Hawaii has no native frogs. Every frog you hear or see on the islands is an introduced species, and the most famous by far is the coqui (Eleutherodactylus coqui), a tiny tree frog endemic to Puerto Rico that arrived in Hawaii accidentally in the late 1980s, stowed away in shipments of nursery plants.1Pacific Science. Biology and Impacts of Pacific Island Invasive Species. 5. Eleutherodactylus coqui, the Coqui Frog (Anura: Leptodactylidae) What began as a handful of hitchhikers has become one of the most contentious biological invasions in the United States, reshaping forest ecosystems, dividing communities, and testing the limits of wildlife management on islands that evolved without any amphibians at all.
How the Coqui Got to Hawaii
The coqui is a nocturnal, ground-dwelling frog about the size of a quarter. In its native Puerto Rico it is a beloved cultural symbol, but in Hawaii it is classified as an invasive pest. The frogs arrived hidden in potted tropical plants shipped from nurseries in Puerto Rico and Florida. By the time anyone noticed, breeding populations had established on the Big Island (Hawaii Island) and on Maui. Genetic studies point to two separate introductions, one on each island, both tracing back to source populations near San Juan, Puerto Rico.2PubMed. Strong founder effects and low genetic diversity in introduced populations of Coqui frogs Despite losing a large chunk of their genetic diversity during the journey, the frogs thrived. Researchers found that extensive human-assisted movement of plants continued to shuffle frog populations among sites across Hawaii, erasing the genetic isolation-by-distance pattern that exists in Puerto Rico and effectively turning nursery trucks into frog transit systems.
Population Explosion
Freed from many of their native predators and parasites, coqui populations in Hawaii grew astonishingly dense. In well-established forested areas of the Big Island, population estimates run about three times as high as what researchers report from forests in Puerto Rico.3Journal of Herpetology. Population Densities of the Coquí, Eleutherodactylus coqui (Anura: Leptodactylidae) in Newly Invaded Hawaii and in Native Puerto Rico Part of the explanation is parasite loss. When a species moves to a new range, it tends to leave behind the parasites and pathogens that keep its numbers in check back home. The Hawaiian coqui populations fit this pattern cleanly, carrying far fewer parasites than their Puerto Rican relatives while reaching densities far beyond anything recorded in the native range.4Biological Invasions. Parasite loss and introduced species: a comparison of the parasites of the Puerto Rican tree frog, (Eleutherodactylus coqui), in its native and introduced ranges
At densities of thousands per hectare, the sheer biomass of coqui in some Hawaiian forests is staggering. Each frog is small, but collectively they represent a massive new consumer in ecosystems that never had anything like them. That mass of frog bodies has consequences running in several directions at once, from invertebrate communities to nutrient cycling to the food web above and below the frogs.
What They Eat and What That Means for Native Insects
Coqui are opportunistic, generalist predators that feed mainly on leaf-litter invertebrates. A detailed diet study across eleven sites on the Big Island and Maui found that non-native ants and amphipods together made up over half of all prey items consumed, and the frogs appeared to prefer these non-native invertebrates over what was proportionally available in the environment.5Copeia. Diet of the Invasive Frog, Eleutherodactylus Coqui, in Hawaii That sounds almost like good news, an invasive eating other invasives, but the picture is more complicated. The same study found that several groups containing endemic Hawaiian species also appeared in coqui stomachs, including mites, beetles, springtails, and flies. No mosquitoes turned up, which deflates the occasional claim that coqui help control mosquito-borne diseases. Termites barely registered either, contradicting another common hope that the frogs might serve as pest control.
A separate field experiment quantified the community-level damage more directly. Across multiple sites, coqui presence was associated with a roughly 27% reduction in total leaf-litter invertebrate numbers, driven mainly by a 36% decline in mites.6Biological Invasions. Coqui frog invasions change invertebrate communities in Hawaii Interestingly, the frogs actually increased the abundance of flies in the leaf litter, likely because frog carcasses and droppings created new food sources for fly larvae. The net effect is a reshuffling of invertebrate communities rather than a blanket wipeout, but for endemic Hawaiian invertebrates that evolved without any amphibian predators, even a modest new source of predation is a real concern. Management recommendations from these studies tend to prioritize protecting specific sites known to harbor rare endemic invertebrates rather than attempting island-wide eradication.
Nutrient Cycling and Forest Growth
One of the less obvious effects of having thousands of frogs per hectare is what all those frogs do to the soil. Coqui eat invertebrates and excrete nutrient-rich waste, effectively converting insect biomass into a form that plants and soil microbes can use more readily. Field research on the Big Island found that forests with established coqui populations had higher concentrations of nitrogen and phosphorus in rainfall filtered through the canopy, faster decomposition of leaf litter from native ohia trees, and increased new leaf production in strawberry guava, an invasive plant.7Biological Invasions. An invasive frog, Eleutherodactylus coqui, increases new leaf production and leaf litter decomposition rates through nutrient cycling in Hawaii The mechanism appears to be nutrient enrichment rather than changes to the invertebrate community. In other words, the frogs are fertilizing the forest floor. That the main plant beneficiary measured in this study was strawberry guava, itself one of Hawaii’s worst invasive plants, adds an unwelcome twist: the invasive frog may be indirectly boosting the growth of an invasive tree.
