Determinants of community structure: parasites fight the environment, their hosts and each other

This blog post is provided by Joshua Brian and tells the #StoryBehindthePaper for the paper ‘Factors at multiple scales drive parasite community structure’, which was recently published in Journal of Animal Ecology. In the study, they look at how parasite communities differ depending on host species and spatial scale.  

At the heart of ecology are a set of quite simple questions. One of the most fundamental questions is ‘why does something live where it does?’ Despite many decades of work, we still aren’t really able to reliably answer this question. One of the reasons this question is so hard to answer is because community assembly (different species coming together to live in the same place at the same time) is the product of many different processes, which happen at different scales.

This is especially clear to see when we think about parasite communities (one of my major interests along with ecological scale!). Parasites might compete with other parasite species inside a single host, which could change their abundance or ability to exist in that particular host individual. However, parasites also might never successfully infect a certain host – perhaps the host has strong defences, or the parasite chooses another host species instead. Or perhaps some parasites might not even get the chance to try and infect a host – maybe they can’t reach a site where hosts live or can’t survive the broader environmental conditions at the site even if they do reach it (Figure 1).

Figure 1: One of our study sites, the Old West River in north Cambridgeshire – there are worse ways to spend an afternoon… Credit: Josh Brian

If we want to understand how communities assemble (which is important for management, and for predicting how they will change in the future), we need to understand the relative importance of these scale-dependent processes. We saw an opportunity to do so, using freshwater mussels and their parasite communities. We studied three different river sites and two different host species at those sites, dissecting over 400 mussels to look at all the parasites they contained (Figure 2). Importantly, by comparing our results to a range of different simulated communities, we were able to prove that the patterns we found are due to real ecological drivers, and not due to chance.

Figure 2: Some mussels on the river bank, sampled in the name of science. Note that we only sampled non-endangered, highly abundant mussels! Credit: Josh Brian

We found that the three sites were more different than expected by chance, and the two host species were also more different than expected by chance. Therefore, there are definitely important drivers at multiple scales: the three sites seem to differ in suitability for parasite species, maybe suggesting fine-scale environmental differences. Then, within each site, some parasites ‘prefer’ one host species or the other, which could be caused by different defences in the two host species being more effective against different parasites. We also found evidence for competition between different parasite species within a single host individual – basically, parasites have to fight to keep their place at all levels of community organisation!

The result that the different river sites were so disparate in terms of parasite communities was a big surprise, as they are actually all part of the same broader river system! (Figure 3). This suggests that there is some fine scale environmental variation between the sites that we didn’t measure. Alternatively, the sites might differ in terms of other animals, some of which act as important hosts for other life-history stages of our hosts.

Figure 3: The three sites (black dots) are all connected and part of the same river system – so why are the parasite communities so different? Points showing Ely (E) and Peterborough (P) are there too, mainly to show off my skills in R. Credit: Josh Brian.

Our study has shown some clear and defendable patterns in parasite community structure, and emphasises that multiple scales need to be considered simultaneously when thinking about communities. However, while we raise some hypotheses, we still don’t have clear reasons for these patterns – this should be the focus of future work!

Read the full paper here: Brian, J. I., & Aldridge, D. C. (2022). Factors at multiple scales drive parasite community structure. Journal of Animal Ecology, 00, 1–14. https://doi.org/10.1111/1365-2656.13853

About the author: I am a community ecologist, with broad interests in invasion ecology, species interactions and community assembly, and how these processes interact across scales. I am currently a postdoctoral research associate at King’s College London. You can find more details about me and my work here: https://jshbrian.wixsite.com/joshua-brian

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