees such as green ash (Fraxinus pennsylvanica), Russian olive (Elaeagnus angustifolia), chokecherry (Prunus virginiana), with an understory of various shrubs and introduced grasses. The second scenario, applicable to somewhat drier areas, has succession proceed to a shrub community, dominated by wolfberry (Symphoricarpos occidentalis), silverberry (Elaeagnus argentea), and Woods rose (Rosa woodsii). The third scenario, anticipated in the more arid parts of the state, does not have a woody community arise, but instead the grassland becomes choked with an accumulation of litter.
Bird communities change dramatically under these vegetation successions from grassland. The first scenario leads to increases in numbers of many woodland-edge species, such as willow fly-catcher, eastern and western kingbirds, house wren, American robin, gray catbird, and brown thrasher. The second scenario favors species such as clay-colored sparrow and common yellowthroat. Few species likely benefit to any degree from the third scenario. In contrast, virtually all true grassland bird species suffer with any of these successional changes. Ferruginous hawk, willet, marbled godwit, burrowing owl, Sprague's pipit, Baird's sparrow, and chestnut-collared longspur are only a few of those that become reduced in number.
The intrusion of tall, woody vegetation into a prairie landscape affects the bird community in several ways. Most obvious, of course, is the direct loss of prairie plant species, through com-petition for light, water, or nutrients. Subsequent to that can be expected the loss of insect species that used those plants, and which serve as a food base for many birds. Also, certain prairie bird species avoid areas with woody vegetation, even where grasses and prairie forbs remain. Woody vegetation can fragment a grass-land, dividing it into noncontiguous parts that are individually too small to be used by area-sensitive prairie birds. The influ-ence of woody plants can be felt well beyond their sites. Trees and tall shrubs provide nesting sites and hunting perches for raptors, as well as travel lanes and denning sites for mammalian predators. And finally, trees and other tall structures offer vantage points from which brown-headed cowbirds can survey the surrounding area and locate nests to parasitize. Thus, the intrusion of woody vegetation has far-reaching consequences to grassland bird communities.
Overall, succession to woody vegetation, as anticipated under the first two scenarios, would lead to an increased total number of species using an area. This is local species diversity, which is usually enhanced by having a large number of different habitats and habitat edges in close proximity. This should be contrasted with the concept of biodiversity, under which it is attempted to preserve as many species and populations as possible.
How best do publicly-owned wildlife lands in North Dakota contribute to biodiversity? Although these lands in the state could be managed to increase local numbers of woodland and woodland-edge species, the areas are unlikely to make substantial contributions to maintaining their continental populations. Most such species have widespread distributions and are much more common elsewhere. Most of them have relatively large populations, which are not in jeopardy. Many grassland species, however, especially those of the mixed-grass prairie, have few alternatives to the northern plains. Some of them have distributions centered in or near North Dakota, with no major populations elsewhere. Further, many grassland species have suffered population declines as severe as birds of eastern forests, which have received far greater, popu-lar, and scientific attention. Both the lark bunting and grass-hopper sparrow, as examples, have declined 60 percent during the past quarter-century as indicated by results from the Breeding Bird Survey.
The mission of the US Fish and Wildlife Service and other wildlife management agencies is to protect and manage wildlife populations. Its mission is not to pack as many species as possible into the parcels of land it owns as might befit a zoo. Accordingly, the primary interest is in maintaining natural ecosystems and bio-diversity, as opposed to local species diversity. Prairie species, both game and nongame, need protection and they need it in areas such as North Dakota, a grassland state in the heart of their ranges. We submit that management should be directed at grassland (and wetland) species, especially endemics, in preference to those of other habitat affinities and distributions.
Exceptions exist, but most species are best maintained by maintaining in as natural condition as is feasible the ecosystems on which they rely. For that reason, we believe that management of publicly owned wildlife lands in North Dakota should be oriented toward protecting and restoring the most natural ecosystems possible. As a consequence, those actions will serve to protect the biodiversity, although local species diversity will not be maximized. Such an approach may be resisted by those who enjoy the natural values of unnatural habitats, such as bird watching in tree thickets on a grassland national wildlife refuge, but it will favor the long-term protection of the widest array of game and nongame bird species. Compromises may be needed, such as restricting woody vegetation primarily to riparian areas, which would increase local diversity and allow associated public uses but would still permit restoration of native communities in most of the area.
Managers are often responsible for large areas of degraded grassland. Restoration of those habitats to a more natural condition will result in a local reduction in the number of species using those areas. Those losses should be considered by managers and the public as an acceptable trade-off in favor of preserving the natural biodiversity, including game and nongame species, of the Northern Great Plains.