Eco-evolutionary Trophic Dynamics and the Loss of Top Predators

Humanity is at odds with the world’s top predators. Ecologists have long recognized the importance of top predators for the functioning of food webs. Decades of work have revealed that top predator removal can impact the biomass of primary producers. This is because, as top predators decline, prey populations increase, initiating trophic cascades. Traditional trophic cascades are mediated by the demographic and behavioral responses of prey populations. But predator removal may also have important effects on prey evolution. When predators are present and prey density is low, natural selection may favor prey traits that are important for predator escape ability. However, when predators are eliminated and prey density increases, natural selection may shift, now favoring traits that are important for competitive ability. This shift in natural selection may modify important trophic interactions.

Mike Kinnison, Ben Wasserman and I investigated the impact of predator loss on the evolution and ecology of prey. We took advantage of a historical introduction experiment involving Trinidadian guppies. In 1976, John Endler introduced about 200 guppies from a site with the top fish predator Crenicichla to a site lacking predators. Much is known about how this introduced guppy population has evolved in terms of color patterns and life history traits. However, little is known about how trophic morphology and feeding rates have changed in response to predator loss. We hypothesized that the absence of Crenicichla would lead to increased guppy density and heightened intraspecific competition. Due to trade-offs between gathering resources and avoiding predators, we predicted that the population released from predation would display heightened feeding rates compared to the high-predation source population.

Our results confirmed this prediction. The introduced population and a nearby natural low-predation population both displayed greater guppy densities and higher individual level consumption rates than the high predation source population. In addition, morphometric analysis revealed that both head and body shape have evolved to facilitate heightened resource acquisition. Results from prior experiments in mesocosms suggest that heightened feeding rates in low-predation guppy populations may cause them to have stronger top-down effects on algal biomass compared to high-predation populations.

Traditionally, the loss of top predators has been considered from a strictly ecological point of view. Our results suggest that predator loss may drive prey evolution, which itself may have important ecological effects – in this case, amplifying the strength of trophic cascades. If our results reflect a common response of prey populations to the loss of top predators, then a full assessment of the ecological impacts of top predator removal must carefully consider the effects of prey evolution.

This study recently appeared in PLoS ONE: http://dx.plos.org/10.1371/journal.pone.0018879.