Abstract
Critical thermal limits form an increasing component of the estimation of impacts of global change on ectotherms. Whether any consistent patterns exist in the interactive effects of rates of temperature change (or experimental ramping rates) and acclimation on critical thermal limits and warming tolerance (one way of assessing sensitivity to climate change) is, however, far from clear. Here, we examine the interacting effects of ramping rate and acclimation on the critical thermal maxima (CTmax) and minima (CTmin) and warming tolerance of six species of springtails from sub-tropical, temperate and polar regions. We also provide microhabitat temperatures from 26 sites spanning 5 years in order to benchmark environmentally relevant rates of temperature change. Ramping rate has larger effects than acclimation on CTmax, but the converse is true for CTmin. Responses to rate and acclimation effects are more consistent among species for CTmax than for CTmin. In the latter case, interactions among ramping rate and acclimation are typical of polar species, less marked for temperate ones, and reduced in species from the sub-tropics. Ramping rate and acclimation have substantial effects on estimates of warming tolerance, with the former being more marked. At the fastest ramping rates (>1.0°C/min), tropical species have estimated warming tolerances similar to their temperate counterparts, whereas at slow ramping rates (<0.4°C/min) the warming tolerance is much reduced in tropical species. Rates of temperate change in microhabitats relevant to the springtails are typically <0.05°C/min, with rare maxima of 0.3-0.5°C/min depending on the site. These findings emphasize the need to consider the environmental setting and experimental conditions when assessing species' vulnerability to climate change using a warming tolerance approach.
Highlights
Ectotherm physiological performance and fitness are directly affected by temperature
Among the many significant outcomes of this work, two are notable in the context of environmental change: (i) the finding that thermal tolerance limits can provide accurate means to estimate geographical ranges (Bozinovic et al, 2011; Overgaard et al, 2014) and warming tolerances (WTs; sensu Deutsch et al, 2008) as a proxy for species vulnerability to climate change; and (ii) indications that tropical and sub-tropical species may be substantially more at risk from rising temperatures than their temperate counterparts, with some complexity about this pattern and the assumptions made to derive it (Deutsch et al, 2008; Diamond et al, 2012; Hoffmann et al 2013; Sunday et al, 2014)
Other investigations have shown that critical thermal limits are reduced when individuals are exposed to slow ramping rates (Chown et al, 2009; Peck et al, 2009; Allen et al, 2012), probably as a consequence of mounting heat damage over long time periods (Cossins and Bowler, 1987)
Summary
Ectotherm physiological performance and fitness are directly affected by temperature. Recent impetus for understanding the variation in thermal traits has come from the need to forecast the response of populations to changing climates and the ways. The first concern is the way in which experimental rates of temperature change (hereafter ‘ramping rate’) affect estimates of these limits and the extent to which they show heritable variation (Terblanche et al, 2007; Mitchell and Hoffmann, 2010). Perhaps key among the emerging perspectives is the requirement for an understanding of how rates vary in field conditions (Hoffmann, 2010; Woods et al, 2015) and the extent to which the effects of varying rates might, if at all, be consistent among different treatments, taxa and environmental settings (Terblanche et al, 2011; Sørensen et al, 2013; Rezende et al, 2014; Hangartner and Hoffmann, 2015)
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