Coqui Research Articles

Invading nonnative frogs use different microhabitats and change physiology along an elevation gradient.

The coqui frog (Eleutherodactylus coqui) was introduced to the island of Hawai'i in the 1980s, and has spread across much of the island. There is concern they will invade higher elevation areas where negative impacts on native species are expected. It is not known if coqui change behavior and baseline physiology in ways that allow them to invade higher elevations. We investigated where coqui are found across the island and whether that includes recent invasion into higher elevations. We also investigated whether elevation is related to coqui's microhabitat use, including substrate use and height off the forest floor, and physiological metrics, including plasma osmolality, oxidative status, glucose, free glycerol, and triglycerides, that might be associated with invading higher elevations. We found coqui have increased the area they occupy along roads from 31% to 50% and have moved into more high-elevation locations (16% vs. 1%) compared to where they were found 14 years ago. We also found frogs at high elevation on different substrates and closer to the forest floor than frogs at lower elevations-perhaps in response to air temperatures which tended to be warmer close to the forest floor. We observed that blood glucose and triglycerides increase in frogs with elevation. An increase in glucose is likely an acclimation response to cold temperatures while triglycerides may also help frogs cope with the energetic demands of suboptimal temperatures. Finally, we found that female coqui have higher plasma osmolality, reactive oxygen metabolites (dROMs), free glycerol, and triglycerides than males. Our study suggests coqui behavior and physiology in Hawai'i may be influenced by elevation in ways that allow them to cope with lower temperatures and invade higher elevations.

Invasive frogs show persistent physiological differences to elevation and acclimate to colder temperatures

The coqui frog (Eleutherodactylus coqui) was introduced to the island of Hawai'i in the 1980s and has spread across much of the island. Concern remains that this frog will continue to expand its range and invade higher elevation habitats where much of the island's endemic species are found. We determined whether coqui thermal tolerance and physiology change along Hawai'i’s elevational gradients. We measured physiological responses using a short-term experiment to determine baseline tolerance and physiology by elevation, and a long-term experiment to determine the coqui's ability to acclimate to different temperatures. We collected frogs from low, medium, and high elevations. After both the short and long-term experiments, we measured critical thermal minimum (CTmin), blood glucose, oxidative stress, and corticosterone levels. CTmin was lower in high elevation frogs than low elevation frogs after the short acclimation experiment, signifying that they acclimate to local conditions. After the extended acclimation, CTmin was lower in frogs acclimated to cold temperatures compared to warm-acclimated frogs and no longer varied by elevation. Blood glucose levels were positively correlated with elevation even after the extended acclimation, suggesting glucose may also be related to lower temperatures. Oxidative stress was higher in females than males, and corticosterone was not significantly related to any predictor variables. The extended acclimation experiment showed that coquis can adjust their thermal tolerance to different temperatures over a 3-week period, suggesting the expansion of coqui into higher elevation habitats may still be possible, and they may not be as restricted by cold temperatures as previously thought.

Climate change drives frog call change in Puerto Rico: Predictions and implications

Acoustic signal production and reception in ectotherms are acutely temperature-dependent. We recorded the advertisement calls of the Puerto Rican coqui frog (Eleutherodactylus coqui) along an altitudinal gradient and repeated the measurements twenty-three years later. We found that over this period, at any given elevation, calls exhibited both significant shortening of their duration and increases in pitch. We also found that the observed differences are consistent with a movement to higher elevations for the population, a familiar response to rising ambient temperature. Using temperature data obtained from four local weather stations over the same time period, we confirmed a significant ambient temperature increase, the magnitude of which closely predicts the observed changes in the frogs’ calls. Physiological responses to long-term temperature rises include reduction in individual body size and thus, population biomass. These changes can have alarming consequences, since coqui frogs form an essential component of the food web in the Puerto Rican rainforest. (Work supported by UCLA Academic Senate via Grant No. 3501.)

