PS 18-252 - Correlated changes over 24 years in the abundances of a Costa Rican poison-dart frog (Oophaga pumilio) and an understory plant (Araceae: Dieffenbachia sp.) used for tadpole rearing

Abstract: There have been marked recent declines in amphibian populations in many locations, especially in tropical forests. Loss of breeding habitat could be among the multiple factors that contribute to reduced populations. Poison-dart frogs (Dendrobatidae) are dependent on small pools of water to raise their tadpoles, often found within plants (phytotelmata) above the forest floor. From 1998 to 2022, we surveyed populations of the dendrobatid Oophaga pumilio and one of its important tadpole-rearing plants (Dieffenbachia spp.) at the La Selva Biological Station in Costa Rica. Both primary and secondary forests were sampled, and showed distinct but changing patterns of relative abundance of frogs and plants. In 1998, there were nearly 5X more frogs in secondary forest than in primary forest; a similar pattern also had been documented in multiple other published studies from La Selva at this time. Dieffenbachia plants were much more abundant in secondary forest than in primary forest in 1998, reflecting the greater frog abundance. However, in 2010 there was no difference in frog abundance between primary and secondary forest. Apparently the frog density in secondary forest had fallen to the lower level previously observed in primary forest. There also was a dramatic decline in the population of Dieffenbachia between 1998 and 2012. By 2022, the abundances of O. pumilio and Dieffenbachia began to return to the pattern seen in 1998. The frogs were again more abundant in secondary forest (3X more than in primary forest), and Dieffenbachia abundance in secondary forest had more than doubled from 2012 to 2022. These results suggest that O. pumilio populations respond to the abundance of a plant that provides a vital microhabitat for tadpole rearing. We hypothesize that the changes in Dieffenbachia in turn have been driven by consumption by collared peccaries (Pecari tajacu). Peccary abundance greatly increased at La Selva in the early 2000s when we detected large declines in Dieffenbachia populations. By 2022, the large peccary herds typically observed a decade earlier had disappeared, which may have allowed the Dieffenbachia population to recover. Overall our results suggest that the rapidly changing populations of O. pumilio at La Selva may have resulted from an indirect effect from fluctuating peccary populations, mediated by herbivory on the abundance of Dieffenbachia, a plant that provides locations for tadpole rearing. This highlights the vulnerability of frogs and any other species that depend on specific plants for reproduction.