tropical forests move upwithclimate due to lowland dieback

One of the primaImagery 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.

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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.

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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

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Species can’t keep upwithclimate

When I teach or give seminars about climate change, I often start by reminding the audience that there are only four possible responses of any species to climate change: species can adapt, individuals of a species can acclimate, species can shift their geographic distributions, or failing in these species can become committed to extinction. I then usually go on to dismiss adaptation as a non-viable option due to the fact that climate is changing so fast and adaptation happens so slow. Now I have some support for my dismissal of adaptation.

Ignacio Quintero and John J. Wiens (of Yale and U. Arizona, respectively) have just published a new paper entitled, “Rates of projected climate change dramatically exceed past rates of climatic niche evolution among vertebrate species” the journal Ecology Letters. This article describes a study looking at past rates of evolution in the climatic niches of species. More specifically, the authors used time-calibrated phylogenies and estimates of species’ current climatic distributions/niches to calculate the time since divergence and most likely ancestral climatic niche for many different pairs of sister species (540 species in 17 clades of terrestrial vertebrates). For each species, the rate of climatic adaptation was then calculated as the difference between its current and ancestral climatic niche divided by time since divergence.

Based on this simple analysis, Quintero and Wiens find that rates of past climatic evolution were ‘slow’ across all the taxa examined. For example, in the case of mean annual temperature, the mean rate of evolution was generally less than 1°C change per million years. Given that global mean temperatures thave increased by approx. 0.6°C over the past 30 years and expected to increase by more than 4°C over the next century, 1°C of evolution per million years is about 10000 to 100000 times too slow.

As indicated above, the reported analyses are very simplified. This is fully acknowledged by the authors and indeed nearly the entirety of their discussion is spent addressing potential sources of error. In addition to several potential minor sources of error, the authors identify three potential major sources of error:

1) The assume a constant rate of evolutionary change through time. It is possible that species are capable of much faster bursts of evolution than indicated by the analyses which looked at the average rate across long periods of time including any periods of stasis.

2) Subspecies or populations may be capable of faster evolutionary change than entire species.

3) The species may not have been evolving at their maximum possible rates in the past. This is my biggest complaint with the study. Rate of evolution will depend on many factors including eh strength of selection. If climate change was slower in the past, selection pressure would have been less resulting in slower evolution. Now that climate change is fast, selection pressure is greater and species may be able to evolve faster. But even with the greater selection pressure, I think it is inconceivable that species can evolve the 100000 times faster that will be required to keep pace with future climate change.

Despite these and other concerns, the results of this study very strongly support the contention that climate change will simply be too fast for species to respond to through adaptation. If species can’t keep upwithclimate through evolution, then it becomes more and more likely that their only real option to escape extinction will be to move upwithclimate to higher elevations or latitudes.

–Ken Feeley

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