• Energy Research

Rapid species turnover in tropical mountains has fascinated biologists for centuries. A popular explanation for this heightened beta diversity is that climatic stability at low latitudes promotes the evolution of narrow thermal tolerance ranges, leading to local adaptation, evolutionary divergence and parapatric speciation along elevational gradients. However, an emerging consensus from research spanning phylogenetics, biogeography and behavioural ecology is that this process rarely, if ever, occurs. Instead, closely related species typically occupy a similar elevational niche, while species with divergent elevational niches tend to be more distantly related. These results suggest populations have responded to past environmental change not by adapting and diverging in place, but instead by shifting their distributions to tightly track climate over time. We argue that tropical species are likely to respond similarly to ongoing and future climate warming, an inference supported by evidence from recent range shifts. In the absence of widespread in situ adaptation to new climate regimes by tropical taxa, conservation planning should prioritize protecting large swaths of habitat to facilitate movement.

SignificanceGlobal warming is predicted to constitute an “escalator to extinction” for species that live on mountains. This is because species are generally moving to higher elevations as temperatures warm, and species that live only near mountaintops may run out of room. However, there is little evidence that high-elevation populations are disappearing as predicted. Here, we show that recent warming does indeed act as an escalator to extinction for birds that live on a remote Peruvian mountain. High-elevation species have shrunk in range size and declined in abundance, and several previously common species have disappeared. We suggest that high-elevation species in the tropics are particularly vulnerable to climate change.

AbstractMany species are responding to global warming by shifting their distributions upslope to higher elevations, but the observed rates of shifts vary considerably among studies. Here, we test the hypothesis that this variation is in part explained by latitude, with tropical species being particularly responsive to warming temperatures. We analyze two independent empirical datasets—shifts in species’ elevational ranges, and changes in composition of forest inventory tree plots. Tropical species are tracking rising temperatures 2.1–2.4 times (range shift dataset) and 10 times (tree plot dataset) better than their temperate counterparts. Models predict that for a 100 m upslope shift in temperature isotherm, species at the equator have shifted their elevational ranges 93–96 m upslope, while species at 45° latitude have shifted only 37–42 m upslope. For tree plots, models predict that a 1°C increase in temperature leads to an increase in community temperature index (CTI), a metric of the average temperature optima of tree species within a plot, of 0.56°C at the equator but no change in CTI at 45° latitude (–0.033°C). This latitudinal gradient in temperature tracking suggests that tropical montane communities may be on an “escalator to extinction” as global temperatures continue to rise.