Demographic dynamics of the afro-tropical pig-nosed frog, Hemisus marmoratus: effects of climate and predation on survival and recruitment

We studied the population ecology of the West African pig-nosed frog, Hemisus marmoratus, to understand the relative contributions of adult survival and recruitment to population growth rate in savannah frogs using mark-recapture modelling. We marked a total of 821 adult frogs over 6 years and recaptured 74 at least once between years. Between-year adult survival was sex-specific and varied between 0.06 and 0.53 for males and 0.07–0.41 for females. Adult survival was significantly associated with annual rainfall and is cause for concern if rainfall declines further in the study region as predicted by changes in the global climate. There was a significant interaction between rainfall and sex with dry weather having a stronger negative effect on males than females. Pig-nosed frogs experienced boom and bust years with a single decline more dramatic than increases. Recruitment (in situ and immigration; 0.67–0.88) was substantially more important than adult survival (0.12–0.33) in determining realised population growth. In situ recruitment was highly variable between years with 1–36% of eggs and tadpoles released by females into the pond surviving to metamorphosis. Years of low tadpole survival were associated with high numbers of predatory tortoises. Thus, like other pond-breeding anurans, pig-nosed frogs showed highly variable juvenile recruitment, low adult survival and density-independent effects on population growth by predators and weather.