Amphibians closer to the pole respond stronger to global warming
Submitted by editor on 13 October 2014.
By Tobias Uller
In a warmer world, spring will be earlier. The increase in temperature is not the same across the globe, however, and the highest rate of warming is found at high latitudes. It is therefore not very surprising that, in a meta-analysis of 59 studies from the northern hemisphere , we found that amphibian populations at higher latitudes have advanced their breeding date more than populations closer to the equator.
More interestingly, our data suggest that this is only partly due to the differences in warming itself - populations at higher latitudes are also more responsive to temperature change than populations at lower latitudes. As a consequence, the effect of warming becomes exaggerated in the north.
Why do populations in the north respond stronger for a given increase in temperature? There are several possible reasons. First, populations in the north may be at the margins of their range and hence be continuously swamped by locally adapted genes from more southern populations. As a result, northern populations become more responsive to warm temperature than expected from local conditions (Phillimore et al. 2010). However, the latitudinal effect was similar across species with different range limits, suggesting there is an alternative explanation.
This explanation could be local adaptation. A shorter activity season and generally cooler water temperatures may exercise stronger selection on breeding date to ensure that tadpoles can metamorphose before the onset of autumn. When conditions are right, there may be no reason to hold back. Indeed, another recent meta-analysis showed that the shape of reaction norms can evolve rapidly (Murren et al. 2014). The fact that we found no phylogenetic signal in our data strongly supports fast evolution of local adaptation within species, in particular since many populations are the result of range expansion after the latest ice age. Data from the southern hemisphere would be useful to explore this further.
Finally, populations closer to the equator may not make use of temperature because it is a poor cue. In warmer climates, rainfall and other environmental cues may be better predictors of good breeding conditions than temperature. It remains unclear how animals incorporate new types of inputs into an existing physiological system - an interesting problem for ecological and evolutionary developmental biologists to solve. Our study of amphibians shows that this is also of interest to ecologists. Because humans modify some parts of the environment (e.g., temperature) and not others (e.g., photoperiod), we anticipate that we will better understand responses to anthropogenic change if we pay more attention to how individuals acquire and process information from their environments.
Is breeding earlier good or bad for amphibians? Early breeding typically increases survival and recruitment so we may expect frog populations to increase. However, one must not forget that there are several additional consequences of climate change, such as the effect of warmer winters on breeding condition, an increase in disease susceptibility and the indirect consequences of shifts in the phenology of other species in the ecosystem. Long-term studies of amphibian population dynamics will be needed to make robust predictions.
References
Murren, C. J., H. J. Maclean, S. E. Diamond, U. K. Steiner, M. A. Heskel, C. A. Handelsman, C. K. Ghalambor et al. 2014. Evolutionary Change in Continuous Reaction Norms. American Naturalist 183:453-467.
Phillimore, A. B., J. D. Hadfield, O. R. Jones, and R. J. Smithers. 2010. Differences in spawning date between populations of common frog reveal local adaptation. Proceedings of the National Academy of Sciences of the United States of America 107:8292-8297.