Examination of the relative strength of trophic interactions on species abundance

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University of New Brunswick


Trophic cascades occur when the predation of prey has indirect effects on taxa at lower trophic levels in the same food chain (top-down effects), or the direct effect of primary productivity on herbivores has indirect effects on taxa at higher trophic levels (bottom-up effects). To assess whether top-down or bottom-up effects are stronger and/or more common across natural systems, I obtained the mean correlation between interacting species-pairs, then quantified the proportion of positive versus negative correlations. I found weak support for bottom-up effects. From my analyses of 3-species food chains, I found no support for either top-down or bottom-up cascades. There was, however, evidence of species that reduced prey abundance but increased predator abundance, preventing the possibility of cascading effects in either direction. If ‘blocker’ species are ubiquitous in natural food webs, it would be one explanation for why I didn’t find evidence that trophic cascades were strong or common.