American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Predation risk reverses the potential effects of warming on plant-herbivore interactions by altering the relative strengths of trait- and density-mediated indirect interactions”

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Nathan P. Lemoine

Elevated temperatures reduce the role of fear in herbivore foraging decisions

Feeling the heat: rising temperatures increase herbivore exposure to predation risk

A grasshopper (<i>Melanoplus</i> sp.) sitting on a blade of prairie grass (<i>Sorghastrum nutans</i>).<br />(Credit: Lauren Baur)
A grasshopper (Melanoplus sp.) sitting on a blade of prairie grass (Sorghastrum nutans).
(Credit: Lauren Baur)

Foraging is risky business for herbivores because it exposes them to significant predation risk. At the same time, however, herbivores must forage in order to maximize both their growth rates and number of offspring. This trade-off between feeding and hiding from predators dictates much of herbivore behavior and is determined by an herbivore’s metabolic rate and energetic needs. For ectothermic (i.e. cold-blooded) herbivores, like grasshoppers, caterpillars, or many fish species, metabolic rates increase with rising temperatures. Climate warming might therefore fundamentally alter the amount of predation risk herbivores are willing to accept in order to avoid starvation. In other words, rising temperatures will pose a dilemma to ectothermic herbivores: do they forage more intensely but risk being killed more frequently, or do they skip foraging to avoid predation but face increased starvation rates?

A new modeling experiment by Nathan Lemoine, a research scientist at Colorado State University, demonstrates the consequences of rising temperatures on insect herbivores. At cool temperatures, herbivores have low metabolic rates and, as a result, there are few consequences for herbivores who skip a foraging bout to avoid predation. At high temperatures, herbivores experience considerably greater metabolic demands. Skipping foraging to avoid predation therefore imposes a great cost: the surviving herbivores are smaller and have fewer offspring. Furthermore, herbivores that choose to forage suffer incredibly high predation rates, with herbivore mortality rates nearing 100% in some scenarios.

These results have profound implications for our understanding of how climate change will impact terrestrial ecosystems. The landscape of fear might cease to exist in the future as the threat of starvation overwhelms the threat of predation for herbivores, who suffer dramatically reduced population sizes due to increased predation rates. As a result, herbivore control of plant primary production might be substantially weaker in the future. Read the Article