Incorporating Parasites Into Community Ecology

I’ve said it before and I’ll say it again until I retire*: parasitism is THE most interesting (and arguably the most successful) life history strategy on the planet. Parasites are present in every ecosystem on the planet, and it is incredibly unlikely that any study system or ecological community is parasite-free. So why don’t we talk about them more?

As a disease ecologist, my work focuses on parasites and their place in the natural world, so I think about these organisms a lot. My PhD was centered on incorporating parasites into food webs to understand how they affect species interactions (and how species interactions in turn affect them). Failing to consider parasites can lead scientists to miss important aspects of an ecosystem and draw false conclusions.

Yet most ecological studies – even those which look at entire communities – fail to consider parasites and their effects on other organisms. I can’t blame them, parasite ecology can be difficult to get your head around. So today, I want to try and give ecologists everywhere some tips on incorporating parasites into their work.

They’re Everywhere

Before getting into the nitty-gritty, I just want to provide a quick example of how parasites can affect results. If you want to know how many arthropods a fish eats, it will be worthwhile to ensure that those arthropods are parasite-free, as many parasites are known to alter the behavior of their arthropod hosts to ensure that a fish eats them. If you compare two lakes with wildly varying parasite populations, you could easily attribute differences in the fish population to things like temperature or algal growth, without realising that the parasites were the problem.

This is an easy thing for me to point out, but it is much harder to incorporate into a study. Lots of parasites are small, and plenty are endoparasites, meaning that they live inside of their host organisms. So unless you cut open your study organisms, you won’t even know they are infected! Solving this problem requires a detailed knowledge of your study system (which scientists have), but it also involves expanding that knowledge to the other organisms that the focal organism interacts with. This will entail a lot of work, and most people (myself included) don’t have the taxonomic knowledge required to identify and recognize the myriad parasitic species out there. Bringing onboard a parasite or disease ecologist, or at least consulting one at varying stages of the study (DON’T leave it until you’re halfway through writing up the results) could be a huge help here.

Do I ALWAYS need to think about parasites?

This is a tough one. For me, I think that parasites should ALWAYS be accounted for. They can have some serious effects on their hosts, and as a result anything you try and study about a host may actually be a parasite-mediated behavior. But, if we are to tackle this problem practically, we can’t really expect to ALWAYS account for them. Instead, it will be best to try and account for parasites when it makes sense to do so. For example, if you are studying the effects of pesticide on reproduction of amphibians, it will be worth making sure that the amphibians aren’t also already parasitized, because parasites are known to reduce the fecundity (how many offspring a parent has) of a host. A parasitized amphibian may be even less fecund than a non-parasitized amphibian, giving you a biased result. 

Conversely, if a researcher is interested in studying the effects of fragmentation on mammal community structure, they won’t necessarily need to account for parasites (though I would still make the argument that they could, but that’s another topic).

Are parasites being incorporated more these days?

As I said before, a large focus of my PhD was understanding how parasites fit into food webs. I have read a ton of papers on various ecological concepts and experiments, many of which fail to account for or even mention parasites. All that being said, parasites are increasingly recognized for their role in ecology and evolution, with more and more papers including them. For example, I recently broke down a paper investigating if parasites had a role in the evolutionary history of a group of fishes in Africa. So, things are on the up and up for parasites, and I am looking forward to the day that parasites are held to the same lofty heights as predators.

Big jumps are ahead

Parasites are here to stay, whether we like it or not. If we are to truly understand the natural world, we need to ensure that we are including all of the natural world. It’s a lofty goal, and one that is impossible without parasite ecology. 

One promising avenue for the future of parasite research is environmental DNA (eDNA), which is DNA that organisms shed into the environment throughout the course of their daily lives. For example, whenever dogs defecate in the park and their owners leave it (shame on them) that feces would contain DNA from the dog, and thus serve as a source of eDNA. Using eDNA allows for the researchers to collect information on the organisms in an environment without actually having to sample and locate every single organism. Because parasites are often quite small and hard to detect, eDNA provides a useful tool for parasite detection.

So, to wrap all of this up, what’s the biggest step you can take in incorporating parasites into community ecology? Simply acknowledging them from the start, and taking steps to speak to the right people and find out how they could fit into your study system.

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.

*I’ll never really retire, they’ll just stop paying me eventually

Title Image Credit: Adam Hasik

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