Predation risk is a function of alternative prey availability rather than predator abundance in a tropical savanna woodland ecosystem

Eric J Nordberg,Lin Schwarzkopf

doi:10.1038/s41598-019-44159-6

Abstract

Typically, factors influencing predation risk are viewed only from the perspective of predators or prey populations but few studies have examined predation risk in the context of a food web. We tested two competing hypotheses regarding predation: (1) predation risk is dependent on predator density; and (2) predation risk is dependent on the availability of alternative prey sources. We use an empirical, multi-level, tropical food web (birds–lizards–invertebrates) and a mensurative experiment (seasonal fluctuations in abundance and artificial lizards to estimate predation risk) to test these hypotheses. Birds were responsible for the majority of attacks on artificial lizards and were more abundant in the wet season. Artificial lizards were attacked more frequently in the dry than the wet season despite a greater abundance of birds in the wet season. Lizard and invertebrate (alternative prey) abundances showed opposing trends; lizards were more abundant in the dry while invertebrates were more abundant in the wet season. Predatory birds attacked fewer lizards when invertebrate prey abundance was highest, and switched to lizard prey when invertebrate abundance reduced, and lizard abundance was greatest. Our study suggests predation risk is not predator density-dependent, but rather dependent on the abundance of invertebrate prey, supporting the alternative prey hypothesis.

Highlights

The complex interactions between predators and their prey are instrumental in shaping the composition and structure of animal assemblages[1]
Changes in predation risk in response to predators may, be explained by one of two hypotheses: the shared predation hypothesis, which suggests that all prey groups are at high risk when predator populations are large[12,21]; or the alternative prey hypothesis, which states that predators switch prey groups when primary prey densities drop below the density of alternative prey[22,23]
Artificial lizards were attacked nearly twice as frequently in the dry season compared to the wet season, corresponding to low and high invertebrate prey abundances, respectively

Summary

Introduction

The complex interactions between predators and their prey are instrumental in shaping the composition and structure of animal assemblages[1]. Solely dependent on the abundance, or density of predators, as the abundance of prey populations are important factors determining predation in multiple-prey systems[12,13,14]. Understanding the relative importance of these processes in determining predation risk is critical to translating the predictions of predator–prey models to explain the outcomes of real population dynamics and community processes. Few studies incorporate both predator and prey densities and their impacts on direct measures of predation risk (but see[30,31,32]). We simultaneously quantified the abundance of predators and alternative prey to test if predation risk was proportional to predator abundance or if predation risk to lizards was more closely related to the abundance of alternative prey

Methods

Results

Conclusion