Do Coqui Compete with Native Birds?
When the coqui invasion was first recognized, one concern was that a massive new insectivore in the forest might compete with Hawaii’s already-struggling native birds for food. The evidence gathered since then suggests this fear was largely unfounded. A study comparing coqui density and bird abundance at multiple sites found that no native bird species showed lower numbers in areas with more coqui. In fact, the only statistically detectable association was a positive one: non-native bird abundance increased with coqui density, rising from an estimated average of about 57 per plot in frog-free areas to roughly 90 at the highest coqui densities observed.8The Condor: Ornithological Applications. Invasive coqui frogs are associated with greater abundances of nonnative birds in Hawaii, USA Native birds showed no response in either direction.
The reason competition has not materialized probably comes down to foraging zones. Coqui mostly hunt on the ground and in the leaf litter, while most surviving native Hawaiian birds forage in the canopy and understory. A dietary analysis confirmed this, finding that over 90% of the coqui’s diet consisted of mites, amphipods, and cockroaches, while native honeycreepers and white-eyes relied more heavily on spiders and caterpillars from higher vegetation layers.9Biological Invasions. Different prey resources suggest little competition between non-native frogs and insectivorous birds despite isotopic niche overlap The dietary overlap between bird species was actually greater than the overlap between any bird species and the coqui. The frogs and the birds occupy a similar position in the food chain in a broad sense, but they are eating different things in different places. At most, the competition is weak.
Feeding the Enemy
If coqui are not starving native birds, they may be doing something worse: feeding the animals that prey on native birds. Hawaii’s forests are already plagued by invasive mongooses and rats, both of which eat bird eggs and nestlings. Research on the Big Island found that coqui presence was associated with about 30% higher mongoose abundance and roughly 17% lower Pacific rat numbers.10Biological Invasions. Invasive coqui frogs are associated with differences in mongoose and rat abundances and diets in Hawaii Diet analyses confirmed that mongooses were the primary mammalian consumers of coqui, which may explain the mongoose population boost. The rat decline is harder to interpret; frogs could be competing with rats for invertebrate prey, or the higher mongoose numbers could be suppressing rat populations through predation. Either way, the possibility that a frog invasion increases the abundance of one of Hawaii’s most destructive nest predators is troubling, even if the pathway is indirect.
Climbing Higher
For years, cold temperatures at higher elevations were seen as a natural barrier limiting the coqui’s spread. That barrier appears to be eroding. A resurvey of road corridors on the Big Island found that coqui now occupy about 50% of the surveyed roadside area, up from 31% fourteen years earlier, and they have moved into substantially more high-elevation sites, from 1% to 16% of surveyed locations.11PubMed. Invading nonnative frogs use different microhabitats and change physiology along an elevation gradient Frogs at higher elevations behave differently than their lowland counterparts, perching closer to the forest floor where air temperatures tend to be warmer, and they show physiological shifts: higher blood glucose, likely a cold-acclimation response, and elevated triglycerides that may help fuel energy demands at cooler temperatures.
Lab experiments back up the field observations. Extended acclimation trials showed that coqui can adjust their cold tolerance over a period of about three weeks, suggesting their thermal limits are more flexible than previously assumed.12PubMed. Invasive frogs show persistent physiological differences to elevation and acclimate to colder temperatures This is particularly relevant for conservation because many of Hawaii’s remaining intact native forests sit at higher elevations where they were thought to be beyond the frog’s reach. If coqui continue to push upslope, they could begin altering invertebrate communities and nutrient cycles in some of the most ecologically sensitive habitats left in the islands.
The Noise Problem
Ecology aside, many residents know the coqui primarily as a noise problem. Males call loudly at night with a two-note “co-QUI” that can exceed 80 decibels at close range, roughly the volume of a lawnmower. In areas with dense populations, the collective chorus is relentless from dusk until dawn. This has real consequences for property values and quality of life, particularly on the Big Island, where the frogs are most established. A survey of over 700 property owners found that willingness to participate in coqui control activities depended heavily on whether people already had frogs on their land. On the Big Island, where most respondents had direct experience with coqui, attitudes toward specific control practices were the strongest predictor of whether someone intended to help manage frogs. On other islands, where frogs were less common, social pressure from neighbors and community norms mattered more.13PubMed. Predictors of Participation in Invasive Species Control Activities Depend on Prior Experience with the Species
Habituation is a real factor in public engagement. Some long-term residents in heavily invaded areas report that they have simply gotten used to the noise and no longer see the frogs as a problem worth fighting. This creates a management challenge: the people living closest to the densest populations, and therefore best positioned to take action, are sometimes the least motivated to do so. Meanwhile, residents on islands where coqui are still uncommon may be more alarmed but have less opportunity to act.
Control Methods and Their Limits
Hawaii’s lack of native amphibians initially seemed like it could be an advantage for biological control. Because no native frogs would be harmed, the chytrid fungus Batrachochytrium dendrobatidis, which has devastated amphibian populations worldwide, was floated as a possible tool. The fungus had been documented infecting coqui in Puerto Rico since at least 1978 and was thought to contribute to population declines at higher elevations there.14Biological Conservation. Infection of an invasive frog Eleutherodactylus coqui by the chytrid fungus Batrachochytrium dendrobatidis in Hawaii However, deliberately introducing a pathogen carries its own risks. