The limit to the distribution of a rainforest marsupial folivore is consistent with the thermal intolerance hypothesis

Abstract

Models of impacts of climate change on species are generally based on correlations between current distributions and climatic variables, rather than a detailed understanding of the mechanisms that actually limit distribution. Many of the vertebrates endemic to rainforests of northeastern Australia are restricted to upland forests and considered to be threatened by climate change. However, for most of these species, the factors controlling their distributions are unknown. We examined the role of thermal intolerance as a possible mechanism limiting the distribution of Pseudochirops archeri (green ringtail possum), a specialist arboreal folivore restricted to rainforests above an altitude of 300 m in Australia's Wet Tropics. We measured short-term metabolic responses to a range of ambient temperatures, and found that P. archeri stores heat when ambient temperatures exceed 30°C, reducing water requirements for evaporative cooling. Due to the rate at which body temperature increases with ambient temperatures >30°C, this strategy is not effective over periods longer than 5 h. We hypothesise that the distribution of P. archeri is limited by interactions between (i) the duration and severity of extreme ambient temperatures (over 30°C), (ii) the scarcity of free water in the rainforest canopy in the dry season, and (iii) constraints on water intake from foliage imposed by plant secondary metabolites and fibre. We predict that dehydration becomes limiting for P. archeri where extreme ambient temperatures (>30°C) persist for more than 5 h per day over 4-6 days or more. Consistent with our hypothesis, the abundance of P. archeri in the field is correlated with the occurrence of extreme temperatures, declining markedly at sites where the average maximum temperature of the warmest week of the year is above 30°C. Assuming the mechanism of limitation is based on extreme temperatures, we expect impacts of climate change on P. archeri to occur in discrete, rapid events rather than as a slow contraction in range.

Tree-hugging koalas demonstrate a novel thermoregulatory mechanism for arboreal mammals

“…Animals restricted to the arboreal environment cannot exploit these thermoregulatory options-however, our study reveals, for the first time, that analogous cool microclimates can be available to these species in the form of cool tree trunks. For koalas, tree-hugging behaviour greatly reduced predicted heat loss requirements, and water savings from this behaviour could be critical for the survival of this species during heat waves when water availability is limited [14], or under high humidity when evaporative cooling is inefficient [2]. Our results are consistent with, and may help explain, previous studies of koalas in more northern populations that found seasonal [18] and weather-dependent differences in tree use, with koalas using non-food trees more frequently during hot days [6,19].…”

“…In the wild, however, koalas rarely drink and during dry periods may not have access to free water (e.g. dew on leaves)-potentially restricting this species' capacity to sustain high levels of evaporative cooling; [14]. We therefore predicted that in the wild koalas would use behavioural adjustments to minimize their reliance on evaporative cooling.…”

Predicting organismal vulnerability to climate warming: roles of behaviour, physiology and adaptation

“…Endotherms have high physiological capacities to buffer environmental variations [60], but are not immune to extreme heat waves [38,116,117], which may increase in frequency, intensity and duration as climate warms [118]. For endotherms facing such conditions, perhaps the UCT (figure 2)-or perhaps ambient temperature at the onset of panting-might be useful proxies of risk.…”

Behavioural responses to heat in desert birds: implications for predicting vulnerability to climate warming

“…To date, this approach has focussed predominantly on single species [7][8][9][10], limiting inference over the potential impact of climate change on communities. Despite the difficulties of building and parameterising complete mechanistic models, empirical mechanistic studies have provided convincing insights into how particular species and populations are affected by climate, particularly in response to high temperatures (e.g., [11][12][13][14][15][16][17][18][19]). …”