Testing the parasite-mediated competition hypothesis between sympatric northern and southern flying squirrels (2022) O’Brien et al. 2022, International Journal for Parasitology: Parasites and Wildlife, https://doi.org/10.1016/j.ijppaw.2021.11.001

Image credit: Stephen Durrenberger, CC BY-NC-SA 2.0, Image Cropped

The Crux

One consequence of climate change is that organisms move to new habitats, as they try and track suitable environmental conditions. This can result in closely related species coming into contact with one another, which in turns drives competition among these organisms. Competition between these organisms can manifest as either direct competition (where two organisms directly compete with one another for food or habitat), but it can also manifest as apparent competition.

Apparent competition happens when species A serves as a food source for predators or parasites, which increases the numbers of predators/parasites in the environment. This increase in predators or parasites then puts more pressure on species B. Apparent competition via parasitism was actually a major driver for the decline of red squirrels in the UK, as the introduced grey squirrel brought along squirrelpox virus that had severe effects on the red squirrels.

If one species is more tolerant to a parasite than another, this can result in competitive exclusion, where one species outcompetes the other species to such an extent that the outcompeted species goes locally extinct. This is particularly important when a climate-mediated range expansion brings two species together that share parasites. Today’s authors sought to quantify how infection by parasites affected a vulnerable population after a range expansion by a potential reservoir species.

Did You Know: Parasite tolerance

A common response to parasitism is resistance by the host, whereby the host actively fights off parasitic infection. A great example of this is what the damselflies and dragonflies that I worked with during my PhD do to fend off mites (think ticks, but for insects). The mite inserts a feeding tube into the insect host, and the host will encase that feeding tube in melanin via an immune response, resisting the mite.

Tolerance of a parasite is when a host reduces the costs associated with more severe infections. Think of two animals (a horse and a donkey), both infected by one parasite. When the horse is moderately infected it loses a bit of weight, and when the donkey is moderately infected it also loses just a bit of weight. Makes sense, right? But, when the horse is severely infected it loses a LOT of weight, yet the donkey still just loses just a bit of weight. In this example, we would say that the donkey is more tolerant, because it suffered less when the infection was more severe.

What They Did

The authors trapped both northern (Glaucomys sabrinus) and southern (G. volans) flying squirrels in and around Petersborough, Canada. Both species often carry the nematode parasite, Strongyloides robustus. S. robustus is often found in other host species, so the authors also trapped eastern grey squirrels (Sciurus carolinensis), red squirrels (Tamiasciurus hudsonicus), and eastern chipmunks (Tamias striatus). When trapped, sciurids (the name for the group of animals including squirrels and chipmunks) were either tagged with an ear tag or a transponder, and their age, weight, sex and species was noted. The authors also collected fecal samples, allowing them to quantify the parasite load.

With this data, the authors tested two things. First, they quantified what effect the parasites had on squirrel body mass. Second, they used statistical models to test for parasite-mediated competitive exclusion. Specifically, they tested to see if the presence of other sciurids, parasites, and woodland characteristics explained the presence or absence of the northern or southern flying squirrels.

What They Found

In total, the authors captured 229 sciurids, 25 of which were northern flying squirrels and 62 of which were southern flying squirrels. For the effects of parasites on body mass, the authors found that there were no differences in the body mass between the infected and non-infected southern flying squirrels, while the infected northern flying squirrels had lower body mass than the non-infected individuals. The southern flying squirrles are therefore more tolerant to infection, meaning the system is asymmetrically tolerant. Additionally, the authors found that the parasite was present in all trapped species except for the chipmunks.

For the statistical models of the relationships between squirrel presence and other squirrel species, parasites, and woodland characteristics, there were no significant relationships between any of the predictor variables and the dependent variables in the models, meaning that none of the variables used could explain changes in squirrels presence or absence.

Problems?

I know from personal experience how tough a field-based study can be, and though I haven’t tried trapping animals for my work I have heard plenty of horror stories about how difficult it is, on top of the tough field study. The authors were interested in quantifying the effects of parasite-mediated competitive exclusion on the northern flying squirrel by the southern flying squirrel, yet they were only able to trap 25 northern flying squirrels. This is no fault of their own, the data is what the data is, and I also know about how tough it is to work with limited data. But, it is important to keep that fact in mind, because it has potential implications for their results. While they did not find any significant relationships between the squirrel presence/absence and any of their predictor variables, that may be because of the limited data. A more clear pattern could become evident if more data is collected, which is an exciting thought and highlights the importance of further study in this system.

So What?

There are two interesting takeaways from this study. First, the authors showed evidence for asymmetric tolerance to a parasite in this system, which is super cool! Though there was no evidence for parasite-mediated competitive exclusion, this asymmetric tolerance is an important first step for such competitive exclusion to happen, so while it’s not happening now it may in the future.

Second, the authors showed that the parasite of concern is present in other species and can parasitize more than flying squirrels. This means that parasite-mediated competition could be happening, but it requires an understanding of the ENTIRE sciurid community, and not just two closely related species of flying squirrel. Ecology is an incredibly complex beast, and today’s study is a great example of how an attempt to simply things to better understand them can actually result in making things even more complicated.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who saw a flying squirrel once. It was super cool. 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.