Some (Don’t) Like it Hot

Do latitudinal and bioclimatic gradients drive parasitism in Odonata? (2021) da Silva et al., International Journal for Parasitology.

Image Credit: Adam Hasik, image cropped

The Crux

If there is one thing that people know about me and my research it’s that I love parasites. They’re everywhere, and more than half of all animals are parasites. They also make ecosystems more stable and link organisms within food webs to one another. For example, some parasites connect prey animals and their predators by making it easier for the predator to find and/or eat the prey. Though they can be found all over the world, there are a variety of environmental factors that make it more likely for a parasite to be found in a given environment. Today’s study focuses on one particular hypothesis related to the effects of the environment, the latitudinal diversity gradient (LDG, see Did You Know).

Did You Know: Latitudinal Diversity Gradient

The latitudinal diversity gradient is a well-known phenomenon whereby species richness increases the closer you are to the equator. In other words, there are more species in the tropics than their are at higher latitudes. There are many hypotheses for why this pattern exists, but one I want to draw attention to is the increased productivity at the equator. Primary producers are organisms like plants that make their own energy via photosynthesis, and because there is more sun and warmer temperatures at the equator, this creates an enormous amount of available food for animals to feed off of and adapt to, thus leading to a greater amounf of species.

There are a variety of environmental factors that contribute to the LDG, such as how hot it is, how much rainfall an area gets, etc. But for simplicity’s sake, I’ll just say “climatic variables”. Regarding parasites and the LDG, studies have found that ectoparasites (parasites found on the outside of the host) follow the “normal” pattern (e.g., more parasites closer to the equator), while endoparasites (parasites found inside of the host) show differing patterns and their diversity appears to be influenced by other mitigating factors. Today’s authors wanted to use these two types of parasites to ask if there are indeed climatic gradients to the patterns of parasitism by ecto- and endoparasites.

What They Did

For their host-parasite system the authors used odonates (dragonflies and damselflies, my favorites) and their water mite ectoparasites and gregarine (think worms) endoparasites. Because water mite distribution (where they are found) and population numbers are dependent on their hosts the authors predicted that mite prevalence (the percentage of the host population with parasites) would increase towards the equator, as there are more odonates near the equator. Gregarines are trophically-transmitted parasites (they infect their host by being eaten), which is not likely to be affected by climatic variables, so the authors predicted that there wouldn’t be a gradient to gregarine prevalence.

The authors searched for all studies that investigated odonate parasitism (either water mites or gregarines), with a focus on latitude and climate. They then analyzed which climatic and environmental variables predicted parasitism by the two parasites along these latitudinal gradients.

An example of mite parasitism on an odonate (orange bluet, Enallagma signatum, photo credit: Adam Hasik).

What They Found

Contrary to their predictions, the authors found that water mite prevalence increased away from the equator. Despite this surprising result, water mite prevalence was affected by environmental variables like temperature and how many plants there were in the local environment (though this was a small effect). Gregarine prevalence had no relationship with latitude, per the authors’ predictions.


This study was limited by what data was available, and unfortunately that meant that there was little data from the tropics, and no data from the southern hemisphere. This is not an issue with the study itself, but is instead indicative of how studies should focus on other areas in the future.

So What?

I love a good experiment, but studies like this one where researchers collect data from previously published work is super important because it helps to reveal large-scale patterns. With these findings, others can now focus on areas that this study wasn’t able to cover, in addition to asking more questions off the back of this study.

Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and is frantically preparing for a move to the other side of the world. 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.

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