PARENT SESSION
Oral Session # 32: Herbivory II: Grasslands.
Presiding: K Suttle
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 200.
Compositional constancy in grazed plant communities increases with herbivore size.
Bakker, Liesbeth*,1, 2, Olff, Han3, 1 Wageningen University, Wageningen, The Netherlands2 University of Nebraska, Lincoln, Nebraska3 University of Groningen, Haren, The Netherlands
ABSTRACT- An important current discussion is how the loss of biological diversity will affect community structure and ecosystem functioning in grasslands. Several recent experimental and theoretical studies have shown important links between plant diversity, the stability in species composition of plant communities and the reliability of grassland ecosystems to perform certain critical functions, e.g., to maintain productivity. In most grassland ecosystems, vertebrate herbivores are key elements without which community structure and ecosystem functioning cannot be understood. However, herbivores have hardly been incorporated in studies on the relation between diversity, compositional stability of plant communities and ecosystem functioning. We experimentally investigated the effects of different-sized herbivores on the spatio-temporal stability of grassland community composition and biomass by stepwise exclusion of vertebrate herbivores from large to small body size at two nutrient levels. We found that large herbivores had a strong stabilizing effect on plant community composition, whereas excluding them (leaving only small herbivores) induced strong variability in plant species composition, both in space and time. Seven years of nutrient addition did not change the vegetation in the presence of large herbivores, due to selective grazing in the fertilised plots. Small herbivores in contrast, did not counteract the effects of nutrient addition, resulting in strong compositional changes in the vegetation. Furthermore, large herbivores suppressed the expression of small-scale soil differences in the vegetation, resulting in homogeneous vegetation, while the presence of small herbivores enhanced vegetation heterogeneity. The temporal stability of the vegetation composition increased towards higher plant species richness, and large herbivores increased plant species richness more than small herbivores. We conclude that different-sized vertebrate herbivores have important and unique effects on plant diversity, compositional stability and thus possibly ecosystem functioning in grasslands and therefore should be incorporated in studies that link these aspects.
Key words: cattle, biodiversity, european rabbit, meadow vole