One of the primary ways that climate change is predicted to affect the natural world is through changes in the geographic distributions of species. For example, increasing temperatures are expected to force species to ‘migrate’ to higher elevations and/or higher latitudes into areas that were previously too cold for them. Evidence has been rapidly accumulating showing the expected migrations of species in North America and Europe. In contrast, very few studies have documented species migrations of tropical species. This is despite the fact that tropics house the majority of earth’s species, that these tropical species are expected to be especially sensitive to climate change (due to greater specialization on stable climates), and that tropical species are known to have migrated in response to past climate change.
In 2011, Feeley et al. published the first-ever study showing evidence of contemporary species migrations in tropical trees. Using data from repeated censuses of tree plots situated along a steep elevational gradient in the southern Peruvian Andes, they documented patterns of compositional change through time that were consistent with expectations of upward species migrations. Despite the strength of their findings, the question remained as to whether these results were driven by idiosyncratic factors such as land use change, succession or regional climate patterns, and thus specific to the study region, or if they reflect the effects of global warming and thus are more generalizable to the greater tropics.
In a new paper, “Compositional shifts in Costa Rican forests due to climate-driven species migrations” Feeley show that forests in Volcan Barva, Costa Rica, are likewise showing strong evidence of upslope migrations. More specifically, Feeley et al. used herbarium collections data to characterize the ‘preferred’ temperatures or elevations of thousands of Costa Rican tree species. They then used the relative abundance of the different tree species in 10 1 ha tree plots established along the Volcan Barva to calculate each plot’s ‘Community Temperature Score’ or CTS. A plot has a high CTS has a high relative abundance of species with lowland affinities (i.e., species that prefer hot climates); in contrast a plot with a low CTS has a high relative abundance of species with highland affinities (i.e., species that prefer cold climates). Feeley et al. then tracked how the CTS of the Volcan Barva plots changed during the course of 10 years of annual plot censuses. Mirroring their results from the Andes, Feeley et al. found that nearly all of the Costa Rican plots had increasing CTS. This means that the relative abundance of lowland species in the plots increased through time – exactly as predicted under climate-drive upward species migrations.
The fact that the two studies by Feeley et al. (2011 and 2013) show such similar results despite a separation of thousands of kilometers, zero overlap in species, and different methods and personnel being used to collect and analyze the data, strongly suggests that species migrations are a general phenomenon in tropical forests and thus that the most likely explanation is a large-scale driver such as global warming.
A very important, and often overlooked, consideration is that apparent species migrations can be driven by several different processes including “range skew” (i.e., no movement along the species’ trailing or leading edges but a shift in the relative abundance of individuals at different elevations within the range), “range shifts” (i.e., the leading and trailing edges of the species’ distribution migrate at the same pace), “range expansion” (i.e., the leading edge moves faster than trailing edge), or “range contraction” (i.e., the leading edge moves slower than the trailing edge). Depending on which of these processes is occurring, predictions for the future of ‘migrating species’ will vary from positive (under range expansions), to neutral (under range shifts) to dire (under range contractions). Most studies, including Feeley et al. 2011, only look at changes in the mean elevation/temperature of species and/or changes in the overall relative abundance of species at a site, making it impossible to distinguish between the four possible underlying processes. In their new analysis of Costa Rican forests, Feeley et al. go the extra step and determine the individual contributions of tree mortality, recruitment and growth to the observed changes in the species composition of the study plots. They find that most of the observed changes are driven by mortality. In other words, the plots are increasing in their relative abundance of lowland species but this is actually due to the dieback of highland species rather than the encroachment of lowland species. This suggests that the compositional shifts are driven by range contractions. If range contractions continue in the future it will spell trouble for these species, and if generalizable it will spell trouble for tropical, and hence global, biodiversity.
Feeley K.J., Hurtado J., Saatchi S., Silman M.R., and Clark D.B. 2013. Compositional shifts in Costa Rican forests due to climate-driven species migrations. Global Change Biology, Available Online. DOI: 10.1111/gcb.12300