Predicting and Detecting the Impacts of Climate Change on Montane Fauna in Australian Tropical Rainforests

Global climates are changing rapidly and biological responses are becoming increasingly apparent. Some models indicate that climate warming impacts will largely consist of latitudinal and altitudinal shifts in potential species' distributions while others suggest that the complete disappearance of critical climate types and dependent species are possible. While early investigations have focused on high latitudes where warming has been most pronounced, there is an increasing recognition of the need to understand impacts in biodiverse tropical regions that support many spatially and climatically restricted species. The vertebrate fauna of the Wet Tropics biogeographic region of north-eastern Australia provides a unique opportunity to address this deficiency. First, endemic species are typically associated with cool, wet and relatively aseasonal upland environments; characteristics considered to have predisposed the fauna to being particularly vulnerable to future global climate change. Second, recent projections of species responses to future climate scenarios suggest strong systematic change in the extent and location of species' distributions with even small amounts of warming. Altitude is effectively a surrogate for turnover in temperature in the region and impacts of increasing temperature are expected to be most noticeable along this gradient. Focusing on two ecologically disparate taxa (ie. rainforest birds and microhylid frogs) empirical abundance patterns across altitudinal gradients and predicted altitudinal range shifts are used to: (1) predict changes in total population size; (2) estimate change in total population size relative to distribution area; and (3) quantify the sensitivity and bias of widely used measures for assessing contemporary range shifts along altitudinal gradients. According to the most conservative model scenario, 41 of 55 (74 %) rainforest birds and 6 of 6 (100%) microhylid frogs examined are predicted to become threatened as a result of projected mid-range warming expected within the next 100 years. Extinction risk varied according to where along the altitudinal gradient a species was most abundant. Upland species were most affected and are likely to be immediately threatened by even small increases in temperature. In contrast, there is a capacity for the population size of lowland species to increase, at least in the short term. In the context of climate warming impacts, a documented decline in distribution area will not necessarily reflect the extent of change in total population size. For 9 out of 12 species of regionally-endemic birds and 5 out of 6 species of microhylid frogs, total population size is expected to decline more rapidly than distribution area with increasing temperature. For both groups combined, only 4 species (22%) showed either a comparable or slower decline in population size with change in distribution area. The disproportional loss in population size suggests that extinction risk associated with climate change can be more severe than that expected from decline in distribution area alone. Detection and measurement of climate induced change is crucial if we are to improve predictions of future change and attempt to mitigate impacts to biodiversity. Randomisations of available data suggest that the mean position of presence records allows for a smaller minimum detectable range shift than change measured at upper or lower range boundaries. For a moderate survey effort of 96 surveys, measurements of change in the mean altitude of 34 rainforest birds has the capacity to provide strong inference for mean altitudinal range shifts as small as 40 m across the species assemblage. Randomizations also demonstrate that range shifts measured at range boundaries can be potentially misleading when differences in sampling effort between contemporary and historical data sets are not taken into account. In conclusion, abundance data collected across climatic gradients will be fundamental to gaining an understanding of population size change associated with climate warming. Complex spatial variability in species' abundance along with area constraints of mountain systems undermine the ability of documented change in distribution area to predict important change in population size. There is therefore an urgent need to not only track changes in location and extent of distribution area but also spatial patterns in the relative abundance of species within current and future distributions. Randomisations suggest that change in the mean altitude of species will allow for the detection of smaller range shifts than change measured at range boundaries. As detection limit is dependent on sampling effort, analyses of statistical power should be considered an integral first step in designing future monitoring programs