Biological responses to climate change II- Extinctions

Despite it is extremely hard to attribute a single cause to an extinction event, there are documented cases of climate change induced extinctions. For example in spite of habitat protection, 67% of the cloud forest specialist South American harlequin frogs (genus Atelopus) already went extinct and many more are critically endangered (see IUCN red list). Interestingly despite these are mountain restricted species, the reason for their extinction is not the loss of habitable climatic space, as we may think. Instead, altered dynamics of the Chytridiomycosis disease caused by the fungus Batrachochytrium is responsible for the mass decline of these species. Pound and colleagues (2006) showed that the increased cloudiness resulting from warming shields the forest from heat, and provides the moist conditions required for the survival, growth and reproduction of  Batrachochytrium. This example reminds me again how complex the earth-system and ecosystems are. Higher temperature causes increased evaporation which in turn results in more clouds. Clouds reflect sunlight, protect from too high temperatures and aids the reproduction of a fungus, which after penetrating the frog's skin completes it's life cycle and kills the frog. How many similar pathways exist, and what small fraction of these will ever be discovered? Staggering.... 

In marine ecosystems, the greatest challenge is faced by corals. They are not only extremely sensitive to increase in SST (sea surface temperature), but are also affected by ocean acidification caused by increased atmospheric CO₂ concentration. Whereas ocean acidification is rather a future threat than a current one, increased SST and climatic extremes in particular are already causing major problems. It has been estimated for example, that the extreme El Nino event of 1997-1998 bleached 16% of corals globally, and 95% in the Indian Ocean. The good news is that there is evidence suggesting corals are capable of adapting to some extent, partly by shifting their range and partly through genetic responses. Hughes and colleagues (2003) therefore suggests that if adequate protection is provided to coral reefs (through setting up Marine Protected Areas or No-Take MPAs), their resilience will increase which will enable them to cope with increasing SST.

Atelopus varius, one of the many harlequin frog species. It is not extinct yet, but it is critically endangered, and it's population have declined drastically in recent years in response to the spread of Batrachochytrium fungus. Source: IUCN Red List.

Stressed from heat, corals expel zooxanthellae, their symbiotic partners. Zooxanthellae are crucial for the survival of corals, without their pigmented symbionts corals loose their colour and eventually die (Hughes et al 2003). Source of image: Wikipedia.

Lastly, it is important to notice that climate change often drive populations of species to extinction. In essence, range shifts discussed in the previous posts are dynamic processes resulting from multiple colonization and extinction events. If extinction events occur on the equatorial side of a species range but colonisation and expansion is not possible on the poleward side, range contraction and ultimately extinction of the whole species occurs.Stressed from heat, corals expel 

Bibliography:
Hughes, T. P., A. H. Baird, D. R. Bellwood, M. Card, S. R. Connolly, C. Folke, R. Grosberg, O. Hoegh-Guldberg,5 J. B. C. Jackson, J. Kleypas, J. M. Lough, P. Marshall, M. Nystro ̈m, S. R. Palumbi, J. M. Pandolfi, B. Rosen, J. Roughgarden. 2003. Climate Change, Human Impacts, and the Resilience of Coral Reefs. Science 301:929-933

Parmesan, C. 2006. Ecological and Evolutionary Responses to Recent Climate Change. Annual Review of Ecology, Evolution and Systematics. 37:637-69

Pounds, Alan J., Mart ́ın R. Bustamante, Luis A. Coloma, Jamie A. Consuegra3, Michael P. L. Fogden1, Pru N. Foster, Enrique La Marca5, Karen L. Masters6, Andre ́s Merino-Viteri, Robert Puschendorf7, Santiago R. Ron2,8, G. Arturo Sa ́nchez-Azofeifa, Christopher J. Still & Bruce E. Young. 2006. Widespread amphibian extinctions from epidemic disease driven by global warming. Nature. 439:161-167