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Two Decades of Changing Bird Abundances on Mt. Kilimanjaro

Paper Reviewed
Dulle, H.I., Ferger, S.W., Cordeiro, N.J., Howell, K.M., Schleuning, M., Böhning-Gaese, K and Hof, C. 2016. Changes in abundances of forest understory birds on Africa's highest mountain suggest subtle effects of climate change. Diversity and Distributions 22: 288-299.

Writing as background for their work, Dulle et al. (2016) state that "the shortage of long-term empirical datasets hinders the possibility of informed predictions on how tropical mountain bird species may respond to the effects of both climate change and habitat disturbance by land use changes." In an effort to relieve this data shortage, the team of seven researchers set out to conduct just such an analysis on Mt. Kilimanjaro, Tanzania. Specifically, they recorded the abundances of understorey bird species along elevational gradients there from August to December of 2011, replicating the exact protocol of Cordeiro et al. (1994), who performed a similar analysis on Mt. Kilimanjaro two decades earlier in 1991. Dulle et al. then calculated the differences in bird species abundances between these two periods, during which time mean minimum air temperatures increased by approximately 2.6°C. And what did those calculations reveal?

Dulle et al. report there was a significant increase in the mean abundance of understorey bird species of 13 percent over the two decades, which increase was more strongly manifest at higher elevations. This finding was somewhat unexpected, as the authors had originally hypothesized that rising temperatures "should have led to declines in mean bird abundance at lower elevations and increases at higher elevations." Although bird abundance indeed increased at higher elevations, it did not decrease at lower ones; it also increased.

Another somewhat surprising discovery was that "both warm- and cold-adapted species appear to have benefited from increases in minimum temperatures at high elevations." Dulle et al. had not fully anticipated this outcome either. They had expected warm-adapted bird species would increase in response to the observed temperature rise, while they hypothesized that cold-adapted species would decrease. Their measurements, however, showed that "both warm- and cold-adapted species appear to have benefited from increases in minimum temperatures" between the two periods (emphasis added), as both experienced abundance increases.

Lastly, the authors also expected the abundance of bird habitat specialists (as opposed to generalists), as well as those bird species that favored an insect-centered diet (as opposed to herbivorous or omnivorous), would decrease across the 20-year interval. In contrast, their data revealed that the abundance of forest specialists and insectivores remained stable between sampling periods, whereas the abundance of bird generalists, herbivores, and omnivores all increased.

The combined results of Dulle et al.'s analysis reveal that, contrary to alarmist expectations, rising temperatures and ongoing habitat disturbance did not negatively impact bird abundances on Mt. Kilimanjaro. In fact, they appear to have had no impact, as in the case of forest specialists and insectivores, or they led to enhanced abundances, as in the case of both warm- and cold-adapted species, forest generalists, herbivores and omnivorous bird species.

Posted 29 August 2015