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Influence of Agriculture on Aquatic Invertebrate Communities of Temporary Wetlands in the Prairie Pothole Region of North Dakota, USA

Discussion


Our study clearly suggests that current agricultural practices have had major negative impacts on aquatic invertebrate communities of temporary wetlands in the PPR of North Dakota. Several practices associated with modern agriculture may have impacted invertebrate egg banks by lowering numbers of ephippia and other invertebrate eggs; the remains of snails and ostracods were also reduced in cropland wetlands. Agrichemicals have been demonstrated to cause significant increases in aquatic invertebrate mortality (Borthwick 1988, Grue et al. 1989). Additionally, the physical effects of cultivation, such as increased erosion and sedimentation, probably reduced invertebrates in cropland wetlands. Suspended silt and clay can be toxic to zooplankton and reduce foraging and food assimilation rates of aquatic invertebrates (Robinson 1957, McCabe and O'Brien 1983, Newcombe and MacDonald 1991). Increased suspended sediments also clog filtering apparatuses of invertebrates and may reconfigure aquatic food webs by shading primary producers (Gleason and Euliss 1996). Sedimentation buries seed banks in wetlands (Jurik et al.1994, Wang et al. 1994) and presumably invertebrate egg banks (Euliss et al. 1999). Tillage of uplands surrounding wetland basins increases water-level fluctuations and probably impacts invertebrate communities through altered water chemistry and hydroperiod (Euliss and Mushet 1996a).

Agricultural practices may also have less obvious impacts on invertebrate communities in wetlands. Wetland drainage in the PPR has been focused primarily on shallow temporary and seasonal wetlands within agricultural fields. This increases the distance between wetlands, thereby increasing the distances aquatic invertebrates must travel to repopulate wetlands. Hence, repopulation following natural drought cycles may be prevented or delayed (Euliss et al. 1999). Dispersal by wind (Pennak 1989), in the digestive tracts of birds (Proctor 1964, Proctor et al. 1967, Swanson 1984), and by clinging to vertebrate and invertebrate fauna (Segersträle 1954, Rosine 1956, Fryer 1974, Peck 1975, Swanson 1984) may also be diminished if the distance between remaining wetlands is increased.

Temporary wetlands dry quickly during most years, and most farming operations cultivate their basins and adjacent upland areas. Kantrud and Newton (1996) found lower plant species diversity and greater percentages of unvegetated bottom in the wet meadow zones of agricultural wetlands, and Kantrud and Stewart (1977, 1984) found poor bird use of tilled prairie wetlands. Our results suggest that agricultural practices have limited invertebrate communities as well. Of all the wetland classes in the PPR, temporary wetlands are most directly influenced by farm operations; hence, their condition may provide a general indication of wetland health within an area. In much the same fashion as "indicator species" in biological assemblages, temporary wetlands may alert managers of potential problems in an area that may include more permanent wetland classes. Future work should address the relationship between the condition of temporary wetlands and the condition of other habitats in an area, including more permanent wetlands.

Our study demonstrates that aquatic invertebrate communities of temporary wetlands in the PPR have been altered by agriculture. Future studies are needed to clarify causal mechanisms and the ecological implications on other wetland-dependent communities, such as waterfowl that utilize these wetlands to satisfy nutritional requirements (Swanson et al. 1977). We suggest that future studies be initiated collaboratively with agricultural interests to facilitate development of interdisciplinary approaches that promote land management practices beneficial to both wildlife and agricultural interests (Gleason and Euliss 1998).


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