Animals of wildly different sizes may have different likelihoods of extinction, but it could all depend on their range sizes (Image Credit: Harvey Barrison, CC BY-SA 2.0, Image Cropped)

Constraints on vertebrate range size predict extinction risk (2019) Newsome et al., Global Ecology and Biogeography, http://doi/epdf/10.1111/geb.1309

The Crux

To act to prevent a species going extinct, we have to know that it’s at risk of extinction. Ecologists and conservationists simply don’t have the time or resources to make sure that all species remain safe. So having reliable methods of predicting species extinction risk is crucial.

On a global scale, the relationship between a species size and the area that it is found in (geographical range) has been studied intensively since ecology’s inception, both in existing and prehistoric species. Initial research showed that in general, the larger a species is, the larger its range size needed to be, with large species that had relatively smaller range sizes more prone to extinction. However more recent work has shown (naturally) that there are exceptions to this, with mammals viable range size actually decreasing up to a certain ‘breakpoint’, after which the size grows again.

This week’s researchers wanted to know whether or not the relationship between range size and body size could be a more useful predictor of extinction risk than traditional predictors, such as range size by itself.

What They Did

The researchers found body and range sizes for over 23,000 different types of species. These included six different groups: amphibians, birds, bony fish, cartilaginous fish, mammals and reptiles. The species range data came from the International Union for Conservation of Nature, and details the entire range across which the species can potentially be found.

These range sizes were then compared to body sizes. The researchers used quantile regression, a method which (in this case) analyses the lowest and highest ends of the relationship between the two variables. Theoretically, species which appear along the lower end of the relationship (ie. which have fairly low range sizes for their size) should be more prone to extinction.

Did You Know: Accounting for Phylogeny

The more closely two species are related, the more likely they are to have similar responses to an environmental variable. So whereby we might attribute similar responses between two species to a certain variable (like body size, in this case), the similarity might just be because they’re so closely related. Or the responses might differ wildly, and we miss it because we’re looking at far too diverse a range of species instead of one defined group. Because of this, ecological modellers often use ‘phylogenetic distance’ (basically a measure of how closely two species are related) to account for bias in models. Here, the most obvious example is the breaking up of species into different groups. If all the species were lumped together, we might simply think that body size corresponded to range size in one simply fashion. However because the animals were broken up into groups, the authors were able to tease apart the relationships.

What They Found

As expected, for most of the groups, range size increased with body size. However there were instances where the aforementioned ‘breakpoints’ (points until which the relationship was reversed or changed slightly) occurred, most notably for mammals and fish.

More importantly, for all groups except cartilaginous fish, range size relative to body mass proved to be a very accurate predictor of a species’ extinction risk, better so than simply looking at a species range. Species which inhabited relatively small range sizes considering their body size were more likely to be threatened by extinction.

Problems

Mapping a species range is not as easy as it may seem, and this paper faces the same problems as any research which depends on an accurate assessment of where a species is. It uses area of occupancy, which maps everywhere a species could be found. However this does not necessarily represent the area that a species actually uses, as there will be areas within this range that could be ecological wastelands for a species. One advantage of this approach though is that it means the study incorporates historical distribution of species, as opposed to the ranges they may currently be restricted to by recent human activity.

So What?

Assessing a species’ extinction risk independently is an arduous and expensive process, which is why the IUCN has only been able to evaluate extinction risk for under 5% of the world’s identified species. Developing a simple method for identifying extinction risk like this is therefore extremely important.

I’ve also written before about our need to start approaching threatened species differently (see this earlier article). Too often species’ conservation efforts only start to gather momentum when we’re down to a tiny number of individuals left. Tracking extinction trajectories like this means that we can start conservation efforts early, when it’s not too late to save a species.