Invasive frogs do not affect native bird communities in Hawaii
Non-native species can influence community and ecosystem dynamics. The effects of non-natives on natives can be of particular concern in insular communities where many natives are endemic and threatened with extinction. The introduction of the Puerto Rican coqui frog (Eleutherodactylus coqui), hereafter the coqui, to Hawaii has the potential to disrupt sensitive, native communities, especially at higher elevations. Previous studies have shown that the highly abundant coqui can change arthropod communities where it invades. It has been hypothesized that the frogs may affect bird communities through competition for arthropod prey. The objective of our study was to determine whether coqui frogs change both native and non-native bird abundances and richness. We measured bird and coqui abundance in three replicate plots on either side of 15 coqui invasion fronts across the island of Hawaii. To account for different detection probabilities across sites, we used N-mixture models to determine whether coqui presence and abundance were top predictors of bird abundance compared to other environmental variables measured.
We observed 5707 individuals of 21 bird species, of which 16 were non-native and five were native. While coqui abundance and presence were not important explanatory variables for native abundance or richness, our top model for total non-native bird abundance suggests that they increase with coqui abundance. For individual species, coquis were not the top predictors for the three natives with sufficient observations for analysis. However, top models for two non-natives, the Common Myna (Acridotheres tristis) and Chinese Hwamei (Garrulax canorus), suggest their abundances increase with coqui presence. While our results suggest that coquis are associated with increased abundances of non-native birds in general, and we controlled for habitat differences in our study design, we cannot reject the possibility that non-native birds and coquis respond to similar habitat characteristics. Alternatively, coquis could act as an additional food source or change arthropod and plant communities in a way that favors non-natives. We only observed native birds in three of our 15 sites, and no analysis revealed a change in their abundance as a result of coqui frogs. Results suggest that, at this time, native bird communities have not changed as a result of coqui frogs.