Predator removal and introduction affects the abundance of organisms in non-adjacent trophic levels through trophic cascades. Trophic cascades arise through behavioral mechanisms and the change in predation rates that arise when the number of interacting predators and prey is altered. These shifts, however, also alter fitness by changing demographic rates and the fitness value of specific traits. Thus, the loss or gain of a top predator will shift not just the abundance of organisms across trophic levels, but also fitness gradients, changing the pattern of selection across trophic levels and leading to eco-evolutionary trophic cascades. We tested this idea using Gillespie eco-evolutionary models (GEMs). We used a four-trophic level model parameterized for a typical temperature pond food chain and for which predator-prey interactions were dependent on the body size of both predator and prey. We induced a trophic cascade by removing the top predator and then evaluated both the trophic cascade with and without the evolution of body size.
Results/Conclusions
Our model showed relatively stable dynamics over the course of a year with the predator intact. Without evolution, removal of the top predator (piscivorous fish) caused a standard trophic cascade, with an increase in zooplanktivorous fish, a decrease in zooplankton, and an increase in phytoplankton. When we allowed evolution to proceed at each level, predator removal induced substantial shifts in body size, but the trophic cascade was only slightly altered at trophic levels 2 (zooplankton) and 3 (zooplanktivorous fish). The increase in abundance of phytoplankton, however, was reduced, meaning that the effect of evolution on the trophic cascade was transmitted across trophic levels to the basal resource. The small change in abundance in the middle trophic levels suggests that cryptic dynamics may arise in some trophic levels while more obvious eco-evolutionary dynamics arise in other levels. In conclusion, our results indicate that trophic cascades are likely to be accompanied by rapid evolution that alters the expected consequences of the loss or gain of a top predator.