Natural systems in the Great Plains are currently stressed by a variety of agents. These agents include: fragmentation of grasslands through land conversions to agriculture, cities, and roads; sedimentation and water pollution from fertilizer and pesticide runoff; and introduction of invasive species through human activities and natural encroachment. Other stresses include: altered hydrology (water cycles) due to the impoundment of water in dams and reservoirs and diversion of water for irrigation and other uses; and changes to natural runoff from watersheds by human activities that alter the natural efficiency of watersheds and the permeability of soil surfaces (e.g., infilling and soil compaction). Increasing human demands on natural systems for consumable wildlife opportunities (such as hunting and fishing) and other recreational opportunities are also likely to stress natural systems that are being affected by climate change.

The pattern of remaining native species in the Great Plains is probably associated with the regional pattern of agriculture and urban development. In the agriculturally rich, eastern portions of the Great Plains, the native habitats are already absent or highly fragmented. In the central portion of the Plains, small areas of these habitats are still present but are largely disconnected. In the western edges, the native habitats are often more continuous, although the recent rapid expansion of urban areas along the Rocky Mountains is increasingly fragmenting these native communities as well. In that persistence of species often depends on sufficient habitat and connection between habitat patches, these natural communities are highly vulnerable to additional stresses.

Invasive species are currently a significant issue on the Great Plains. Invasive species are plant or animal species that have been introduced into an environment (often accidentally) in which they did not evolve. Such species usually have no natural enemies present to limit their reproduction and spread. Invasive species typically have high reproductive rates, fast growth rates, and good dispersal mechanisms. The costs and weed-associated losses from invasive species in crop and forage production in the agricultural sector are currently nearly $15 billion annually.

In native ecosystems, invasive species can compromise an ecosystem's ability to maintain its structure or function. For example, in grassland ecosystems, riparian areas next to streams and rivers are rich in native plant species and highly productive. These riparian areas also provide critical habitat for species living in the surrounding drier ecosystems, offering shelter and forage. Such riparian ecosystems can be easily invaded by invasive species, providing pathways for the exotic plants to become established and migrate into upland sites in grassland ecosystems. Natural disturbance regimes such as fire and grazing (first by buffalo and now by cattle) are seen as important in maintaining the native species. Changes in the frequency, intensity, spatial pattern, or scale of these disturbances through changes in climate could hasten the replacement of native species with exotics.

In the discussion that follows, potential environmental impacts of invasive species and changes in biodiversity in the Great Plains are examined; discussions of some of the social and economic impacts follow. General principles to guide strategic development of social responses to climate change conclude this discussion.

The projected climate changes are very likely to alter the biodiversity of the Great Plains. The “new -- composition of species that could arise in response to the changing climatic conditions could or could not be detrimental to the ecological services on which society draws, but the rapid rate of change in climate is likely to be disadvantageous to native species. Invasive species have a history of exploiting habitats left vacant by native species. As climate changes, the indirect effects of weeds and pests seem likely to bring surprising challenges.

Some native species are unlikely to be able to adapt fast enough to the changing climate regimes, resulting in a lowered competitive edge and weakened resistance of ecosystems to infestations by invasive plants and animals. Potential impacts include shifts in the relative abundance and distribution of native species, significant changes in species richness and communities, and local extinctions of native species. Subtle changes in the diurnal (day/night) or seasonal patterns of temperature have also been shown to affect plant community composition. In the short-grass steppe, for instance, the slight warming of nighttime temperatures seen in the last 20 years has been linked to the decline of blue grama grass, the dominant grass of the short-grass prairie. Increased average temperature and annual precipitation in the central Great Plains could make it possible for invasive plants, such as kudzu and Johnson grass, now found further south, to migrate north. Additional land-use pressure on these native systems is likely if agricultural practices extend into these areas as a result of more favorable climatic conditions or increased demand for agricultural products.

Changes, such as in the nutritional value of plants or changes in the timing of insect emergence, could also contribute to a decline in bird populations in the Great Plains. At the same time, longer growing seasons and the possibility of increased plant productivity could increase the region's attractiveness for birds. Grassland bird species are currently declining due to a loss of habitat. Further alteration of their habitat from climate change would be a likely additional stress on these declining populations.

Changes in precipitation, temperature, and the hydrological (water) cycle are likely to affect aquatic systems and the terrestrial animals dependent on these ecosystems. The abundance of wetlands is closely tied to the year-to-year variations in climate, both precipitation and temperature. Changes in yearly precipitation in the northern regions are likely to have significant impacts on breeding duck populations. Changes in precipitation and river flow are also likely to aggravate land-use conflicts and competition for water supply including conservation needs.