Disturbance And Grassland Biodiversity

Disturbance is such an intrinsic property of grassland ecosystems that it could be argued that the true disturbance is a lack of disturbance. It has been suggested that degradation of Australian grasslands may be as much a consequence of improper fire regimes as of overstocking (WCMC 5992), and the trcelcssness of North American prairies was due in significant part of both lightening-causcd and Amerindian-set fires (Sauer 1952). Perhaps rather than characterizing environmental fluctuations in grasslands as disturbance, we should recognize them as integral stochastic factors. Chief among these in pre-Colonial grasslands were grazing and browsing by both large and small mammals, abundant seed-eating and insectivorous birds, stochastic precipitation on seasonal, interannual and decadal times, fire, trampling, and nutrient harvest over large areas accompanied by deposition in small areas due to foraging, defecation and urination by grassland animals. In a thorough examination of the literature on the effects of grazing on species composition changes in the Earth's grasslands, Milchunas and Lauenroth (1993) concluded that those changes were associated with, in order of decreasing importance, the intrinsic above-ground productivity of a grassland, the evolutionary history of grazing at each location, and the level of consumption. Thus, high primary productivity, generally associated with grasses of greater stature, was associated with greater changes in species composition when grazed as the tall species were replaced by shorter, more grazing-tolerant, grasses. There can be little doubt that stochastic environmental fluctuation has been a fundamental feature contributing to grassland biodiversity (McNaughton 1983).

Large-scale environmental modification of habitats by humans, particularly in Europe and North America, has been instrumental in range expansion of grassland species in historical times. Once reduced to small pockets of distribution in the Eurasian steppe, the steppe marmot (Marmota bobac) has expanded throughout farmlands since the 1940s, and many steppe animals expanded into Europe as it was deforested and portions were converted into pasture (WCMC 1992). Similarly, tremendous range expansions by the brown-headed cowbird [Molothrus ater) and coyote (Canis latrans) have carried them far beyond their native Great Plains in North America, and brood parasitism by the cowbird is believed to be a significant contributor to songbird declines in the cowbird's newly exploited habitats (Trail and Baptista 1993).

Thus, disturbance has disparate effects on grassland biodiversity. Environmental fluctuations intrinsic to the grassland climatc and the co-existing biota are fundamental to grassland biodiversity. Conversely, the transformation of grasslands to cultivated croplands has obliterated such once-extensive grasslands as North America's tall-grass prairies and parts of the Eurasian steppe. Overstocking and other improper management policies have degraded grasslands on all continents. Exotic diseases have also had drastic effects upon the biodiversity and function of grassland ecosystems, modifying their organization substantially far beyond the susceptible organism as the consequences are transmitted through food weds (McNaughton 1992). Finally, expansion of cultural pastures into previously forested regions had led to major range expansion of some grassland organisms, sometimes contributing to detrimental changes in the biodiversi-ties of invaded communities.

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