Are parasites really ecologically necessary?

Frontiers in Ecology and the Environment “A world without parasites: exploring the hidden ecology of infections”

I was really excited to read this article because this is an issue that I have thought a fair bit about. As you will know if you have read my posts before, I am not the biggest fan of the pop-ecological idea that “every organism has its place”. Instead, I am more inclined to think that different organisms are more integral to ecological stability than others. I don’t doubt that removing any species will have some impact on its ecological community, but I tend to think that many species can be removed without fundamentally altering the basic structure and function of the local ecosystem. Parasites are a prime candidate for being “peripheral” to ecosystem function because they appear to take and never give: parasites are potent consumers but generally do not provide essential resources to any other organisms… or at least that’s how things look at first glance.

This article is a big thought experiment that seeks to understand the ecological role of parasites by imagining how their complete eradication might change ecosystems. We aren’t going to eradicate all parasites, so the article’s premise in totally unrealistic… but in this unreal premise lies the potential to better understand the nature of parasites.

There are individual-level effects of parasites that need to be considered. Parasites may exact a cost on their host, but it is also possible that a mild parasite keeps more harmful parasites at bay via competitive exclusion. As such it is important to recognize that eradication of a particular parasite might lead to worse outcomes for some host species. In addition, host species have evolved to calibrate their immune systems to the parasitic environment of their ancestors, so a sudden drop-off in parasite presence can sometimes lead to auto-immune diseases.

At the population level, loss of parasites can change population stability in two ways: host population release and boom-and-bust dampening. Loss of a parasite may allow a species population to grow to unusually-high densities; this may be happening as humans eliminate or at least mitigate many of our biggest parasite threats. But interestingly parasites also may intensify boom-and-bust population cycles by speeding up the “bust” periods, and so the disappearance of parasites could dampen overall population oscillations.

At the community level we really start to see the impact of parasites. They mediate competitive interactions just as predators do, so a parasite-free world might also be a less diverse world. They may also stabilize communities by acting as a drag on the most prevalent species. Parasites give advantage to invasive species when invasives are immune, but may also act as a barrier to invasives that are particularly susceptible. Perhaps the most interesting impact of parasites is on host behavior; frequently, parasites have evolved to manipulate their host into being eaten. This means that parasites subsidize the predators of their hosts, increasing their effective drag on host populations.

Having already gotten rid of almost all of our predators, we are now locked in a battle with our parasites. We will most surely come to a stalemate in this battle when it comes to fast-evolving microbes, but it is possible that we may eradicate some of our parasites. As the authors of this article also suggest, I am not ready to argue for the conservation of the parasites that infect us. But we need to be mindful of the fact that ecologically parasites are far from peripheral.