Predicting Future Range Shifts - What can models tell us?

Species are already responding to climate change by moving upwards and polewards, and scientist are keen to know to what extent is this trend going to continue in the future. To address this question, several so called species distribution models (SDMs) have been developed. Ecological Niche Models are some of the most widely used ones. These models infer the habitat requirement of species by looking at correlation between their distribution and climatic conditions. Once the climatic requirement, the so called ‘climatic envelope’ of the species is established, its distribution can be projected into the future with altered climatic conditions.

There are numerous examples in the literature assessing potential future distribution changes under different climate change scenarios. For example Peterson and colleagues (2002) modeled the response of Mexican fauna, using 1870 species and two climate change scenarios for year 2055. They identified clear trends of northward range shifts, and for many species drastic range contractions and fragmentation. The severity of range contraction (which is highly dependent on assumptions about migration ability) was used to estimate extinction probability, which showed a more complex spatial pattern across the country. Species turnover (the sum of extinction and colonization events) have been found to be highest in the North. An other study by Bakkenes and colleagues (2002) looked at 1400 higher plant species, and found similarly high species turnover. They predict that on average a third of all plants will disappear from each area. Again however, there is considerable spatial variation in extinction patterns: The northern and western coastal zone, the Balkans seem to be climatically stable and therefore will loose less species, whereas extinctions are likely to be most severe in Spain, France, the Black Sea and Belorussia.

Whereas a general trend of northward shift was clearly identified, patterns of extinction were not that clear varying considerably across the country. Species turnover (sum of colonization and extinction event) have been predicted to be highest in the north where up to 45% of species The reliability of these models however have been questioned and they are widely criticized. The reason being is that several assumptions of these models may not hold true. Firstly, biotic interactions are not incorporated in these models, despite both the current and future range of species can be highly restricted by competition and other interactions. Second, it assumes conservation of niches, that is the habitat requirement of a species will not change. However, there are numerous examples of adaptation to novel environmental conditions through rapid evolution. Thirdly many (though not all) models ignore colonizing ability of species, which may be restricted by the biology of species as well as by geographic boundaries (Pearson and Dawson 2003). 

Despite all the criticism, I think and important point to remember is that these models do not attempt to actually predict the future distribution of species. They merely identify potentially suitable areas. Whether a species will occur in a particular area or not will of course be determined by many natural and anthropogenic factors, not only by climate. Whereas these models are still far from perfect, they can provide valuable insight in how climate change may impact macro-scale distribution of species in the future.

Bibliography

Bakkenes, M., Alkemade, J.R.M., Ihle, F., Leemans, R. & Latour, J.B., 2002, Assessing effects of forecasted climate change on the diversity and distribution of European higher plants for 2050, Global Change Biology, 8(4), pp. 390-407.

Pearson, R.G. & Dawson, T.P., 2003, Predicting the impacts of climate change on the distribution of species: are bioclimate envelope models useful? Global Ecology and Biogeography, 12(5), pp. 361-71.

Peterson, A.T., Ortega-Huerta, M.A., Bartley, J., Sánchez-Cordero, V., Soberón, J., Buddemeier, R.H. & Stockwell, D.R., 2002, Future projections for Mexican faunas under global climate change scenarios, Nature, 416(6881), pp. 626-9.

ps.: I recommend reading Pearson & Dawson's review, it's an excellent paper on the theory behind ecological modelling, as well as its achievements and limitations.