Image Credit: Judy Gallagher, CC BY 2.0, Image Cropped

Predators weaken prey intraspecific competition through phenotypic selection (2020) Siepielski, Hasik et al., Ecology Letters, https://doi.org/10.1111/ele.13491

The Crux

We are all familiar with predator-prey relationships in nature, those in which one organism (a predator) kills and consumes another (the prey). Besides these direct effects on prey via consumption, predators can also impose indirect effects on their prey. An indirect effect is one in which the predator changes some aspect of the prey, such as their behavior or the way that they look, but these changes are brought about just by the predator being around. These predator-mediated effects are known to affect the relationships between prey organisms themselves, such as how prey organisms compete with one another, whether its for food, mates, or other resources.

Predators are known to affect how active their prey are, and this selection on activity results in a trade-off between how much prey can grow and their risk of predation. Being more active can allow you to find and eat more food, but that also means that a potential predator is more likely to see you. Today’s paper used larval damselflies and their fish predators to study how selection of fish on their damselfly prey based on the damselfly activity rates affected competition between the damselflies.

What We Did

We conducted two experiments to understand how selection on activity rates affected density-dependent competition. First, we tested damselflies in the lab to quantify their activity rates by placing them in individual plastic trays and recording how often they moved over a period of three hours. After quantifying activity rates, we then split these damselflies into four groups, each one corresponding to differing selection on activity rate (i.e. the first group was least active, the fourth group was the most active). This artificial selection mimicked selection imposed by fish predators in nature, with the least active group representing selection by fish on low activity rates. Once the groups had been established damselflies from each group were split up into cages, each of which had a density of either one, two, four, or ten damselflies. Each cage contained damselflies from one of the four activity groups. 

After studying selection in the lab, we then performed a similar experiment in the field. Using submerged cages in six lakes that differed in their fish predator density we once again stocked these cages with either one, two, four, or ten damselflies. In both the lab and field experiment competition was quantified by measuring growth over a period of three weeks.

Did You Know: Density Dependence

Density dependence means just what it sounds like, that the effect depends on the density of the organisms themselves. In this study, the effect of predators was density dependent, but so was competition itself. Predators imposed selection on the damselflies, but only up to a point. After that point the effect of predators plateaued and no longer increased. Competition was density dependent because it increased with density for the more active groups, the more damselflies there were in a givencl cage the stronger the competition.

Fish predators like this bluegill select for reduced activity rates because they are visual predators. The more active the damselfly, the more likely it is to be seen and subsequently eaten (Image Credit: Scott Harden / CC BY-SA 4.0, Image Cropped).

What We Found

Competition among the damselflies increased with density in both the lab and field experiments. However, this competition was only evident in the active groups in the lab experiment and in the lakes with low fish predation in the field. This indicates that selection by fish on damselfly activity rates decreased the competition between the damselflies themselves.

So What?

The fact that predators affect their prey is not a surprise, and many previous studies have shown that these effects can be both direct and indirect. This study showed that predation affected among-prey competition due to selection on the prey activity rates, and that this affect was density-dependent. These results imply that future studies need to not only consider the (direct) consumptive effects of predators, but also their selection on prey. The natural world is incredibly complex, and this study has opened the door for new ways of thinking about the joint dynamics of predation and competition. 

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, or follow him on Twitter here.