Coping near thermal maximum: Demographic responses of the common frog species, Eleutherodactylus coqui, to experimental climate warming

The effects of climate warming on phenological patterns and species distributions have been described for many species but understanding how changes in behavior and demography lead to these shifts is less understood. We followed a population of the common frog, Eleutherodactylus coqui, as part of an in-situ field warming experiment (Tropical Responses to Altered Climate Experiment; TRACE), in a humid tropical forest using capture-mark-recapture methods. We assessed how growth, body size, and survival varied by warming treatment, sex, and developmental stage of frogs. In warmed plots (+4°C above ambient), adult female frogs grew faster but attained a smaller body size than frogs in control plots, suggesting a fitness tradeoff. Growth rates of adult males and juveniles did not vary between treatments, but body size was smaller in warmed plots. Survival rates did not differ significantly between warmed and control plots for juveniles, females, or males. Our results suggest that demographic responses to warming are complex and likely to vary between sexes and stages within a population. We conclude that an increase of 4°C remains within the tolerable temperature range for E. coqui at lowland sites, at least within the spatial and temporal scale of our experiment. However, this temperature increase brings this frog population perilously close to its high temperature threshold, beyond which performance is likely to decline. If this threshold is reached and the species is unable to acclimate or adapt, long-term persistence could be threatened. For a common, generalist species like E. coqui, identifying this tipping point is crucial for understanding demographic consequences in Puerto Rico as well as for its invasive potential in locations where it has been introduced.