Occupancy and probability of detection of the introduced population of Eleutherodactylus coqui in Turrialba, Costa Rica

The Puerto Rican Common coqui frog (Eleutherodactylus coqui) has a long history as an invasive species in places such as Hawaii. Since its introduction in Costa Rica, scarce information is available to understand why and how the habitat in the Turrialba town is suitable for the species. Our goal was to analyze the habitat selection of E. coqui to identify if there are key habitat features that explained its success there. We measured 9 site variables that may affect the habitat selection of E. coqui in 92 survey units of 10 m radius distributed over a 500 m radius from its introduction point. We registered the presence/pseudo-absence data of E. coqui and environmental variables in each survey unit during eight surveys. We ran occupancy models to determine the influence of the variables on the habitat selection and to estimate its detection probability. We found that sites near the introduction point, containing abundant vegetation, bromeliads, and palms have a higher probability to be occupied by E. coqui. The habitat selection in Costa Rica shares characteristics with the populations of Puerto Rico and Hawaii. But, unlike the case in Hawaii, in Costa Rica this species has maintained a limited dispersal because the potentially higher biotic resistance, as well a sedentary behavior. However, the microhabitat conditions used by E. coqui in the study site are common throughout the country. Therefore, active management in new populations and environmental education programs to avoid human transportation of the species is critical to reduce its dispersal.

Metabarcoding Approaches in Amphibian Disease Ecology: Disentangling the Functional Contributions of Skin Bacteria on Disease Outcome.

Molecular technologies have revolutionized the field of wildlife disease ecology, allowing the detection of outbreaks, novel pathogens, and invasive strains. In particular, metabarcoding approaches, defined here as tools used to amplify and sequence universal barcodes from a single sample (e.g., 16S rRNA for bacteria, ITS for fungi, 18S rRNA for eukaryotes), are expanding our traditional view of host-pathogen dynamics by integrating microbial interactions that modulate disease outcome. Here, I provide an analysis from the perspective of the field of amphibian disease ecology, where the emergence of multi-host pathogens has caused global declines and species extinctions. I reanalyzed an experimental mesocosm dataset to infer the functional profiles of the skin microbiomes of coqui frogs (Eleutherodactylus coqui), an amphibian species that is consistently found infected with the fungal pathogen Batrachochytrium dendrobatidis and has high turnover of skin bacteria driven by seasonal shifts. I found that the metabolic activities of microbiomes operate at different capacities depending on the season. Global enrichment of predicted functions was more prominent during the warm-wet season, indicating that microbiomes during the cool-dry season were either depauperate, resistant to new bacterial colonization, or that their functional space was more saturated. These findings suggest important avenues to investigate how microbes regulate population growth and contribute to host physiological processes. Overall, this study highlights the current challenges and future opportunities in the application of metabarcoding to investigate the causes and consequences of disease in wild systems.

Warming increases activity in the common tropical frog Eleutherodactylus coqui

Tropical ecosystems are expected to experience climate warming, with predicted increases in drying and heat extremes in the coming years. Understanding how these changes will affect terrestrial vertebrates such as amphibians is limited. The Tropical Responses to Altered Climate Experiment in the Luquillo Experimental Forest in northeastern Puerto Rico allows us to study how the tropical forest responds to warming within a replicated plot design. From September 2018 to August 2019, we used mark-recapture sampling to investigate how the spatial population ecology of the common coqui frog (Eleutherodactylus coqui) is impacted by experimentally increasing surface temperatures by 4 °C above ambient. We compared estimates of baseline detection, space use, and the density of frogs in control and warmed plots. Coqui space use and population density did not differ between control and warmed plots. However, coqui detection probabilities were higher in warmed plots, suggesting an increased level of activity relative to individuals in the control (unwarmed) plots. Frog detection increased in all plots with increased precipitation. Our results suggest that, at least in the short-term, the density of an ecological generalist frog like E. coqui does not change as a response to increased surface temperatures. However, short-term responses to warming such as changes in behavior may lead to changes in population dynamics in the long-term. Our research highlights the need to consider mutiple repsonses in order to understand the effects of climate warming on tropical vertebrates.

