Relative thermal tolerances of species in a community depend on warming rates

Paul Bendiks Walberg, Ecology and Evolution, Rutgers University, New Brunswick, NJ

Background/Question/Methods
Periods of extreme environmental warming are expected to increase in frequency, severity, and duration over the next hundred years, exposing a rising number of fragile ecosystems to their destructive effects, and increasing the number of species at risk of extinction globally. Climate change may result in the loss of a significant proportion of the world’s biodiversity, necessitating the development of strategies to mitigate the effects of increasing temperature on ecological stability and function. However, frameworks used to identify species most at risk of extinction are complicated by a limited understanding of whether survival depends more on the means or extremes of environmental temperature, as well as uncertainty regarding whether faster rates of warming increase or decrease temperature of extinction. In this study, freshwater communities of four rotifers and six ciliates in long-term coexistence were exposed to increasing temperature at four rates of warming differing by four orders of magnitude, roughly corresponding to the wide range of warming rates occurring during stochastic and recurring patterns of temperature variability observed in natural systems. The identities of surviving species were recorded every +0.5°C, allowing examination of the effect of warming rates on the temperature of local extinction of all ten community members.
Results/Conclusions
A surprisingly consistent positive relationship between warming rate and temperature of local extinction was observed for all ten species: temperature of extinction fell 8-12°C as warming rates were slowed to +0.5°C week-1. This consistency differed from mixed rate effects identified in previous studies examining similarly large ranges of warming rate. More importantly, in several cases the relative thermal tolerances of community members flipped as warming rates slowed (i.e., one species in the community survived to a higher temperature than another species at a high rate, but to a lower temperature than the same species at a lower rate). Several ecological mechanisms may have been responsible, singly or together, for changes in relative thermal tolerances of community members at different rates of warming, including a) differences among species in their sensitivity to both the intensity and duration of heat exposure, b) the effects of warming rates on temperature-dependent interspecific interactions, and c) differences in opportunities for adaptation among species and across warming rates. These results suggest the critical importance of considering effects of warming rates when predicting the absolute and relative risk of extinction of species in a community during extreme high-temperature events and heat waves.