Prey diversity constrains the adaptive potential of predator foraging traits

Abstract

Predators are generally under selective pressure to get better at foraging, leading to steeper functional responses and stronger predator-prey interactions. Yet strong interactions can de-stabilize food webs, and most interactions across ecological communities are thought to be weak. This conflict between evolutionary and community expectations for the strength of predator-prey interactions represents a fundamental gap in our understanding of how the evolution of foraging plays out in food webs.Here we help to resolve the conflict by showing analytically that the expectation for the evolution of steeper functional responses is relaxed in communities with diverse prey types. We simulate communities with varying prey richness and show that increasing prey richness can indeed constrain the adaptive potential of predator foraging traits, but that at low prey richness predators can evolve to have a stronger interaction with prey that have high net energy yields. Our results also indicate that handling time plays a role in determining whether predators may evolve to have a stronger interaction with abundant prey, suggesting that the evolution of keystone predator modules in food webs is most likely when handling times are negligible. Our results also provide a new mechanism predicting more diffuse interactions in diverse tropical communities relative to more species-poor communities at higher latitudes.

Ecological boundaries and constraints on viable eco-evolutionary pathways

“…At high carrying capacities, the simulations without evolution suggested that some parameter combinations that lead to predator-prey cycles may nevertheless still lead to persistent populations because populations do not reach low enough sizes to be susceptible to extinction via demographic stochasticity. In terms of the effects of demographic stochasticity in generating variation in evolution trajectories among populations, this is because the selection gradients on the predator’s space clearance rates and handling times depend on the densities of the prey (Rosenzweig 1973, Amarasekare 2022, DeLong and Coblentz 2022). Because demographic stochasticity can alter population abundances relative to deterministic expectations, two predator populations with exactly the same distribution of traits could experience different selection gradients because their prey population abundances are stochastically different.…”

“…We explicitly model the prey’s birth and death rates and their density dependence to allow for a stochastic birth-death process and facilitate the use of the eco-evolutionary modeling approach we employ (see below). This form of logistic growth is equivalent to the classical model of logistic growth with intrinsic growth rate r = b – d and carrying capacity (DeLong and Coblentz 2022; See Supplemental Information S1). The predator’s dynamics are described as: where e is the conversion efficiency of prey into predators, m is the per capita mortality rate of the predator, and all other parameters are defined above.…”

“…To model predator evolution in the Rosenzweig-MacArthur model, we used Gillespie Eco-evolutionary Models (GEMs, (DeLong and Gibert 2016, Luhring and DeLong 2020, DeLong and Coblentz 2022, DeLong and Cressler in press)). GEMs work by adding an evolutionary component to the Gillespie algorithm for stochastic simulations of ordinary differential equations (ODEs; Gillespie, 1977).…”

“…Amarasekare 2022, DeLong andCoblentz 2022). Because demographic stochasticity can alter population abundances relative to deterministic expectations, two predator populations with exactly the same distribution of traits could experience different selection gradients because their prey population abundances are stochastically different.…”

High variation in handling times confers 35-year stability to predator feeding rates despite altered prey abundances and apparent diet proportions

“…However, handling and hence detection times may be far more changeable for other types of predator-prey interactions depending on the species’ biological attributes and aspects of the feeding process on which feeding surveys rely (e.g., whether feeding events are observed directly or by the examination of gut contents (Novak et al ., 2017)). For some species, such as those involving more specialized predator-prey pairs (DeLong & Coblentz, 2021), handling times could be just as labile as species abundances and prey preferences, and could in fact respond to these as well (Okuyama, 2010; Stouffer & Novak, 2021). In such contexts where the consistency of detection times may be weak, detection-time variation will need to be large for the statistical mechanism of correlated ratios to contribute to feeding-rate stability.…”