The path of least resistance: how native species can tell us how aliens will spread

Predicting how alien species will spread is a longstanding challenge. By breaking away from the traditional focus on niche-based models, we demonstrate the ‘environmental resistance’ model which can predict patterns of alien spread without any information on an invading species’ ecological niche.

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The number of alien species (those that humans have introduced to an area outside of their native range) is increasing rapidly, with no signs of slowing. It is therefore becoming ever more important to anticipate how an alien species will spread after introduction to a novel location. During my MSci degree at University College London (UCL), I worked on this topic in Alex Pigot’s research group at UCL’s Centre for Biodiversity and Environment Research (CBER). This began as a somewhat different project to what it is now. Initially – taking advantage of the comprehensive geographic range data for hundreds of alien bird species compiled by our collaborators Tim Blackburn and Ellie Dyer (also in CBER) – we ran a series of simulations to investigate the relative importance of stochasticity and environmental determinism in shaping alien birds’ ranges. In other words: once introduced, do aliens spread in random directions, as expected if spread is mainly determined by stochastic dispersal events, or are patterns of alien range expansion predictable based on the climate?

Ring-necked parakeet, Psittacula krameri (Photo: Rebecca Lovell)

Unsurprisingly, the results of my MSci dissertation showed that climatic gradients were a good predictor of alien spread. But importantly, this was highly dependent on the choice of climate variables used to model spread. Using the optimal combination of climate variables for a particular species and location could predict spread reasonably well. Conversely, choosing the wrong variables could result in predictions worse than a model of random dispersal. Because we generally don’t know the climate variables limiting an alien species’ range until after it has spread, this model is of little practical use in mitigating invasion risks – we only know which stable doors to shut once the horses have already bolted. To get around this, most studies use the set of conditions occupied in a species’ native range to predict spread in its alien range. But crucially, we found that the optimal combination of climate variables for predicting spread often differed between species’ alien and native ranges – there appeared to be regional variation in the factors limiting range expansion. This meant that using the combination of climate variables that best predicted a species’ native range was no better at predicting alien spread than was using a model of random dispersal. While based on a different approach, these results add to an ongoing and lively debate about the (lack of) conservatism in species’ niches and the ability to transfer ecological niche models.

This left us with a bit of a conundrum: while climate does appear to control alien range expansion, without detailed information on a species’ fundamental niche, predicting patterns of spread remains a challenge. We were keen to try and find a way around this problem, so after my graduation we decided to take this project further. Given that the importance of different climate variables appeared to vary from place to place, we thought that this should be reflected in the way the native species in a locality are distributed. For instance, in one region temperature may be the primary limiting factor, and species ranges should therefore be aligned along gradients in temperature. In other regions, different combinations of climatic gradients may be more important and species ranges should be distributed accordingly. We began to wonder, could we actually use the patterns of how native species have spread across the landscape to directly predict how a newly introduced alien species would expand? 

Canada goose, Branta canadensis (Photo: Tim Blackburn)

It turns out that this idea wasn’t a new one. Over forty years ago Eduardo Rapoport, in a book entitled ‘Areography’, proposed the idea that the ‘environmental resistance’ of the landscape to the spread of an alien species could be estimated based on the biotic similarity of native species communities. Specifically, he hypothesised that once an alien species had established, it would then tend to spread to places that are more similar in their native species composition (lower environmental resistance) to the site where it initially established – if it is good for the native species it is also likely to be good for the alien. But, despite the attractive simplicity of this approach, to our knowledge it has not been explored since. We tested the environmental resistance model by re-running our simulations, but this time with alien species spreading according to the patterns of native biotic similarity rather than climate. We found that this environmental resistance model was able to predict alien birds’ ranges better than both a model of random dispersal and our original models where dispersal was guided by climate variables.

The fact that the environmental resistance model works is important because it suggests that we can still make reasonable predictions even in cases where we don’t know anything about an alien species’ niche. Put simply, the sites most at risk from invasion will be those that have a more similar composition of native species to areas that have already been invaded. This could provide a simple and quick approach for predicting invasion risk that complements existing niche-based models. While our path to finding these results was unanticipated and not a direct one, when it comes to alien species, they seem to predictably take the path of least resistance.

Our paper is available here https://doi.org/10.1038/s41559-020-01376-x

A free ReadCube version is available here https://rdcu.be/cebhx

Rebecca Lovell

PhD Student, University of Edinburgh