Is the Enemy of My Enemy My Friend?

Natural enemies have inconsistent impacts on the coexistence of competing species (2021) Terry et al., Journal of Animal Ecology.

Image Credit: Alandmanson, CC BY 4.0

The Crux

In nature, organisms are often competing with other organisms for food, mates, or even just for a place to call home. This competition comes in two forms: interspecific competition (meaning competition between two different species) and intraspecific competion (meaning competition within the same species). These two forms of competition play into the phenomenon known as mutual invasibility (see Did You Know), which is a necessary component of coexistence. If two organisms coexist, one species will not outcompete the other and drive it extinct, and thus the two species will coexist over time.

Because competition plays such a strong role in species coexistence, any factor that affects competition between two species has the potential to also affect coexistence. Today’s authors wanted to ask how an antagonistic species interaction (specifically, interactions with a parasitoid) affected coexistence in rainforest flies.

Did You Know: Mutual invasibility

Mutual invasibility is the defining characteristic of modern coexistence theory. What this means is that when a species is rare, it has a higher population growth rate than it does when it is common. This happens when a species limits its own population growth rate more than it does another species’ population growth rate. In other words, when a species is common, the strength of intraspecific competition is stronger than the strength of interspecific competition. Put simply, for two organisms to coexist they must be able to “invade” the community when rare, but their growth is limited when they are more common.

What They Did

The authors first set up multiple experimental treatments with six different species of Drosophila flies, with each treatment including two species at varying densities. The varying densities allow the authors to examine the effects of competition, as all treatments were supplied with the same amount of food, so increasing density while keeping the food level constant results in an increase in competition for that food.

In this system, parasitoid wasps will lay an egg inside of developing flies. To capture this parasitism, the authors first allowed adult flies in the various treatments to breed and lay eggs. Afterwards, the adults were removed. The eggs then hatched, the larvae competed with one another, and then pupated to begin their transformation into adult flies. It was during the pupation stage that parasitoid wasps were released into the boxes so that they could parasitize the fly pupae. Once the adult flies emerged from the pupae, the authors counted and identified their species to determine how many of which species emerged from each treatment.

Figure 1 from today’s paper shows the workflow of the experiment, starting with the founding of the experimental treatment and ending with counting and ID’ing (Image credit: Terry et al. 2021 JAE, Image cropped)

What They Found

Not surprisingly, treatments that were exposed to parasitoids experience lower population growth rates. This makes sense, as a parasitoid by definition kills the host. Increasing parasitism will therefore decrease the number of surviving flies, and therefore how much the population can grow. Despite this impact of parasitoids on host population growth rates, there were no consistent relationships between parasitism and coexistence among the various treatments. That is, in some treatments parasitism made coexistence possible between the two competing species, in others parasitism made coexistence impossible, while in the remaining treatments there were such strong differences in competitive ability between the two species that coexistence was impossible, regardless of whether or not parasitoids attacked.


The experimental design that the authors utilized meant that only one generation of flies were produced and they were not able to track how changes in parasitoid density may have affected the apparent competition between the competing fly species. Apparent competition is when two species (fly A and fly B) are vulnerable to the same natural enemy (the parasitoid). Because the parasitoid attacks both fly species, increases in either fly A or fly B will increase how many parasitoids can exist, which will then increase the pressure on the other fly species. So, if the authors had continued the experiment for multiple generations there is a possibility that the results would have varied due to the more detectable signature of apparent competition.

So What?

Though they did not find a clean and consistent result, studies like today’s are important because they capture the “messy” nature of ecological data. Few things in nature are clean cut, and by publishing papers like what today’s authors did allows us to get a better and more accurate picture of how natural systems operate. What’s more, they provide a great starting point that inspires future researchers to further investigate the questions that remain.

Dr. 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, see his personal website here, or follow him on Twitter here.

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