Rapid upslope shifts in New Guinean birds illustrate strong distributional responses of tropical montane species to global warming

Temperate-zone species have responded to warming temperatures by shifting their distributions poleward and upslope. Thermal tolerance data suggests that tropical species may respond to warming temperatures even more strongly than temperate-zone species, but this prediction has yet to be tested. We addressed this data gap by conducting resurveys to measure distributional responses to temperature increases in the elevational limits of the avifaunas of two geographically and faunally independent New Guinean mountains, Mt. Karimui and Karkar Island, 47 and 44 y after they were originally surveyed. Although species richness is roughly five times greater on mainland Mt. Karimui than oceanic Karkar Island, distributional shifts at both sites were similar: upslope shifts averaged 113 m (Mt. Karimui) and 152 m (Karkar Island) for upper limits and 95 m (Mt. Karimui) and 123 m (Karkar Island) for lower limits. We incorporated these results into a metaanalysis to compare distributional responses of tropical species with those of temperate-zone species, finding that average upslope shifts in tropical montane species match local temperature increases significantly more closely than in temperate-zone montane species. That tropical species appear to be strong responders has global conservation implications and provides empirical support to hitherto untested models that predict widespread extinctions in upper-elevation tropical endemics with small ranges.Temperate species are responding to anthropogenic temperature increases by rapidly shifting geographic distributions to track their climatic niche (1–3). These shifts appear to be increasing in pace—a recent metaanalysis concluded that species are shifting their distributions poleward and upslope much faster than previously estimated (1, 2). Range shifts are less studied in tropical regions however (1, 4, 5), despite being home to the vast majority of biodiversity (6). Notwithstanding strong latitudinal bias in empirical studies, climate change-driven range shifts are predicted to cause widespread extinctions in both temperate and tropical species within the next century (7–10).With scarce empirical data, models of tropical species’ response to temperature increases predict a wide range of responses (11). At one extreme, tropical species may be relatively unaffected, as the magnitude of temperature increases is relatively low in the tropics (12). Alternately, vulnerability to warming temperatures could be highest in the tropics if tropical species are physiologically specialized to narrow thermal niches (13–18). Such thermal specialization has been documented in tropical ectotherms (16, 17), but it is unclear whether similar patterns may apply to tropical endotherms, whose distributional shifts in response to warming may result from indirect rather than direct impacts of temperature increases (5).We resurveyed geographically and faunally independent elevational gradients in New Guinea nearly a half-century after they were first surveyed. The original transect surveys were conducted by J. Diamond to determine bird species’ elevational limits on Mt. Karimui (July–August 1965) (19) and Karkar Island (May 1969) (20). These environments differ significantly: Mt. Karimui is located in New Guinea’s biodiverse Central Ranges and harbors a diverse resident avifauna of ca. 250 resident landbirds (19), whereas Karkar Island is a small oceanic island off New Guinea’s north coast with a depauperate flora and fauna (ca. 50 resident landbirds) dominated by highly dispersive taxa (20) (Fig. 1).Open in a separate windowFig. 1.Map of resurvey sites in Papua New Guinea. The elevational transects recently revisited by the authors are marked by dashed lines (Mt. Karimui: 1,130–2,520 m; Karkar Island: 800–1,600 m). Mt. Karimui is an extinct volcano in the southern Central Ranges of New Guinea, whereas Karkar Island is an oceanic island located 10 miles from the New Guinean mainland. These elevational gradients were originally surveyed by Diamond in the 1960s [Mt. Karimui: 1965 (19); Karkar Island: 1969 (20)], and remain covered in primary forest.We used elevational limits measured during historical transects and modern resurveys to investigate New Guinean montane birds’ response to warming temperatures. We predicted that species have moved upslope relative to historical range limits. Given that tropical species are hypothesized to be especially sensitive to temperature increases (either directly or via indirect ecological interactions), we additionally predicted that the magnitude of upslope shifts would closely match predicted shifts based on local temperature increases. We simultaneously tested two additional hypotheses, investigating whether upslope shifts at the leading range margin outpaced upslope shifts at the trailing range edge (21), and whether species’ dietary preferences influenced upslope shifts (22, 23). We then used our data in conjunction with recent tropical resurveys to test the tropical-species-are-strong-responders hypothesis, predicting that upslope shifts measured in tropical resurveys match predicted upslope shifts significantly more closely than for temperate-zone resurveys.