Ecological drift alters trophic dynamics

Background/Question/Methods

Ecological drift causes species abundances to differ from their deterministic means through random sampling processes. Although ecological drift has important implications for the structure of communities, the strength of ecological drift is likely to differ between communities. Theory predicts that communities composed of small populations should experience stronger ecological drift as demographic stochasticity increasingly governs population dynamics. Predator communities should be more strongly structured by ecological drift, both because they are composed of small populations, and because they should be influenced by the drift of their prey. Drift should also be stronger in small and isolated communities owing to small population sizes and lack of demographic rescue respectively. Ecological drift is expected to manifest as increased beta diversity, and homogenization of local communities (reduced alpha diversity), particularly when those communities are small, isolated, or occupy a high-trophic position.  Predator extinction mediated by ecological drift can subsequently affect species interactions by triggering top-down trophic cascades, particularly in small and isolated communities.

We tested these hypotheses by creating replicated old-field communities that varied in size and degree of isolation, then sampling the associated arthropod community while measuring percent herbivory. We used statistical methods to separate drift from the deterministic effects of habitat size and isolation.

Results/Conclusions

We found that predator communities are more susceptible to ecological drift than their prey, particularly in small and isolated patches. Small, isolated communities supported increased beta diversity and reduced alpha diversity (per unit area), evidence of an important role for ecological drift in structuring these communities. Herbivory was highest in small and isolated communities, but the effect of isolation disappeared at larger habitat sizes. Together, these results suggest that increased drift in small and isolated communities leads to measurable trophic cascades. Our results offer insights into how the relative influence of ecological drift on community structure can create predictable patterns of diversity and species interactions.