Climate related extinctions - future fear or the present state?

Determining what exactly will happen under climate change scenarios, and whether what is happening currently is attributable to climate change, is extremely difficult but very interesting subject. Pounds et al in 1999 reported a loss of an endemic toad species in Costa Rica (Bufo periglenes) due to an increase in sea surface temperatures, the first species extinction attributed to climate change. Since then great losses in population abundances have been linked to the increased stress anthropogenic activity has placed on the planet.

The exact effects of climate change on species and proposed extinction rates are inherently uncertain and difficult to predict due to the many variables involved, and the attribution of losses to climate change is problematic as all correlation/causation arguments are. One of the most discussed effects of climate change is range shifts leading to declining population abundances, in which species migrate to a different latitude to maintain their environment at the right climate (Thomas et al, 2004). This recent paper from John Wiens indicates that climate-related local extinctions have indeed been happening in a number of species. Through an examination of studies focusing on range shifts as a response to warming climate, Wiens finds climate-related extinctions in 47% of the 976 species studied. Findings also showed more frequent impacts of climate on biodiversity in tropical areas.

Minimum change scenarios for climate change predict extinctions of 18% as a direct impact of climate change with maximum change scenarios leading to extinction of ~35% (Thomas et al 2004), but as discussed the expected losses in species populations are much greater than this. Whilst declines in population abundances does not equal extinctions, it obviously makes them a lot more likely.

Limited physiological tolerances to temperature is perhaps one of the most obvious factors leading to species loss, but according to this paper, very few studies suggest a direct relationship between temperature tolerance and local extinction. So whilst Wiens details a worrying account of current local extinctions, perhaps he is missing the bigger picture. Studies investigated by Cahill et al in 2012 indicate extinctions directly caused by species interactions leading to a loss of prey and disease, and extinctions directly caused by stress of abiotic factors leading to desiccation in trees and oxygen limitation in aquatic species.  As such it appears for these assessments at least to not be a straightforward correlation between temperatures and biodiversity loss as a result of climate change, more a complex chain of events triggered by the alterations in global climate. 4 studies even reported extinctions being the result of ‘natural climatic oscillations’, a phenomenon which occurs anyway without the influence of the human race, but which could also be subject to change through anthropogenic activity.

This range of extinction mechanisms causes only furthers the unpredictability of climate change induced extinctions. The complex nature of species interactions and ecosystem functions results in a wealth of different trajectories for biodiversity. Furthermore, whilst extinctions may be caused by human habitat modification, the presence of climate change as an exacerbating variable can’t be ignored.