Colonisation alters ecological community structure by modulating food web interactions

Trophic cascades occur when predators benefit resource populations by directly consuming species in mid trophic levels. Yet, predicting trophic cascades has been difficult despite decades of well-developed theory; predictions from mathematical models receive only mixed empirical support and cascades are often weaker than expected. Here, we combine models with laboratory and field experiments to show that an interaction between two ubiquitous features of real communities – colonisation and omnivory – masks cascades in ecological systems. Namely, differences in colonisation rate habitats experience affect food web structure and the availability of different resources comprising a top omnivorous predator’s diet. This in turn drives shifts in the foraging strategies of individual omnivores, which leads to apparent dampenings or reversals of cascades. We demonstrate that these changes occur on rapid timescales and have drastic effects on the structure of real communities. We hypothesize that such non-independence between colonisation and species’ foraging will help explain ecologists’ difficulties predicting predator effects in ecosystems, despite abounding theory. There is a pressing need for a more spatiotemporally dynamic theory of species interactions.