Differences in spatial versus temporal reaction norms for spring and autumn phenological events

For species to stay temporally tuned to their environment, they use cues such as the accumulation of degree-days. The relationships between the timing of a phenological event in a population and its environmental cue can be described by a population-level reaction norm. Variation in reaction norms along environmental gradients may either intensify the environmental effects on timing (cogradient variation) or attenuate the effects (countergradient variation). To resolve spatial and seasonal variation in species’ response, we use a unique dataset of 91 taxa and 178 phenological events observed across a network of 472 monitoring sites, spread across the nations of the former Soviet Union. We show that compared to local rates of advancement of phenological events with the advancement of temperature-related cues (i.e., variation within site over years), spatial variation in reaction norms tend to accentuate responses in spring (cogradient variation) and attenuate them in autumn (countergradient variation). As a result, among-population variation in the timing of events is greater in spring and less in autumn than if all populations followed the same reaction norm regardless of location. Despite such signs of local adaptation, overall phenotypic plasticity was not sufficient for phenological events to keep exact pace with their cues—the earlier the year, the more did the timing of the phenological event lag behind the timing of the cue. Overall, these patterns suggest that differences in the spatial versus temporal reaction norms will affect species’ response to climate change in opposite ways in spring and autumn.

Countergradient variation in reptiles: thermal sensitivity of developmental and metabolic rates across locally adapted populations

Environmental temperature is a key driver of variation in physiological developmental rates in reptiles. Cooler temperatures extend development time and increase the amount of energy required to achieve hatching success, which can pose fitness consequences later in life. Yet, in locally-adapted populations, genetic variation often opposes environmental variation across ecological gradients, known as countergradient variation (CnGV). It is therefore not only the presence, but the absence of phenotypic variation that can reveal insights into the mechanisms underlying local adaptation across environmental gradients. While evidence for genetic variation opposing environmental variation in physiological rates has been summarised in other taxa, the generality of CnGV variation in reptiles is yet unknown. Here I present a summary of studies measuring developmental time and metabolic rates in locally-adapted populations across thermal clines for 15 species of reptiles across 8 families. CnGV in developmental time is found to be common, while no clear pattern emerges for the thermal sensitivity of metabolic rates across locally-adapted populations. CnGV in developmental time may be an adaptive response in order to decrease the costly development in cool climates, however empirical work is needed to disentangle plastic from genetic responses, and to uncover potentially general mechanisms of local thermal adaptation in reptiles.