Lizard and frog removal increases spider abundance but does not cascade to increase herbivory

AbstractInsectivorous vertebrates, especially on islands, can exert top‐down control on herbivorous prey, which can transfer through a food chain to reduce herbivory. However, in many systems insectivorous vertebrates feed on more than one trophic level, especially consuming arthropod predators, and this intraguild predation can diminish trophic cascades. Our goal was to determine, using an exclosure experiment, the relative importance of anole lizards and coqui frogs in controlling spider and arthropod abundances as well as herbivory rates in the understory of the Luquillo Experimental Forest, Puerto Rico. We found that exclosures removing both anoles and coquis doubled spider abundance compared to exclosures with anoles and coquis at natural densities. The effect of coquis on spiders was greater and occurred more quickly than that of anoles, potentially because of the higher natural densities of coquis and removal of both vertebrates produced no interactive effects. We found support for the idea that anoles, but not coquis, reduce foliar arthropod abundances on one of the two studied plant species. However, there was also evidence that anole removal decreased herbivory, the opposite of what we would expect if there was a trophic cascade. Potential explanations include that anoles reduced predatory arthropods on foliage more than they reduced herbivorous arthropods. Results highlight that the food web in tabonuco forest is not simple and that there are complex and dynamic relationships among vertebrate insectivores, predatory arthropods, and herbivorous arthropods that do not consistently result in a trophic cascade.Abstract in Spanish is available with online material

Determining vocal interactions and call timing in chorusing frogs: The contributions of Peter Narins

Numerous species of frogs call synchronously in a chorus by timing their calls to avoid call-overlap with conspecific neighbors. Although synchronous calling is most likely maintained by a call oscillator, several species of frogs including the Puerto Rican treefrog Eleutherodactylus coqui can adjust the timing of the oscillator period over a wide range so as to maintain call synchrony with competing stimuli. Here we describe some of the playback experiments performed by Peter Narins and his co-worker Randy Zelick on the timing capabilities of the oscillator in the coqui frog, particularly the ability to rapidly alter timing even when the silent window had a random onset and was short in duration. Narins recognized that characterizing calling behaviour in isolation, with synthetic stimuli, did not fully describe the acoustic challenges faced by calling males in natural assemblies. Consequently, Narins and his co-worker Jeffrey Brush obtained individual call timings from groups of two to five vocally interacting coqui frogs in a natural assembly. They showed that some males actively avoided call overlap with two, maybe three neighboring conspecifics, while some actively jammed a neighbor. The impact of these seminal studies, and the technical challenges that were overcome by Narins are discussed.

Occurrence of Rat Lungworm (Angiostrongylus cantonensis) in Invasive Coqui Frogs (Eleutherodactylus coqui) and Other Hosts in Hawaii, USA

The rat lungworm (Angiostrongylus cantonensis) has emerged as an important human and animal health concern in Hawaii, US. Although the life cycle of the parasite requires both rat and gastropod hosts, other animals acting as paratenic hosts, such as frogs and centipedes, have been identified as sources of infection. We investigated the occurrence of rat lungworm infections in potential paratenic hosts in Hawaii to provide information on how they might be involved in transmission of angiostrongyliasis. We confirmed the presence of rat lungworm in 87% (21/24) of introduced Puerto Rican coqui frogs (Eleutherodactylus coqui) in Hilo, Hawaii, by real-time PCR. Additionally, four Cuban greenhouse frogs (Eleutherodactylus planirostris), two cane toads (Rhinella marina), and three centipedes (Scolopendra subspinipes) were found to be infected. In the frogs and toads, multiple tissue types were positive, including stomach and intestine, muscle, liver, heart, and brain, indicating larval migration. We identified rat lungworm infections in frogs, toads, and centipedes in Hawaii and highlighted the lack of knowledge of the role paratenic hosts may be playing in the transmission and life cycle maintenance of rat lungworm in Hawaii.

