Climate change, land use changes, and species invasions are all recognized as key threats to biodiversity but are often examined separately or at small scales. We discussed the recent paper “Will climate change promote future invasions?” by Bellard et al. 2013 in Global Change Biology, which focused on all three of these issues by looking at the interacting effects of land use and climate change on distributions of 100 invasive species. This study is unique because it is the first to examine land use and climate change effects on invasive species distributions at a global scale.
Bellard et al. used global data on current and future land use and climate to create species distribution models for the 100 of the world’s worst invasive species list created by the International Union for the Conservation of Nature (IUCN). They identified Europe, northeastern North America, and Oceania as future hotspots of invasion but also found that invasive species may decline in some regions such as tropical forests and tropical woodlands. As well, their results indicated that changes in range distributions may vary across taxonomic groups, with a decrease in range sizes for amphibians and birds, and an increase for invertebrates.
Although using the IUCN list seems like it could be a reasonable way to select focal species at a global scale, we began our discussion by talking about how this may have influenced results. In particular, the global distributions of richness of invasive species under current and future scenarios (Figure 1a, 1c) are very similar and we wondered to what extent future distributions were driven by current distributions. The IUCN list is not based on quantitative criteria but instead the species on the list are chosen because they have large impacts on biodiversity or humans, threaten a variety of different ecosystems or taxonomic groups, or illustrate important issues relating to biological invasions. Since the species are not chosen representatively or randomly, we were not confident in the paper’s conclusion that future hotspots of invasion will occur in Europe, northeastern North America, and Oceania. The analysis also did not account for the arrival of new species which could be very important in determining hotspots of future richness. The authors do indicate that their results should be interpreted only with respect to the species examined, but we wished this point had been made clearly throughout the paper rather than mainly in the discussion section.
We felt the species distribution modelling methods were appropriate and that the analysis was generally well done. However, it would have been nice to include different sets of climatic variables for different species or taxonomic groups, or at least to test that the variables were appropriate for different species since species’ responses to climatic factors are likely to vary. As well, although the examination of the different taxonomic groups produced some interesting results, there were only two species of reptiles and three species each of amphibians and birds, which makes it difficult to reach general conclusions about those groups.
We also noted that the land use dataset categorized land based on natural habitat categories, with only a few categories to cover all types of human use. Because invasive species often spread via human transport, land use intensity/human activity is an important factor that ideally would have been included. For example, there was only one category for water bodies as a whole, but undoubtedly invasive species are much more common near ports or along shipping routes. In addition, we wished they had included probability maps rather than only binary data and that there had been more discussion about uncertainty in the predictions.
We thought it would have been helpful to have more discussion addressing the fact that invasive species distributions are rarely at equilibrium. As well, the conclusions were somewhat generic, although we admitted we are also sometimes guilty of this. On a more positive note, the figures in this paper were great, as they were both informative and attractive!
Overall, we thought this study was a good initial step towards examining effects of climate change and land use on global distributions of invasive species, but there are some areas that could be improved upon in future research.
References
Bellard C, Thuiller W, Leroy B, Genovesi P, Bakkenes M, Courchamp F. 2013. Will climate change promote future invasions? Global Change Biology 19:3740-3748.
Gallien et al. 2012. Invasive species distribution models – how violating the equilibrium assumption can create new insights. Global Ecology and Biogeography 21: 1126-1136.