Differences among individual predators alter their population feeding rates on prey

Coblentz, Kyle; Merhoff, Stephanie; Novak, Mark

Coblentz et al. show that individual differences among predators, such as the whelk Nucella ostrina shown here with its barnacle prey Balanus glandula, can alter the populationlevel strengths of predator-prey interactions.
Coblentz et al. show that individual differences among predators, such as the whelk Nucella ostrina shown here with its barnacle prey Balanus glandula, can alter the populationlevel strengths of predator-prey interactions.

No two individuals are exactly alike. This is as true for other organisms as it is for humans. Despite this fact, ecologists often treat individuals as identical out of convenience or necessity. However, recent developments in ecological theory have identified conditions under which individual differences may play important ecological roles. One of these is when the relationship between the strength of the interaction between two species and a quality of a species that differs among individuals is not linear. Because of a mathematical fact known as nonlinear averaging, differences among individuals can weaken or strengthen the interactions among species with potential consequences for the dynamics of populations over time and whether or not two species may be able to coexist with one another. Here we sought to measure how strong this nonlinear averaging effect was due to differences among individual sea shore snails (Nucella ostrina) in two properties that influence their feeding rates on a barnacle (Balanus glandulus) and a mussel (Mytilus trossulus): their so-called attack rates and handling times. We find that differences among individual snails in their attack rates reduced population feeding rates by up to nine percent. We also estimated that handling time differences among individuals increased population feeding rates but not as much as they were decreased by attack rate differences. Furthermore, we find that individual differences in attack rates can combine with differences in the environments experienced by the individuals (the amount of prey available) to reduce population feeding rates by up to twenty nine percent. This combined effect was greater than the effect of individual differences in attack rates or differences in the environment alone. Overall, our study shows that individual differences can have a measurable impact on the strength of predator-prey interactions in the field. Nevertheless, how large this impact is depends on a combination of the average characteristics of species, how different individuals are from one another, their environments, and how these different components interact with one another.

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