Biotic interactions are more often important at species’ warm versus cool range edges, Paquette & Hargreaves, 2021 Ecology. https://doi.org/10.1111/ele.13864

Image credit: Trey Ratcliff, CC BY-NC-SA 2.0

The Crux

In nature, we usually refer to the given area in which a species is found as a species range. The size of these vary, even between species that are very similar in appearance. For example, many of the dragonflies and damselflies I worked with during my PhD research could be found all over the state of Arkansas, but others had more limited ranges, and could only be found in the more southern lakes that I visited. Often, species are limited to these areas because the environmental conditions, such as temperature, are favorable to them, and the change in those conditions beyond the boundaries of their range will lead to them suffering. Knowing which factors limit the range of a given species is important for management policies, as knowing the temperature limits can inform predictions about the effects of climate change, while knowledge of natural enemies (like predators) can help with the containment of invasive species.

Previous work on the constraints experienced by species at their range limits tend to focus on abiotic factors (temperature, precipitation, etc.), as these data are easily quantified and there are very extensive records available. However, biotic factors (interactions with predators/competitors, the availability of prey) can also limit the range of a species. Though biotic factors are important, they are more difficult to quantify than abiotic factors, and are often species-specific. That is, the effect of a competitor on limiting the range of one species won’t be the same on another species. Interestingly, biotic interactions may be more important in warmer range limits, while the abiotic may be more important in the cooler range limits. Today’s authors used data from a number of studies to test just that idea.

Did You Know: Misinterpreting Effects

When measuring the effect of an environmental characteristic on a species, it’s always hard to be certain of a direct relationship between the two. Perhaps it is temperature that is driving a species range limit, but it also could be air moisture content, which is often highly correlated with temperature. This can lead to us misinterpreting the relationship between a species and the temperature. The same goes for abiotic factors. We may think that two species co-occurring very infrequently might mean that one is driving out the other, but it may instead be that the species simply have different responses to a local characteristic of the environment.

What They Did

The authors collected data from studies that assessed the causes of species range limits at either cool or warm range limits (the parts of the range where temperatures start to go above or below what a species is comfortable with). They then constructed a series of models that allowed them to understand if and how a given factor (biotic or abiotic) contributed to the range limit of a given species.

They sought to test three separate predictions, namely that 1) biotic factors are more important at warm range limits vs. cool, 2) biotic factors are more important than abiotic factors at warm limits, and 3) that at warm limits, biotic factors limit the species range in the majority of cases (as opposed to not having an effect). Additionally, the authors wanted to understand which range-limiting factors were supported the most often, as prior predictions state that some factors are particularly important (e.g., competition is an important limiting factor at warm limits).

What They Found

The data collected by the authors came from 338 individual studies, which assessed range limits in 654 different groups of organisms. Biotic factors were important more often at warm range limits than cool limits (prediction 1), and contributed to range limits in over 50% of the cases (prediction 3). In general, abiotic factors were supported more than biotic factors at cool range limits, while biotic factors limited species’ ranges more often at warm limits (prediction 2).

Problems?

The data available on species range limits was heavily biased towards abiotic factors (only 27% of the studies included investigated the effects of biotic factors on species range limits). While this isn’t ideal, it speaks more to an issue with how range limits have been assessed than it does an issue with the study itself. The authors rightfully point out that our understanding of species distributions would benefit from future studies of the effects of biotic factors on species range limits.

So What?

In ecology, there are a number of hypotheses and theories for how various processes and interactions occur. Today’s study set out to test a series of these hypotheses, and through a thorough review of the literature they were able to provide evidence supporting all three of them. While the data isn’t perfect, it helps to set the course for future research in this area. What’s more, they found that results differed between the various methods employed by other researchers, highlighting how important it is to standardize sampling efforts in the future.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who worries he may forget what green looks like now that he lives in the desert. 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.