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Disproportionate effects of non-colonial small herbivores on structure and diversity of grassland dominated by large herbivores
- Rebollo, Salvador, Milchunas, Daniel G., Stapp, Paul, Augustine, David J., Derner, Justin D.
- Oikos 2013 v.122 pp. 1757
- Bouteloua gracilis, annuals, body size, botanical composition, cattle, grasses, grasslands, grazing, habitats, herbivores, pastures, plant communities, primary productivity, rabbits, rodents, vegetation structure
- The response of semiarid grasslands to small, non-colonial herbivores has received little attention, focusing primarily on the effects of granivore assemblages on annual plant communities. We studied the long-term effects of small and large herbivores on vegetation structure and species diversity of shortgrass steppe, a perennial semiarid grassland with a long evolutionary history of grazing by large herbivores, but considered marginal habitat for small mammalian herbivores. We hypothesized that 1) small herbivores would affect less common species through selective but limited consumption, 2) large generalist herbivores would affect more abundant species and proportions of litter-bare ground-vegetative components of the general structure through non-selective herbivory, 3) herbivore effects on plant richness increase with increasing aboveground net primary production (ANPP) because rare species would be more abundant in wet years. Plant community composition was assessed over a 14-year period in pastures grazed at moderate intensities by cattle and in exclosures for large (cattle) and large- plus-small herbivores (additional exclusion of rabbits and rodents). Exclusion of large herbivores affected litter and bare ground and basal cover of abundant, common, and rare species. Additional exclusion of small herbivores, did not affect rare components of the plant community, but had indirect effects on abundant species, decreased the cover of the dominant grass Bouteloua gracilis and total vegetation, and increased litter and species diversity. There was no relationship between ANPP and the intensity of effects of either herbivore type on richness. Exclusion of herbivores of two different body-sizes had primarily complementary and additive effects which promoted changes in vegetation composition and physiognomy that were linked to increased abundance of tall and decreased abundance of short species. Both herbivores promoted recovery of short grasses after intense droughts, providing resilience to the shortgrass steppe and contributing to the long-term maintenance of basal cover of total vegetation. Our finding shows that small mammalian herbivores had disproportionately large effects on plant communities relative to their small consumption of biomass. Even in perennial grasslands with a long history of intensive grazing by large herbivores, non-colonial small mammalian herbivores should be recognized as an important driver of grassland structure and diversity.