A role for the local environment in driving species-specific parasitism in a multi-host parasite system (2022) Hasik & Siepielski, Freshwater Biology, https://doi.org/10.1111/fwb.13961

Image credit: Adam Hasik, image cropped

The Crux

Parasites are an ever-present part of every ecological community on Earth, yet there are some species that harbor more parasites than others. In systems where parasitism is density-dependent, meaning parasitism increases with host density, the most common/numerous species will harbor the greatest amount of parasites. Yet there are also cases of species-specificity, whereby parasites specifically target a single host species. In other host-parasite systems, local-adaption plays a role in parasitism dynamics, whereby parasites are better at attacking their local hosts than they are attacking foreign hosts and/or hosts are better at defending themselves from local parasites than foreign parasites.

With all of these different factors affecting how host-parasite systems operate, it is important to identify when and if each one is operating within specific ecological communities. This is especially necessary when ecological communities are comprised of multiple host species and multiple parasite species, all of which can/do interact with one another.

To investigate the above factors, we first conducted a survey of parasitism in damselflies (Enallagma spp.) and their water mite parasites (Arrenurus spp.). From there, we then carried out to field experiments to understand why parasitism operates the way it does within this system.

What We Did

We first carried out a massive field survey of parasitism in northwest and central Arkansas, USA. This involved myself and my colleagues going out to various lakes multiple times a week across an entire flight season (late May to early August) to collect adult odonates (dragonflies and damselflies). Though we captured plenty of odonates, we caught many more damselflies than dragonflies, and of those damselflies we primarily caught four species: E. basidens, E. exsulans, E. signatum, and E. traviatum.

Our preliminary analysis showed that E. signatum were the most heavily parasitized species, and that this parasitism varied among the different lakes that we sampled. Based off of this, we asked two questions, the first of which was is the high level of parasitism due to the fact that E. signatum is also the most common host in these lakes, or are the mites targeting them specifically? The second question was whether the intense parasitism experienced by E. signatum was a product of its immune function, which we know is limited by how much it eats.

To test the first question we set up a response-surface design (see Did You Know?) to test the effects of host density and species on parasitism. We used E. signatum, as it is the most heavily parasitized species and occurs at the highest densities, as well as E. basidens, as it is one of the least-parasitized species and occurs at much lower densities. We used low and high density treatments of each species, plus a mixed-species high density treatment. If the high density, single-species treatments had more parasites, we would know that parasitism in this system is density-dependent. However, if the high density mixed-species treatment also had more parasites, we would know that parasitism increases with density, regardless of host species identity.

To test if the intense parasitism experience by E. signatum was related to poor immune function, we used a reciprocal transplant experiment, moving E. signatum between lakes where parasitism was high (and food low) and where parasitism was low (and food high). We also added a food supplementation treatment, meaning we gave some of the damselflies extra food, to see if that would improve their immune function and decrease parasitism.

Did You Know: Response surface design

Oftentimes in ecology we want to try and pull apart different factors to understand their influence on something, either together or in isolation. With response surface designs, one can test two ideas at once: a species effect and a density effect. At its core, these designs manipulate species and density in a factorial way, meaning that you’ll have every possible combination of species and density.

The final aspect of our study involved testing relationships between various environmental factors, like predation, competition, prey density, air temperature, and water pH, to see if any of those could explain the variation in parasitism. This is crucial, because it could be that other aspects of the local environment affect the host and/or parasite, which would then drive changes in parasitism.

What We Found

In the first experiment, we found that E. signatum was far and away the most heavily-parasitized species, with no effect of density. Specifically, only two of the 250 E. basidens that we used in the experiment were parasitized, and with very few parasites at that. For both species, we found that there were no difference between the low and high density treatments, telling us that parasitism in this system is species-specific, but not density-dependent.

For the second experiment, we did not find an effect of prey supplementation, so the treatments that got extra food had just as many parasites as the ones that didn’t. In the high-parasitism, low-food lake every damselfly was parasitized, but in the low-parasitism, high-food lake we found that the non-local damselflies had fewer parasites than the local individuals. This is evidence of local adaptation, either of the parasite to the host (meaning the local parasites were not as good at attacking the non-local host) or of the host to the parasites (meaning the hosts from the high-parasitism lake have developed enhanced defenses against parasites that mites from the other lake couldn’t overcome).

When relating environmental variables to parasitism, we found that the most important predictor was the pH of the lakes. While we don’t know why this would affect the damselfly hosts, it is known that low pH is bad for the mite parasites. So, as the water becomes more acidic, the parasitism decreases.

Problems?

During the first experiment, a massive storm damaged all of the cages we were using for the experiment. This meant that non-experimental damselflies were able to enter the cages, affecting both the density and species composition of the cages. While this is not ideal, the contamination was minimal, and we showed that our results were the same when taking these invaders into account. This part of the paper was no longer rigorously experimental, but our interpretations were not affected.

So What?

Parasite studies have investigated many different aspects of the host-parasite relationship, but few try to tackle so many aspects at the same time. We showed, through observational and experimental studies, that parasitism is species-specific, not density-dependent, and enhanced by the local environment. While there is still plenty to learn in this system, today’s study has set the stage for future work and provided a strong starting foundation.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who can’t even put into words how many hours went into completing the work for this paper. 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.