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

provide a host of critical ecosystem services [8], support a disproportionate amount of the world’s biological diversity and, harbour many phylogenetically unique species [5,9–12]. Mountain regions contain roughly 87% of the world’s vertebrate biodiversity [13], 54% of which is completely restricted to mountain ecosystems [13]. The global significance of mon- tane ecosystems is heightened in the tropics [14,15]; approximately 50% of the world’s species of plants and vertebrates are believed to be endemic to 34 identified global biodiversity hot- spots [16], 85% of which include large areas of tropical forest or montane cloud forest [17]. Tropical species are considered to be particularly sensitive to climate change [18–22] and the warming rates are relatively high in tropical mountains [18]. Tropical montane birds have been responding faster and more severely to climate warming than temperate species [21,23] Consequently, tropical montane biodiversity is not only globally important but particularly threatened [7,24]. Despite the global significance and high vulnerability of tropical ecosystems, there have been fewer studies demonstrating observed impacts of climate change in the tropics [25,26] but see [22,27,28]. Given that the majority of studies are short-term or lacking abun- dance data [29].and the complexity of tropical ecosystems it is difficult to measure and predict the impacts of climate change relative to other drivers like habitat loss [30]. It is also likely that climate change and habitat degradation are synergistic and baseline measures of ecological

integrity have already shifted [28,31] There is a need for increasing monitoring and improving understanding of the impacts of climate change in tropical montane ecosystems [12]. On mountains, biotic communities and abiotic conditions change abruptly over short dis- tances, with greater elevational than lateral turnover in species composition [32]. Across all elevations, assemblages on mountains with high rates of past temperature change exhibit more rapid diversification, highlighting the importance of climatic fluctuations in driving the evolu- tionary dynamics of mountain biodiversity [12]. Globally, increasing evidence indicates that species are responding to climate change by shifting their geographical distributions [33]. These shifts often follow warming temperatures poleward and upslope [6,10]. Montane species are of particular concern in this respect, as they are expected to experience reduced distribu- tion area, increased population fragmentation, and increased risk of extinction with upslope movement into ever-smaller area [34,35]. The high degree of specialization to narrow temper- ature ranges that montane species typically exhibit has raised concern over their future under climate change [10,35,36]. It is widely expected that montane species will experience further upslope shifts in the future and, in the absence of broad latitudinal shifts due to the geographi- cal features of montane ecosystems, such movements will leave species with less habitable area as they approach mountain peaks [35,37]. Left with nowhere else to go, montane species are

predicted to become increasingly susceptible to the stochastic extinctions or declining popula- tions [38]. This so-called “escalator to extinction” [39] has been predicted, and now observed, in a number of places and taxa around the world [21,22,27,34,40,41]. The rainforests in the Australian Wet Tropics bioregion in north-east Queensland are glob- ally significant for high biodiversity value based on high endemism, evolutionary significance and phylogenetic distinctiveness [42–44]. These biodiversity values resulted in the region being listed as a World Heritage Area in 1988 and being described as the sixth most irreplace- able protected area globally [45] and the second most irreplaceable World Heritage Area [46]. The high endemism and relictual nature of the biodiversity within the region is largely attrib- uted to the influence of historical fluctuations in rainforest area over the Quaternary and the restriction of rainforest to cool, moist, upland refugia [42]. This biogeographic history imposed a non-random extinction filter across the region resulting in most of the regionally- endemic species being cool-adapted upland species [47,48]. It is this biogeographic history, with the resulting concentration of endemic species in the cool uplands, that has made the bio- diversity of the region so unique but highly vulnerable to a warming climate. Predictions PLOS ONE Rainforest birds on escalator to extinction PLOS ONE | https://doi.org/10.1371/journal.pone.0254307 December 22, 2021 2 / 16 Competing interests: The authors have declared that no competing interests exist.