Abstract
Increases in the frequency, duration and intensity of heat waves are frequently evoked in climate change predictions. However, there is no universal definition of a heat wave. Recent, intense hot weather events have caused mass mortalities of birds, bats and even humans, making the definition and prediction of heat wave events that have the potential to impact populations of different species an urgent priority. One possible technique for defining biologically meaningful heat waves is to use threshold temperatures (Tthresh) above which known fitness costs are incurred by species of interest. We set out to test the utility of this technique using Tthresh values that, when exceeded, affect aspects of the fitness of two focal southern African bird species: the southern pied babbler Turdiodes bicolor (Tthresh = 35.5°C) and the common fiscal Lanius collaris (Tthresh = 33°C). We used these Tthresh values to analyse trends in the frequency, duration and intensity of heat waves of magnitude relevant to the focal species, as well as the annual number of hot days (maximum air temperature > Tthresh), in north-western South Africa between 1961 and 2010. Using this technique, we were able to show that, while all heat wave indices increased during the study period, most rapid increases for both species were in the annual number of hot days and in the maximum intensity (and therefore intensity variance) of biologically meaningful heat waves. Importantly, we also showed that warming trends were not uniform across the study area and that geographical patterns in warming allowed both areas of high risk and potential climate refugia to be identified. We discuss the implications of the trends we found for our focal species, and the utility of the Tthresh technique as a conservation tool.
Highlights
Increasingly severe heat waves are an almost universal prediction of climate-change models [1,2,3], there is no universal definition of a heat wave [1]
We suggest that reasons for this include a) a failure to identify relevant Tthresh values, and b) the fact that these values will differ between species such that the definition of heat waves becomes species-specific
Southern pied babblers are arid-zone specialists with a Southern African range centered on the Kalahari Basin, whereas common fiscals are a widespread species which, though common in the periphery of the Kalahari Basin, tends to be absent from its centre [20] (Figure 1)
Summary
Increasingly severe heat waves are an almost universal prediction of climate-change models [1,2,3], there is no universal definition of a heat wave [1]. Intense hotweather events have already led to mass breeding failures of birds [4] and mass mortalities of birds [5,6], bats [7] and humans [8], placing an immediate onus on biologists to define and predict significant heat wave events that have the potential to impact local or regional populations of different species For their analyses, Meehl and Tebaldi [1] defined heat waves using criteria based on societal impacts (human health and economies). A recent study of southern pied babblers Turdoides bicolor in southern Africa’s Kalahari region identified a daily maximum temperature (Tmax) of 35.5°C as a Tthresh for these birds: when Tmax > 35.5°C, the birds were unable to maintain body condition on a day-to-day basis [10] This does not result in any detectable, short-term mortality, but presumably carries a progressively increasing fitness risk as the duration of the event increases
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