My Enemy is Not the Enemy of My Other Enemy
Do predators keep prey healthy or make them sicker? A meta- analysis (2022) Richards et al., Ecology Letters, https://doi.org/10.1111/ele.13919
Ecology is all about understanding how the various parts of the natural world interact with one another. While we tend to think about things like predators, competitors, and parasites as separate entities that have their own effects, it is important to remember that these species interactions can interact with one another. Such interactions will have implications for the dynamics of natural populations.
Of interest is how predators and parasites interact with one another through their shared resources, prey/host species. Specifically, the Healthy Herds Hypothesis (HHH, see Did You Know?) predicts that predators reduce parasitism within the populations of their prey. While the HHH was based on a mathematical model, other theoretical models predict a range of effects, from predators decreasing parasitism to actually increasing parasitism. Because the empirical results from experimental studies show similar variation in their results, today’s authors wanted to determine if there is indeed a consistent, overall effect of predators on the parasitism of their prey.
Did You Know: Healthy Herds Hypothesis
The main thrust of the Healthy Herds Hypothesis is that predators keep their prey populations “healthy”. In this case, healthy means that there are both fewer parasites on average within the population (i.e., lower prevalence), and those animals that are infected tend to have fewer parasites (i.e., lower intensity). Additionally, predators reduce prey population sizes, and for parasites that depend on density-dependent transmission (i.e., parasites transmit to new hosts more in larger populations), this will also limit parasitism within the prey population.
What They Did
The authors used one my favorite statistical techniques, the meta-analysis, or a “study of studies”. Meta-analyses take data from existing studies, standardize the results, and then allow for synthesis of an overall pattern or effect. Specifically, the authors gathered data from empirical studies to understand not only what the general effect of predators on the parasites of their prey (do they increase or decrease parasitism), but also how strong that effect was (did they have a large or small effect).
In addition to these overall questions, the authors asked if variation among studies could explain the differences in the effects or predators. For this aspect of the study, the variables tested were 1) parasite type, 2) what habitat the hosts lived in, 3) whether or not the parasite was trophically-transmitted (meaning does a predator have to eat an infected prey animal for the parasite to complete its life cycle), 4) whether the effect of predators was consumptive or non-consumptive (did the predators eat the prey or just scare the prey?). and finally 5) whether the predators helped to spread the parasites.
What They Found
Contrary to the predictions of the HHH, predators tended to increase parasite prevalence within prey populations. While all of the tested variables were important for explaining this pattern, the two most important were predator-spreader identity (did the predators spread the parasite in the prey population) and host habitat type, with no effect of predators in aquatic environments and a large, positive effect in terrestrial environments.
There was no significant, overall effect of predators on the intensity of parasites in their prey. But, when looking at the variables tested the authors found that the consumptive effects of predators reduced parasite intensity, while the non-consumptive effects increased intensity.
While this is not something that is the fault of the authors, the data they used for the meta-analysis was biased towards positive effects of predators on parasite prevalence. This just means that the papers published on this subject tended to focus on positive effects, and there were fewer negative effects found than would be expected. Interestingly, when the authors removed those biased studies from their analyses, the positive effect of predators on disappeared, highlighting the need for researchers to conduct further studies to test for the ubiquity of this effect.
During the first year of my PhD, I came to learn that an overarching theme in ecology is “it depends”. What this means is that you don’t always get the sexy, direct result that you were hoping for, and today’s paper is a great example of that. While the authors wanted to test the HHH, and mathematical models supported the HHH, they found that the effects of predators on the parasites of their prey is very context-dependent. The wonderful thing about meta-analyses is that they reveal such overall trends in the data, while also highlighting the nuance inherent to ecological systems.
Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who wants to conduct four more meta-analyses, and reading this one only made it worse. 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.