The changing climate is impacting organisms worldwide. Perhaps the most obvious change is in the timing of events; Japanese cherries are blossoming earlier than they have for the last 1000 years, some migrating birds arrive at their summer grounds in the northern hemisphere several weeks earlier than they did only 50 years ago, and others lay two, not one, clutches of eggs per season. However, the directional trend towards earlier phenology conceals substantial variation among populations, species and higher. For example, great tit, Parsus major, and blue tit, Cyanistes caeruleus, populations in Western Europe show considerable variation in their phenological response to climate change, despite climate change itself being relatively uniform across this geographic range. Tree swallows, Tachycineta bicolour, in North America differ geographically in their response to local climate change, with western and more southern populations exhibiting a stronger response to climate change than eastern and northern populations. Finally, the phenological response of both pied flycatchers, Ficedula hypoleuca, and European starlings, Sturnus vulgaris, has been shown to differ between the two species and between geographical locations. Despite considerable evidence for differences in phenological response to global warming between species and regions, the causes of this variation remain poorly understood.
Although mostly studied in birds and plants, the taxon that shows the strongest average shift towards earlier breeding is amphibians. Amphibian populations have, on average, advanced their breeding date at least twice as much compared to other animal taxa for which comparable data exist. However, population changes in breeding phenology in amphibians are also surprisingly variable. For example, only 2 out of 7 studies of common toads report earlier breeding in more recent years, and a recent study of 10 amphibians found significant shifts in only 40% of species. The combination of large average responses and high heterogeneity among populations and species calls for studies that attempt to address the causes of this variation. This will enable better predictions for changes in breeding phenology across different species and across different scenarios of global warming. Population responses to novel environments in general, and global warming in particular, should depend on the species biology (e.g. which environmental cues trigger reproduction), the ecological conditions (e.g. to what extent.
Now, a new article in Ecography by Geoffrey M. While and Tobias Uller (2014) conducted a phylogenetically controlled meta-analysis of breeding phenology of frogs, toads and salamanders to examine the extent of variation in amphibian breeding phenology in response to global climate change. They show that there is strong geographic variation in response to global climate change, with species at higher latitudes exhibiting a more pronounced shift to earlier breeding than those at lower latitudes. The analyses suggest that this latitude effect is a result of both the increased temperature (but not precipitation) at higher latitudes as well as a greater responsiveness by northern populations of amphibians to this change in temperature. The authors suggest that these effects should reinforce any direct effect of increasing warming at higher latitudes on breeding phenology. In contrast, they found very little contribution from other location factors or species traits. There was no evidence for a phylogenetic signal on advancing breeding phenology or responsiveness to temperature, suggesting that the amphibians that have been studied to date respond similarly to global warming.