Long-term changes in populations of rainforest birds in the Australia Wet Tropics bioregion: A climate-driven biodiversity emergency

bird species shown here suggests that any positive influence of increased net primary produc- tivity has been swamped by the negative impacts of increasing temperature. There is existing evidence highlighting the importance of changing rainfall patterns, especially harsher dry-sea- sons [64,70] and decreasing water input from cloud interception [71]. It seems most likely that the driving factor behind many of the changes demonstrated in this study are the increasing frequency and intensity of heat waves [72]. We need to increase our understanding of the impacts of extreme climatic events so we can make more robust predictions than those that rely only on changes in average conditions. There have been clear impacts on biodiversity in almost every ecosystem and taxa across the globe due to anthropogenic climate change [2]; and now the world urgently needs to reduce emissions and adapt as much as possible to minimise future impacts. Our efforts need to be firmly focussed on the difficult question–what can we do? Managing habitats at the land- scape scale via habitat restoration, threat abatement and enhancing dispersal pathways repre- sents one avenue for local adaptation efforts to increase the resilience of biodiversity (Shoo et al. 2013, [73]). The maintenance and restoration of movement pathways and corridors to facilitate species movement into refugial areas will be vitally important [74], however, our results here suggest that facilitating movement also warrants caution. While it is imperative

that many species can move into cool refugia [75], our data demonstrating the observed move- ment of lowland, generalist species into the upland refugia, represents a potential threat to the upland endemics via increased competition with more generalist species. It is clear that montane systems are of paramount importance due to their high biodiversity value, many specialists and endemics and their role as the best-available cool refugia [75] and that these biodiversity hotspots are particularly threatened by climate. It is particularly discon- certing that, even in a fully protected and well-managed system such as the Australian Wet Tropics World Heritage Area, observed impacts are significant and accelerating. Most other tropical, montane biodiversity hotspots across the globe also face additional threats such as ongoing habitat degradation, poaching and urban encroachment. Conclusions Upland bird species, of great conservation importance, are declining in abundance and con- tracting their range to higher elevations in the montane rainforests in the Australian Wet Tropics World Heritage Area in a classic example of the “escalator to extinction” [39]. These species are suffering the combined, and increasing, threat of reduced distribution area, reduced local population density and more fragmented and isolated populations, potentially causing a loss of genetic diversity in many species; factors that increase their vulnerability to extinction [74]. Low-elevation rainforest species that are often more generalist, geographically

widespread and locally common are increasing in abundance and range size, potentially result- ing in yet another negative pressure on the upland specialists and an overall homogenisation of the rainforest avifauna [76]. These changes are likely to be indicative of impacts across all montane ecosystems globally, especially in important biodiversity hotspots associated with tropical mountain ecosystems. The important next step is to determine how we can slow, pre- vent, or adapt to these impacts to prevent the loss of the unique biodiversity of these regions around the world. Methods Study area The Australian Wet Tropics bioregion is composed of mixed tropical rainforest in an area of approximately 1.85 million hectares. The terrain is rugged and dominated by mountain PLOS ONE Rainforest birds on escalator to extinction PLOS ONE | https://doi.org/10.1371/journal.pone.0254307 December 22, 2021 9 / 16