Costs of Inducible Defence along a Resource Gradient

Christer Brönmark,Johan Hollander,Charlotte Lennartsdotter,Thomas Lakowitz,Johan Ahlgren,P Anders Nilsson,Adam Siepielski

doi:10.1371/journal.pone.0030467

Abstract

In addition to having constitutive defence traits, many organisms also respond to predation by phenotypic plasticity. In order for plasticity to be adaptive, induced defences should incur a benefit to the organism in, for example, decreased risk of predation. However, the production of defence traits may include costs in fitness components such as growth, time to reproduction, or fecundity. To test the hypothesis that the expression of phenotypic plasticity incurs costs, we performed a common garden experiment with a freshwater snail, Radix balthica, a species known to change morphology in the presence of molluscivorous fish. We measured a number of predator-induced morphological and behavioural defence traits in snails that we reared in the presence or absence of chemical cues from fish. Further, we quantified the costs of plasticity in fitness characters related to fecundity and growth. Since plastic responses may be inhibited under limited resource conditions, we reared snails in different densities and thereby levels of competition. Snails exposed to predator cues grew rounder and thicker shells, traits confirmed to be adaptive in environments with fish. Defence traits were consistently expressed independent of density, suggesting strong selection from predatory molluscivorous fish. However, the expression of defence traits resulted in reduced growth rate and fecundity, particularly with limited resources. Our results suggest full defence in predator related traits regardless of resource availability, and costs of defence consequently paid in traits related to fitness.

Highlights

In recent years it has become increasingly recognised that, in addition to having constitutional defence adaptations, many organisms respond to predation by modifying phenotypically plastic traits, such as behaviour, morphology or life-history strategies [e.g., 1,2,3]
The phenotypically plastic trait should have superior fitness in environments with predators compared to alternative phenotypes [4,5,6,7], i.e. for prey with predator induced defences the inducible defence should provide a benefit in terms of increased survival probability
We found evidence of R. balthica paying costs in fitness-related growth and reproductive traits when expressing morphological and behavioural defences against predation

Summary

Introduction

In recent years it has become increasingly recognised that, in addition to having constitutional defence adaptations, many organisms respond to predation by modifying phenotypically plastic traits, such as behaviour, morphology or life-history strategies [e.g., 1,2,3]. If the expression of phenotypically plastic traits is constrained by costs, trait expression should depend on the environmental context, including differences in amount of resources available or the density of competitors [5,11,12,13,14,15]. Various models predict that the optimal investment in defence changes along a gradient of resource availability/competitor density. For morphological defences where energy is allocated to build and maintain a defence structure at the cost of decreased growth or development rates, simple allocation models predict that investment in defence structures should be highest at high resource levels and/or at low densities of competitors [5]. State-dependent models predict that behavioural responses to predation threat should be strongest at high resource levels/low competitor density, whereas at low resource levels prey have to be actively foraging in order to avoid starvation and show low levels of behavioural response to predator cues [14,16,17,18,19]

Methods

Results

Conclusion