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

Image credit: Malonecr7, CC BY-SA 3.0, via Wikimedia Commons, image cropped

The Crux

In the natural world, most organisms are limited by the environment as to where they can live. While this can be as drastic as a whale being limited to the ocean and humans being limited to the land, there are also more subtle limitations. That is, black and grizzly bears live in temperate environments, but polar bears are inhabit the arctic where it is MUCH colder. Due to the limitations imposed by the environment, black and grizzly bears cannot live further north.

Historically, most studies have focused on abiotic variables (i.e., non-living), like temperature and precipitation, as there is a clear role for the climate in determining where and when a species can live. However, biotic variables (i.e., living) like predation or competition can also play a role in defining the limits of a species range, though this has proven more difficult to test than abiotic factors, as many tests of biotic variables produce species-specific results. Charles Darwin proposed a framework in 1859 that the importance of biotic interactions would vary predictably with latitude and elevation. That is, at cooler, high-altitude locations abiotic interactions would be more important, while biotic interactions would be more important at warmer, low-altitude locations. Although a number of studies have attempted to test the three predictions (see Did You Know? ) derived from this framework, the results are contradictory and come from data testing different predictions using different data. Today’s authors sought to test all three predictions at once in order to resolve these contradictory results.

Did You Know: The three predictions of Darwin’s framework

Darwin’s framework involves three, non-mutually-exclusive predictions. Specifically, 1) biotic interactions are more important at the warm limits of species ranges than the cool limits, 2) at the warm limits, biotic interactions are more important than abiotic interactions, and/or 3) biotic interactions contribute to most (i.e., more than half) of the limits tested at warm limits, and abiotic interactions will contribute to most of the limits tested at cool limits. See the figure below for a graphical representation of these predictions, as well as a representation of how all three can be true at once. 

What They Did

The authors searched Web of Science (an online database for scientific research) for all papers that investigated the causes of species’ range limits. Their search of the literature returned over 2,300 studies, of which 338 met their specifications for inclusion. Using these data, the authors conducted three analyses.

Analysis 1 focused on testing the importance of biotic vs. abiotic factors at species warm and cool range limits. Analysis 2 involved a more nuanced test of the abiotic and biotic factors involved, as the authors tested what type of abiotic and biotic factors contributed to range limits. The factors included temperature, precipitation/moisture, climate, and soil for abiotic factors and competition, predation/herbivory, pathogens, host/food availability, and biogenic habitat for biotic factors. Analysis 3 focused on the importance of biotic vs. abiotic factors at warm and cool range limits, but also tested if latitude had an effect on the importance.

What They Found

The data collected indicated that cool limits were studied more often than warm limits, more than half of the studies investigated plants, and studies of abiotic factors made up almost three-quarters of the studies included.

The authors found that biotic factors were often more important at warm range limits than abiotic factors (supporting Prediction 1) and that they contributed to warm limits more than half of the time they were tested (supporting Prediction 3). Abiotic factors contributed to cool range limits more often than biotic factors (supporting Prediction 2) and more than half of the time they were tested (supporting Prediction 3).

Not surprisingly, temperature was the most important factor at cool range limits, more so than predation/herbivory or pathogens. In contrast, predation/herbivory and competition were more important at warm range limits than cool range limits. For Analysis 3, the authors did not find any significant results, though they did find that, on average, biotic factors tended to become less important at cool limits at higher latitudes, and abiotic factors tended to become more important at cool limits at higher latitudes.

Problems?

One issue revealed by this study, though not an issue of the study itself, is that the field tends to study abiotic factors when investigating range limits. In addition, pathogens were also poorly studied in relation to range limits, despite how often they are investigated in theoretical studies. These biases can be reduced by investigating more biotic factors (particularly pathogens!) in future studies of species range limits.

So What?

This paper was such a joy to read. Not only is it a robust test of the three predictions put forward by Darwin, it provided a general test of the importance of biotic interactions on species range limits, something that the field has been lacking. Such tests are important for improving our understanding of the natural world, and continue to push the field forward be posing new questions and exciting lines of research for others to take up.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who will never stop loving meta-analyses. 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.