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

about the future of this biodiversity under anthropogenic climate change are grim, particularly for the upland regionally-endemic species and habitat types [5,49–51]. In 2003, species distri- bution modelling of the endemic vertebrates suggested the potential for catastrophic impacts over the coming century with more than 50% of these species predicted to go extinct due to a complete loss of suitable climatic space [5]. These predictions drove a greater research effort in the region in the interim years and there have been extensive region-wide surveys of many ver- tebrate and invertebrate taxa [52–55]. Are the declines in species ranges predicted in 2003 concordant with observed spatial trends in species abundance patterns over subsequent years? Unfortunately, the answer is yes. We present quantitative evidence, based on the long-term monitoring of birds across the entire bioregion, for significant declines and shifts in the spatial distribution of populations. Field monitoring clearly demonstrates that the previously projected impacts are concordant with observed shifts in species abundance. Previous analyses of the climate change impact in the region have relied on either modelled distribution changes using various IPCC emission sce- narios or coarse comparisons of changes pre-2008 compared to post-2008. Here, we examine in high spatial, temporal and taxonomic detail the observed changes in the rainforest bird assemblages of the region between 2000–2016, based on the regional-scale standardised sur-

veys from the Williams Wet Tropics monitoring program (updated from Williams, VanDer- Wal [52]). We use this long-term dataset to test for bird species that have undergone significant changes in local abundance and/or elevational distribution. We predicted that bird assemblages should systematically shift upwards in elevation and that the local abundance of individual species would decline on the lower (warmer) edge of their distribution and increase at the higher (cooler) edge of their distribution [37]. We tested for trends across time in local abundance (site/elevation specific) and assemblage shifts across elevation and used an area- weighted trend to examine trends in total population size. These impacts are likely to be repre- sentative of impacts in many mountain ecosystems across the globe [7]. Results Overall, across all 42 species over the 17 years, there has been a significant decline in local abundance of rainforest birds of approximately 12 ± 1.4% (~ -0.2% per year) (Fig 1A and Table 1). However, this overall trend masks complex, and often contrasting trends, within dif- ferent ecological subsets of the rainforest bird assemblage (Table 1). Habitat generalists increased by more than 50% from 2000 to 2011 and then steadily declined until 2016 (13 spe- cies, 3.3%/year, overall trend 9 ± 4.1%, Fig 1B and Table 1). Local abundances of rainforest spe- cialist species have declined on average by approximately 20% (29 species, -1.7%/year, overall trend -20 ± 1.3%, Fig 1C and Table 1). Regionally endemic species, a subset of habitat special-

ists, showed the strongest decline with a loss of ~34 ± 1.7% in local abundance (10 species, -2.4%/year, Fig 1D and Table 1). Population trends of habitat generalists and specialists were significantly different (trend difference 0.05 ± 0.002, p<0.05). However, these average multi- species trends in ecological groupings also mask variable trends for individual and assemblages in different elevational bands. Species-specific trends in local abundance and total population size (local abundance trends weighted by area within each elevational band) and multi-species indicator groups are included in the interactive supplementary tables and figures (https://doi. org/10.25903/3tmd-sw18). Lowland specialist species have undergone a strong and significant increase of 72 ± 7% (6 species, 3%/year, Fig 2A). Mid-elevation specialists declined by 21 ± 1.4% (16 species, -1.9%/ year, Fig 2B) and upland specialists have undergone declines of 44 ± 4% (13 species, -2.9%/ year, Fig 2C). Population trends for lowland specialists were significantly different from the PLOS ONE Rainforest birds on escalator to extinction PLOS ONE | https://doi.org/10.1371/journal.pone.0254307 December 22, 2021 3 / 16