Invasive Coqui Frogs Serve as Novel Prey for Birds in Hawaii, and Not as Competitors

The Puerto Rican coqui frog (Eleutherodactylus coqui) has been hypothesized to affect bird communities in Hawaii by 1) competing with insectivorous birds for prey, 2) providing prey for predatory birds, and 3) bolstering populations of non-native mammals by serving as prey. No previous studies have collected empirical data on these potential impacts. We investigated the impact of coquis on birds at two scales. For our first research question, we used stable isotope analyses to address whether three species of insectivorous bird, one native and two non-native, and coquis could compete for invertebrate prey. We found that the coquis overlapped in isotopic niche space with all three bird species, which suggests these species occupy a similar place in the food web. However, our Bayesian diet analysis suggests that coquis mostly feed on Acari, Amphipoda, and Blattodea (\u3e90%), and only consume about 2% Araneae, the only diet source they share with birds. This result suggests that coquis do not heavily compete with these bird species for prey. For our second research question, we conducted avian point counts in coqui and non-coqui plots across 15 sites on the island of Hawaii. We modeled whether coqui presence or density explained patterns of insectivorous, vertebrate-preying, and native bird abundance. We estimated abundances of birds in coqui and non-coqui plots using hierarchical Bayesian N-mixture models with random effects. We tested whether habitat variables differed across coqui and non-coqui plots and whether coqui density was correlated with any habitat variable to more confidently attribute changes in bird abundance across coqui and non-coqui plots to the frogs. We found that coquis were associated with greater abundances of vertebrate-preying, generalist insectivorous, and non-native birds in Hawaii. Vertebrate-preying birds showed the strongest association, with a 0.97 probability of abundance being at least 50% higher in coqui plots. Native birds did not show differences in abundance across coqui and non-coqui plots. Because insectivorous bird and native bird abundance did not differ across coqui invasion fronts, our results suggest that coquis primarily affect Hawaiian birds by serving as a food resource for predatory birds, and not as competitors for invertebrate prey