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Warming-Induced Range Shifts of Birds on Tropical Mountains
Forero-Medina, G., Terborgh, J., Socolar, S.J. and Pimm, S.L. 2011. Elevational ranges of birds on a tropical montane gradient lag behind warming temperatures. PLoS ONE 6: e28535.

The authors write that "the tropics contain most of the world's species at risk of extinction (Pimm and Jenkins, 2010), yet few studies evaluate the response of tropical species to climate disruption other than through modeling (Jetz et al., 2007; Marini et al., 2009; La Sorte and Jetz, 2010)." Therefore, they chose to further investigate the subject within the context of this large experimental void.

What was done
Using the same sampling techniques employed by Terborgh and Weske (1975) in 1969, and working at five of the nine localities studied by those earlier investigators, Forero-Medina et al. resampled bird communities at five elevations (690, 1310, 1570, 1970 and 2220 meters) within the Reserva Comunal El Sira on the Cerros del Sira massif in Peru some 41 years later in 2010, seeking to determine what changes might have occurred in the elevations of the several species encountered at the two different sampling times.

What was learned
Of the 55 species encountered in both sampling years, the four researchers report that 36 had moved up in elevation, 12 had moved down, and 7 had not moved in either direction, resulting in an average upward shift of 49 meters for the 55 bird species over the 41-year period, which change in elevation they describe as being "significantly smaller" than the 152-meter increase one would have expected from the amount of warming experienced throughout the region between the times of the two studies.

What it means
In discussing their findings, Forero-Medina et al. first note that in the initial study of Terborgh and Weske (1975) more than one mountain had been involved, and that "the same bird species exhibited different elevation ranges on different mountains, indicating considerable flexibility in the occupancy of habitat and independence of temperature (Diamond, 1970; Terborgh, 1985)." Therefore, they go on to say that "the limited upward elevational shifts reported here are unlikely to be simple responses to increased temperature per se." Instead, they suggest that "birds are likely responding to gradual changes in the nature of the habitat or availability of food resources through their dependence on long-lived elements of the ecosystem (trees), and how the species' competitors respond." And in this regard they note that "the rate of migration of trees is less (45%) than that predicted from the temperature increase of the region," and that "similar lags in the response of trees may be occurring at the Sira, accounting for the lag in response of birds."

Put another way, Forero-Medina et al. conclude that "instead of being directly dependent on temperature, birds may be responding to gradual changes in the nature of their habitat or availability of food resources, and presence of competitors," so that "endothermy [the ability of certain animals to control their body temperature] may provide birds with some flexibility to temperature changes and allow them to move less than expected."

Diamond, J.M. 1970. Ecological consequences of island colonization by Southwest Pacific birds, I. Types of niche shifts. Proceedings of the National Academy of Sciences USA 67: 529-536.

Jetz, W., Wilcove, D.S. and Dobson, A.P. 2007. Projected impacts of climate and land-use change on the global diversity of birds. PLoS Biology 5: 3157.

La Sorte, F.A. and Jetz, W. 2010. Projected range contractions of montane biodiversity under global warming. Proceedings of the Royal Society B Biological Sciences 277: 3401-3410.

Marini, M.A., Barbet-Massin, M., Lopes, L.E. and Jiguet, F. 2009. Predicted climate-driven bird distribution changes and forecasted conservation conflicts in a neotropical savanna. Conservation Biology 23: 1558-1567.

Pimm, S.L. and Jenkins, G.L. 2010. Extinctions and the practice of preventing them. In: Sodhi, N.S. and Ehrlich, P.R. (Eds.) Conservation Biology for All. Oxford University Press, Oxford, United Kingdom, pp. 181-198.

Terborgh, J. 1985. The role of ecotones in the distribution of Andean birds. Ecology 66: 1237-1246.

Terborgh, J. and Weske, J.S. 1975. The role of competition in the distribution of Andean birds. Ecology 56: 562-576.

Reviewed 27 June 2012