Invasive coqui frogs are associated with differences in mongoose and rat abundances and diets in Hawaii

With the increasing rate of species being introduced to areas outside of their native ranges, non-natives are likely to interact in ways that influence each other’s populations. The high densities of invasive coqui frogs (Eleutherodactylus coqui) in Hawaii have been hypothesized to increase non-native mongoose (Herpestes auropunctatus) and rat (Rattus spp.) abundances, and in turn increase bird nest depredation rates. We compared the relative abundances of rats and mongooses and artificial bird nest predation rates at 12 sites that had plots with similar habitat invaded and not invaded by coqui frogs across the island of Hawaii. We interpret our results considering mongoose and rat stomach analyses and camera trap data collected to monitor coqui scavengers. We found that coqui presence was associated with 30% greater mongoose abundance and 17% lower Pacific rat (R. exulans) abundance. Based on our diet analyses and scavenging data, both mongooses and rats consume coquis, but mongooses were the most important consumers of coquis, which may have contributed to their increase in coqui plots. We speculate that coquis are competing with rats for invertebrate prey due to reduced Pacific rat abundance and greater amounts of fruit in rat stomachs collected in coqui-invaded compared to uninvaded plots. We did not observe any difference in bird nest predation rates in coqui-invaded and uninvaded plots. Our results suggest that the coqui invasion may increase or decrease non-native mammal populations, and non-native amphibians may serve as both novel prey and competitors to non-native mammals.

Invasion of Hawaiian rainforests by an introduced amphibian predator and N2‐fixing tree increases soil N2O emissions

AbstractInvasions of introduced species have homogenized ecological communities worldwide, leading to losses of native species and the services they provide. Some of these invaders substantially alter nutrient cycling, which changes conditions for all other organisms, but less is known about the potential influence of these species on nitrogen (N) trace gas emissions that affect atmospheric processes. We used a natural experiment to explore whether the establishment of an introduced nitrogen (N) fixing tree (Falcataria moluccana) and recent invasion of an amphibian predator, the Caribbean tree frog (Eleutherodactylus coqui), into native Hawaiian rainforests have affected soil emissions of nitrous oxide (N2O) and nitric oxide (NO), two atmospherically important trace gases produced by soil microorganisms. Soil N2O and NO emissions and rates of soil N cycling were significantly higher in F. moluccana‐dominated stands compared to native Metrosideros polymorpha (Ohi'a) stands. Additionally, invasion of E. coqui frogs moderately increased soil N2O emissions, primarily in non‐native F. moluccana forests where soil N availability was already elevated. N2O emissions were positively and significantly related to net potential N mineralization, and total N2O+NO fluxes increased with soil nitrate (NO3−) concentration and rates of nitrification. Previous work in these Hawaiian rainforest sites has shown that F. moluccana substantially increases N availability by increasing ecosystem N supply compared to uninvaded stands, and E. coqui accelerates N availability and litter decomposition, although moderately, due to enhanced fluxes of nutrient‐rich waste products. Here, we show that acceleration of nutrient cycling by introduced species can also alter biosphere–atmosphere exchange of N‐oxides.

Body Size and Life History Traits in Native and Introduced Populations of Coqui Frogs

Introduced populations often exhibit rapid phenotypic changes following colonization of new environments. These changes, which often contribute to the successful establishment and spread of introduced species, may result from evolution or phenotypic plasticity. We studied variation in adult body size across elevational gradients in native (Puerto Rico) and introduced (Hawaii) populations of Coquis (Eleutherodactylus coqui). To explore the possible mechanisms underlying variation in adult body size, we conducted a laboratory common-environment experiment to study the effects of temperature (19°C and 25°C), elevation ( 700 m), and area (two in Puerto Rico and one in Hawaii) on five size-related life history traits: clutch size, egg size, hatching size, size at 30 days, and growth rate. In the field in both Puerto Rico and Hawaii, body size was positively correlated with elevation, which is negatively correlated with temperature, but the magnitude of the slope was greater in Puerto Rico than in Ha...

