Evan Rehm of the upwithclimate team has published a letter in the Proceeding of the National Academy of Sciences (PNAS) responding to a recent article by Freeman and Freeman. Evan’s letter is copied below. You can access the Freeman and Freeman’s original article and their reply to Evan here and here.
Rates of upslope shifts for tropical species depend on life history and dispersal mode
The recent paper by Freeman and Class Freeman (1) adds to a growing body of literature documenting the upslope distributional shifts of tropical montane species in response to climate change. Armed with just a handful of studies from the tropics, the authors compare upslope shift rates between tropical and nontropical species and conclude that tropical species are, on average, shifting their distributions faster than temperate species. On the basis of this broad comparison, the authors contend that tropical montane species are more sensitive to temperature changes than are temperate species. Although this is a laudable first attempt to synthesize basic patterns of range shifts in tropical species, I believe that the authors drew spurious conclusions because they overlooked some basic ecological differences between taxa.
Following the example of Freeman and Class Freeman, and incorporating an additional publication on tropical trees in Costa Rica (2), I compared the upslope shifts of temperate and tropical species, but now separating plants from animals. This morenuanced analysis shows that shift rates (observed/expected shift rate) in tropical plants are similar to those of temperate plants (tropical plants: 0.63 ± 0.33, n = 3; temperate plants: 0.51 ± 0.33, n = 6; t4.1 = −0.53, P = 0.625), contradicting the contention that tropical species are in general more sensitive to climate change. For animals, the shift rates for temperate species do lag behind those of tropical species (tropical animals: 1.07 ± 0.62, n = 5; temperate animals: 0.28 ± 0.26, n = 22; t4.3 = −2.74, P = 0.047), but the difference is only marginally significant.
Dispersal ability can play a leading role in determining current and future species’ ranges, especially for plants (3). Animals have the ability to move to new areas when their current locations become climatically unsuitable. Conversely, upslope shifts of plants largely depend on seed dispersal and successful recruitment into new habitats, with large range shifts occurring over several generations (4).
It is not surprising that plants, whether tropical or temperate, are lagging well behind climate shifts, whereas tropical animals in general appear to be keeping pace. If tropical plant species are indeed more sensitive to temperature changes than temperate species, the lagged migration response shown here could mean that tropical plant species will have elevated extinction risks because they cannot adapt to changing temperature conditions in situ, and their migration rates are occurring much slower than what is required to keep pace with climate change.
I caution that making generalizations of how tropical species will shift upslope with increasing global temperatures is not possible at this time. The reality is that our knowledge of species-specific responses remains extremely limited, especially in the hyperdiverse but understudied tropics. For example, we are only now beginning to appreciate the important role that microclimate refugia may play in moderating the effects of climate change, particularly in topographically complex areas such as mountains (5). By acknowledging the differences between taxonomic groups, we see that tropical plants are not shifting their ranges faster than temperate species and are not keeping pace with climate change.
Rehm, EM. 2014. Rates of upslope shifts for tropical species depend on life history and dispersal mode. PNAS 2014 111 (17) E1676; published ahead of print March 21, 2014, doi:10.1073/pnas.1403417111