Document: MAT-3-48-13

Effects of grazers on the correlations between plant traits and habitat variables in ponds.

LEIBOLD, M.A.* ¹, S.PINCA ¹ and V.H.SMITH ²

University of Chicago, Chicago, Il 60637 USA ¹
University of Kansas, Lawrence, KA 66045 USA ²

Abstract:
Patterns of (co-)variation in the ecological traits of species in a species pool are important because these patterns generate the trade-offs that might regulate the outcome of the interactions in different local habitats. Theory based on this idea of trade-offs involving predator-mediated resource competition (i.e. keystone predation) predicts that the distribution of species with different traits along resource gradients will be altered by the presence of higher trophic levels. We examined how correlations between algal plant traits and ambient nutrient levels in the habitat were altered by the presence of zooplankton grazers in experimental pond mesocosms in which nutrient levels were manipulated. We found that grazers qualitatively changed the correlation structure between plant traits and habitat variables. We also measured the same set of correlations in natural ponds and compared it to each of these two experimental situations. We found that the correlation matrix in natural ponds resembled the one we saw when grazers were present but did not resemble the one we saw in the absence of grazers. These results indicate that grazers are a key element determining how algal plants sort out along nutrient gradients in ponds. This is consistent with models of predator-mediated resource competition that predict that many of the important effects of resource supply to a food web will occur as a consequence of indirect effects on higher trophic levels.

Keywords: trade-offs, keystone predation, phytoplankton, herbivory

This abstract is being presented at: 8:30 AM in session:
Oral Session #37: Phytoplankton.