Diverse examples from managing invasive vertebrate species on inhabited islands of the United States

ABSTRACTA wide array of sizes, ecosystems, cultures, and invasive wildlife are represented among inhabited islands. Here, six cases from the United States of America (US) are selected to illustrate the high diversity of invasive animal management issues and objectives. We outline the background, define the problems and management objectives. We identify the management approaches and discuss the results and influences as they specifically relate to inhabited islands. The examples are: (1) Gambian giant pouched rats on Grassy Key, Florida; (2) coqui frogs on Kaua’i, Hawai’i; (3) feral swine on Cayo Costa Island, Florida; (4) rodents and monitor lizards on Cocos Island, Guam; (5) black spiny-tailed iguanas (ctenosaurs) on Gasparilla Island, Florida; and (6) mongooses on Puerto Rico. The outcomes of the programs are discussed, particularly in relation to the impact of human habitation on success.

Estimating density of calling male Eleutherodactylus coqui in Hawaii from audio recordings of the nighttime frog chorus

Analysis of sound pressure data obtained from audio recordings of night-time Eleutherodactylus coqui chorusing was used to estimate population density of calling male coqui frogs in the major Hawaiian island populations. Study plots 400 m2 in size (20 by 20 m; n = 32) were established on the islands of Hawaii and Maui to provide venues for field counting calling male frogs to investigate the ground truth relationship with the recorded sound pressure data. Male coqui frogs emit a two-note ‘co-qui’ advertisement call at regular intervals. Exploiting the temperature-related and relatively consistent male coqui call repetition rate and the average emitted ‘co’ call sound pressure level (70.4 dB at 1 m), cue counts obtained from a correlation process available in the program Raven were also used to estimate the density of calling male coqui. The study methodology included comparing linear regression results between the field-counted frogs and the number of frogs obtained from the cue count method. Both methods revealed a linear relationship between calling male population density and associated average sound pressure of the resulting chorus.

Invasive coqui frogs are associated with greater abundances of nonnative birds in Hawaii, USA

ABSTRACT The global spread of invasive species has created significant challenges for avian conservation. Introduced predators and pathogens have long been recognized for their direct negative effects on birds, but introduced amphibians can reach high densities on islands with no native amphibians, where they interact with native species. The coqui frog (Eleutherodactylus coqui), introduced to the Hawaiian Islands in the late 1980s, could have significant impacts on birds because it is fully terrestrial and achieves high densities. Coquis have been hypothesized to compete with native birds for invertebrate prey, but could also serve as a novel food resource for birds that consume small vertebrates. To test whether coquis measurably affect bird abundance, we conducted point counts of birds in coqui-invaded and adjacent uninvaded plots across 15 sites on the island of Hawaii, USA. We used N-mixture models to estimate the effect of coqui presence and density on the abundances of both native and nonnative bir...

Skin bacterial microbiome of a generalist Puerto Rican frog varies along elevation and land use gradients.

Host-associated microbial communities are ubiquitous among animals, and serve important functions. For example, the bacterial skin microbiome of amphibians can play a role in preventing or reducing infection by the amphibian chytrid fungus, Batrachochytrium dendrobatidis. Evidence suggests that environmental bacteria likely serve as a source pool for at least some of the members of the amphibian skin bacterial community, underscoring the potential for local environmental changes to disrupt microbial community source pools that could be critical to the health of host organisms. However, few studies have assessed variation in the amphibian skin microbiome along clear environmental gradients, and so we know relatively little about how local environmental conditions influence microbiome diversity. We sampled the skin bacterial communities of Coqui frogs, Eleutherodactylus coqui (N = 77), along an elevational gradient in eastern Puerto Rico (0–875 m), with transects in two land use types: intact forest (N = 4 sites) and disturbed (N = 3 sites) forest. We found that alpha diversity (as assessed by Shannon, Simpson, and Phylogenetic Diversity indices) varied across sites, but this variation was not correlated with elevation or land use. Beta diversity (community structure), on the other hand, varied with site, elevation and land use, primarily due to changes in the relative abundance of certain bacterial OTUs (∼species) within these communities. Importantly, although microbiome diversity varied, E. coqui maintained a common core microbiota across all sites. Thus, our findings suggest that environmental conditions can influence the composition of the skin microbiome of terrestrial amphibians, but that some aspects of the microbiome remain consistent despite environmental variation.

