Invasive coqui frogs serve as novel prey for birds in Hawaii, and not as competitors

Non-native amphibians often compete with native amphibians in their introduced range, but their competitive effects on other vertebrates are less well known. The Puerto Rican coqui frog (Eleutherodactylus coqui) has colonized the island of Hawaii, and has been hypothesized to compete with insectivorous birds and bats. To address if the coqui could compete with these vertebrates, we used stable isotope analyses to compare the trophic position and isotopic niche overlap between the coqui, three insectivorous bird species, and the Hawaiian hoary bat. Coquis shared similar trophic position to Hawaii amakihi, Japanese white-eye, and red-billed leiothrix. Coquis were about 3‰ less enriched in δ15N than the Hawaiian hoary bat, suggesting the bats feed at a higher trophic level than coquis. Analyses of potential diet sources between coquis and each of the three bird species indicate that there was more dietary overlap between bird species than any of the birds and the coqui. Results suggest that Acari, Amphipoda, and Blattodea made up >90% of coqui diet, while Araneae made up only 2% of coqui diet, but approximately 25% of amakihi and white-eye diet. The three bird species shared similar proportions of Lepidoptera larvae, which were ~25% of their diet. Results suggest that coquis share few food resources with insectivorous birds, but occupy a similar trophic position, which 1 This chapter was coauthored with Karen H. Beard and Aaron B. Shiels and submitted as a manuscript to the journal Biological Invasions. 22 could indicate weak competition. However, resource competition may not be the only way coquis impact insectivorous birds, and future research should examine whether coqui invasions are associated with changes in bird abundance. Introduction Although most amphibian species are threatened worldwide (Stuart et al. 2004), some species are spreading globally and are significant threats to native wildlife (Kraus 2015). Because they can spread rapidly after introduction (Phillips et al. 2007) and attain high densities (Greenlees et al. 2006), amphibians can have strong ecological impacts in their new range. Non-native amphibians have been shown to change invertebrate communities (Choi and Beard 2012), and through resource competition, reduce native amphibian populations and change amphibian community composition (Kupferberg 1997; Smith 2005; Richter-Boix et al. 2012) . However, few studies have examined whether non-native amphibians compete with other native vertebrate taxa (e.g., Boland 2004). Amphibian invasions are of particular concern on remote oceanic islands, because these islands rarely have native amphibian assemblages (Kraus 2015), and thus, endemic taxa often evolve without amphibian competitors. One such invasion is the Puerto Rican coqui frog (Eleutherodactylus coqui) to the Hawaiian Islands in the late 1980s (Kraus et al. 1999). Coquis are now widespread on the island of Hawaii and have colonized many moist habitats, while they have been controlled or restricted on the other Hawaiian Islands (Beard et al. 2009; Bisrat et al. 2012; Olson et al. 2012). They reproduce through direct development (Stewart and Woolbright 1996), and are terrestrial throughout all life 23 stages. At night, coquis climb onto understory vegetation from diurnal retreat sites to forage on invertebrates, and can change invertebrate community structure and reduce invertebrate numbers where they invade (Choi and Beard 2012). Because their populations can attain extremely high densities, up to 90,000 frogs/ha (Woolbright et al. 2006; Beard et al. 2008), they could reduce prey resources for Hawaii’s native vertebrates. Kraus et al. (1999) first proposed that the coqui could compete with native insectivorous birds for invertebrate prey on the Hawaiian Islands. Coquis may also compete with non-native insectivorous birds, which are abundant in lowland forest habitats (Scott et al. 1986) and where most coqui populations are found (Olson et al. 2012). Beard and Pitt (2005) proposed that coquis could compete with the insectivorous native Hawaiian hoary bat (Lasiurus cinereus semotus) because they both feed nocturnally, and bats move into the lowlands during critical breeding periods (Menard 2001). To assess whether coquis compete with birds and bats for invertebrate prey, overlap between their trophic positions and food resources should be compared. Methods for comparing the trophic position and food resources among different vertebrate taxa present some challenges. Stomach contents and fecal material may not be easily compared between all vertebrate taxa because of differing digestive systems (Bearhop et al. 2004), and stomach contents generally require lethal capture of target organisms, which is undesirable for species of conservation concern. Stable isotope analyses provide a reasonable alternative to traditional stomach content and fecal analysis. For one, the trophic position and the diet of different taxa can be compared on standardized isotope axes (Bearhop et al. 2004), as long as one obtains estimates of the trophic base. Furthermore, stable isotopes reflect the assimilation of prey into the diet 24 over time, in contrast to stomach contents, which do not persist long in the digestive tract. However, there are some limitations to using isotopes in diet analysis. Diet models can show high sensitivity depending on the trophic discrimination factors chosen (Bond and Diamond 2011); stable isotope diet analyses are less precise than stable isotope analyses in identifying prey items to species; and, when assessing competition between species, isotope analysis can have difficulty separating groups when the food web base has similar δ13C signatures (Post 2002). However, for the purposes of comparing the trophic position and general overlap in prey resources, isotopes can help address the likelihood of competition between co-existing species (Beaulieu and Sockman 2012; Shiels et al. 2013). Here we use stable isotope analyses to address three primary questions: 1) What is the relative trophic position, measured using δ15N and δ13C, of coquis and their potential vertebrate competitors, 2) What is the degree of isotopic niche similarity between coquis and potential vertebrate competitors, and 3) What are the potential food sources and contribution of these sources to diet among coquis and potential vertebrate competitors? We use the results to address whether introduced coquis are likely to compete with insectivorous birds and bats in Hawaii.