The Multiple Enemies of Prey Species

The importance of functional responses among competing predators for avian nesting success (2019) Ellis et. al, Functional Ecology, https://doi.org/10.1111/1365-2435.13460

The Crux

In our ever-changing world, natural populations of different species are experiencing changes in both size and range. Part of the difficulty in predicting or responding to these changes is that ecological systems are made up of complex webs of species interactions, all of which have the potential to affect how populations respond to these changes. One of the most important interaction that occurs between species is predation.

Predators can affect the way prey species look, behave, and even where they live (see the Did You Know section).¬†Different predator species can have varying effects on their prey, and as such it is important to consider these differences whenever wildlife managers make policy decisions on how to manage and control endangered populations. The authors of today’s paper were interested in uncovering how different predator species affected prey, using the snowy plover (Charadrius nivosus).

What They Did

Using two different field sites in Utah, the authors set up hundreds of camera traps near the nests of snowy plovers. By doing this, they were able to capture images of predators whenever they attacked the nest and consumed the eggs, and with these images they could determine the type of predator involved and how common it was at a given time in the breeding season.

After collecting all of those data, they then modeled predator-specific hazard risks for the nest mortality. These hazard risks showed the chances that a given nest full of eggs would be predated by different predators, and how that might change both throughout the season and in relation to other predators. Changes in the hazard risk would show what type of functional response each predator had.

Did You Know: Functional Responses of Predators

Functional responses are a way that scientists can visualize a relationship between the size of a prey population and how a predator consumes those animals. Using these, it can become relatively easy to see what kind of effect a predator will have on prey populations, and how that might change as the size of the prey population grows.

Type I functional responses, for example, are those in which a predator’s consumption rate increases in proportion with the size of the prey population. So if a wolf can eat two rabbits out of a population of ten rabbits, that same wolf would be able to eat twenty rabbits out of a population of one hundred rabbits.

What They Found

Throughout the nesting season, the eggs of snowy plovers were vulnerable to a variety of different predators like foxes, coyotes, ravens, and (at one of the two sites) gulls. The risk of predation changed over the course of the season, and this risk also changed when considering multiple predators at the same time.

Regarding the functional response, foxes displayed a Type I response (increasing predation with increases in nest number), coyotes and gulls displayed a Type II response (known as predator satiation, where predators consume less prey as the population grows), and ravens displayed a Type III response (increasing predation at first, but as the prey population continued to grow they eventually consumed fewer prey).

Corvus_corax_laurencei

Predators like this raven have different effects on prey species, and those effects change as the prey population grows and when the raven competes with other predators for those same prey animals (Image Credit: Pkspks, CC BY-SA 4.0).

Problems

The predator species examined in this study (foxes, ravens, gulls, and coyotes) are all generalist predators, and as such they eat a variety of different food. In addition to that, shorebirds like the snowy plover breed in large colonies during a short period, meaning that these predators only have access to the eggs for a limited amount of time and these eggs are not representative of their normal diet. With this in mind, it is hard to generalize the findings of this study to other systems (or even to the predators used in this paper), as this resource pulse of eggs is such a temporary thing that these predators may not behave like this when they are hunting and consuming the food that they normally eat.

So What?

An important takeaway from this study is that predators feeding on the same prey animals will do so in different ways, and the way they feed on prey may differ if the other predators are removed or absent. When making plans for the conservation of an endangered species via predator removal, wildlife managers should base these plans off of how predation by certain species changes in the presence of another, competing predator. If a predator eats more of the endangered prey species in the absence of its competitor, funding spent removing the competing predator may be less ineffective than other methods (e.g., habitat management of the endangered prey species).

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