The threat of invasive species to bats: a review

Abstract Biological invasions are a major driver of biodiversity loss, but no study has described the scope of threats to bats (Chiroptera) by invasive species. We reviewed the literature for negative effects of invasive species to bats and summarised threats according to four categories: predation, disease, competition, and indirect interactions. We identified threats of 37 invasive species to 40 bat species. Ten bat species were threatened by more than one invasion pathway. About 38 percent of cases are speculative and 18 percent circumstantial, many attributed to overlapping ranges, and most accounts do not quantify effects needed to forecast bat population impacts. Evidence of cat predation is frequently cited, constituting the greatest incidence of observational data. Other direct and indirect impacts were documented from goats, dogs, brown tree snake, rainbow lorikeet, rose‐ringed parakeet, yellow crazy ant, giant centipede, palm, burdock, avian cholera, and white‐nose syndrome. Circumstantial evidence suggests impacts by rats, stoats, coqui frog, common wolf snake, little fire ant, kudzu, and Lantana camara. Other impacts by giant centipede, yellow crazy ant, cats, goats, rats, and avian cholera are speculative, as are those from pigs, deer, white eye, common starling, house sparrow, rock dove, barn and little owls, brush‐tailed possum, honeybee, wasp, phytophagous insects, tamarisk, Cinnamomum verum, and Tabebuia pallida. Over 60 percent of bat species reviewed are island‐dwelling, corresponding with evidence indicating that most extinctions occur on islands and invasive species’ impacts are worse for island than mainland populations. Although appreciable bat population reductions owing to invasive species are often unproven, invasions are likely to exacerbate effects of other vulnerabilities. Multiple invaders and synergistic interactions may ultimately lead to species losses. Managers should exercise the precautionary principle by taking action against non‐native species when first detected, even if new species do not appear to be detrimental.

Environmental fluctuations and host skin bacteria shift survival advantage between frogs and their fungal pathogen.

Fluctuating environments can modulate host-pathogen interactions by providing a temporary advantage to one of the interacting organisms. However, we know very little about how environmental conditions facilitate beneficial interactions between hosts and their microbial communities, resulting in individual persistence with a particular pathogen. Here, we experimentally infected Eleutherodactylus coqui frogs with the fungal pathogen Batrachochytrium dendrobatidis (Bd) under environmental conditions known to confer the survival advantage to the host during the warm-wet season, or alternatively to the pathogen during the cool-dry season. We used 16S rRNA amplicon sequencing to quantify changes in bacterial richness and phylogenetic diversity, and identified operational taxonomic units (OTUs) that became overrepresented or suppressed as a consequence of Bd infection. During the warm-wet season, frogs limited Bd infections, recruited putatively beneficial bacteria and returned to pre-infection levels of richness and phylogenetic diversity. In contrast, during the cool-dry season, Bd infections kept increasing through time, and bacterial diversity remained constant. Our findings confirm that infection outcome not only depends on abiotic factors, but also on biotic interactions between hosts and their associated bacterial communities.

Temporal Foraging Patterns of Nonnative Coqui Frogs (Eleutherodactylus coqui) in Hawaii

Abstract The Puerto Rican Coqui Frog (Eleutherodactylus coqui) is a nocturnal, invasive species that was introduced into Hawaii in the 1980s. Because they reach extremely high densities (up to 90,000 frogs/ha), they have the potential to affect invertebrate prey communities. Previously, researchers used frogs collected only at night to characterize their prey. Because Coquis use retreat sites near the forest floor during the day and understory perch sites at night, frogs collected at night might show different amounts and types of prey than would frogs collected in the morning. We analyzed stomach contents of 435 frogs collected in the morning (0300–0600 h) and at night (1900–2200 h) from five sites on the island of Hawaii. Frogs collected in the morning had 1.7 times more prey items and 2.1 times greater prey volume than those collected at night; however, prey composition did not differ between morning- and evening-collected frogs. Across sites, Formicidae (ants) and Amphipoda (amphipods) were the domina...

Different prey resources suggest little competition between non-native frogs and insectivorous birds despite isotopic niche overlap

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 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.