Acute and chronic heat stress risk in Kalahari Desert birds under past, present and future climates

Abstract

Earth’s climate is warming at an unprecedented rate, with climate change affecting virtually every habitat on the planet. Organisms can potentially adjust their physiology, behaviour, morphology and/or geographic distribution in response to changes in climate, although their capacity for such adjustments may be constrained by biotic and/or abiotic factors. Organisms’ niches are part of a dynamic system that responds to changing climates and has resulted in species distributional shifts in the past, and will surely continue to do so in the future. Past climate data combined with species responses to climate change provide a tool for predicting extinction risk and reducing uncertainty for species responses to future climate change. Trajectories of species distributions and extinction risk in response to changing environmental conditions have not considered climate histories and the physiological responses that organisms might have expressed. I developed a dynamic tool interrogating spatial-temporal climate models, allowing identification of areas of acute and chronic heat stress risk in arid-zone birds. Birds are particularly vulnerable to increased air temperatures (Ta) due to their predominantly diurnal lifestyle, small size, and their high energy and water requirements. When exposed to short-term (typically hours) increases in Ta, birds experience acute heat stress where their only avenue of heat dissipation to maintain body temperature below environmental temperatures is evaporative heat loss. The rate of evaporative water loss in response to air temperatures reveals critical thresholds (Tthresh) of lethal dehydration risk. All passerines modelled here experience occasional risk of lethal dehydration under current conditions, with the threat increasing in frequency, severity and geographic area under a high risk climate change scenario (RCP 8.5) by the end of the century. This is most obvious for Burchell’s Starling which will experience approximately 6 – 10 d·y-1 of risk over a large proportion of their range. Similarly Scaly-feathered Finches and White-browed Sparrow-weavers will be exposed to an increase in the number of heat stress days within and around their ranges. When hot weather persists over periods of days to weeks, birds experience trade-offs between foraging and thermoregulation which have chronic, sublethal negative impacts on individual survival and fitness. Currently species in southern Africa are experiencing < 10 consecutive days where Ta > Tthresh, however these conditions are likely to increase by approximately 830% for Southern Pied Babblers and Common Fiscals (Tthresh2), and 283% for Common Fiscals (Tthresh1) in the southern Kalahari Desert region, under large proportion of the breeding season. Given the expected increases in frequency and extent of high temperatures and the inability to maintain body condition at these temperatures, successful breeding attempts are likely to decrease by the end of the century in desert birds. I suggest that these chronic, sublethal effects will be more important in terms of species persistence than the risk of acute mortality events under future climate change.