Nonconsumptive Effects of Predators and Trait‐Mediated Indirect Effects

Abstract

Phenotypic plasticity in species’ traits is ubiquitous across taxa and habitats. This plasticity can affect the qualitative and quantitative nature of species interactions and thereby affect food webs. Empirical work has focused on predator–prey interactions, in part because most prey respond to predator presence through a suite of traits that reduce risk. These responses can incur costs in fitness correlates (e.g. growth and survival) of the prey, i.e. are nonconsumptive effects (NCEs) of predators. Further, these responses can lead to indirect effects transmitted through the prey to other species in the food web, termed trait‐mediated indirect effects (TMIEs). NCEs and TMIEs have been demonstrated in a wide range of communities and across diverse taxa. These effects can contribute substantially to the influence of predators (including through competition, trophic cascades and keystone predation), and therefore can play a critical role in the dynamics and structure of ecological communities.

Key concepts

  • Phenotypic plasticity in species traits is widespread and represented across diverse taxa and ecosystems.

  • To reduce predation risk, organisms respond to predator (including herbivore) presence by modifying traits, including developmental, morphological, physiological, life historical and behavioural characteristics.

  • Phenotypic responses to predators often incur a cost to prey fitness correlates such as growth rate and survival. These costs to fitness are termed nonconsumptive effects (NCEs) of the predator on its prey, to distinguish them from the consumptive effects (CEs) due to predation.

  • Phenotypic responses by prey to predators alter the nature of the interactions between prey and other (third) species. The predator is said to have a trait‐mediated indirect effect (TMIE) on the third species through induced changes in the intermediate species’ (prey's) traits.

  • Predator‐induced changes in prey traits can lead to TMIEs of the predator on prey resources (resulting in trait‐mediated trophic cascades), prey competitors and other predators of the prey.

  • NCEs and TMIEs can qualitatively change the nature of species interactions. For example, predator‐induced changes in prey traits can reverse competitive interactions, and lead to increases in prey growth rates due to strong TMIEs on prey resources.

  • NCEs and TMIEs can contribute strongly, and even dominate, the net effect of predators on prey and net indirect effect of the predator on species the prey interacts with. Thus, they can strongly affect prey population growth rates, community structure and ecosystem processes.

  • Factors, such as temperature, resource growth rates, prey density and prey–competitor density, can strongly influence the magnitude of NCEs and TMIEs.

  • It will be necessary to formally incorporate phenotypic responses of prey to predators into the conceptual foundations of ecology and strategies for the management of ecological systems.

Keywords: nonconsumptive; trait‐mediated; food web; phenotypic plasticity; indirect interaction

Figure 1.

The diagram illustrates the difference between fixed and plastic phenotypic responses to predator risk. Zooplanktons are distributed high in the water column at night (panel a), but migrate to deeper (and darker) water levels to avoid predation risk during the day (panels b and c). Panel (b) illustrates a fixed response to a proximate signal such as light level, i.e. zooplanktons are lower in the water column independent of fish density. Panel (c) illustrates a plastic response, i.e. vertical migration increases as a function of fish density. NCEs will result from the plastic response, but not from the fixed response.

Figure 2.

Diagrams of NCEs and TMIEs. Strait arrows represent consumptive effects. Curved arrows represent a predator‐induced change in prey phenotype. Each figure illustrates a species in a different position (represented by shading of the letter) in a simple food web (or chain) affected by the predator. The figure illustrates NCEs and TMIEs with increasing complexity. First, a predator‐induced change in prey phenotype can affect fitness correlates of the prey (a). These changes in prey traits can also affect the interaction of the prey with other species, causing a TMIE of the predator on these other species, e.g. prey resources (b) or other predators of the prey (c). TMIEs with more links are also illustrated, e.g. with three links in which the predator affects a competitor (i.e. consumes the responding prey's resource) of the prey (d) and with four links in which the predator affects a mutualist (i.e. consumes a competitor of the responding prey's resource) of the prey (e).

Figure 3.

Example of a TMIE, DMIE and their interaction caused by a predator on a prey's competitor. A larval dragonfly predator reduces small tadpole density and induces a change in small tadpole foraging traits (habitat preference and activity levels). These two predator effects cause an increase in periphyton resource levels and large tadpole (competitor) growth. Combinations of treatments allowed teasing apart of the relative contributions of the TMIE and DMIE and their interaction. The DMIE was a small fraction of the total (net) effect of the predator, the TMIE was on the same order of magnitude as the DMIE, and there was a large interaction between the two effects. Reproduced from Peacor and Werner . Copyright 2001, National Academy of Sciences, USA.

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Peacor, Scott D, and Werner, Earl E(Dec 2008) Nonconsumptive Effects of Predators and Trait‐Mediated Indirect Effects. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021216]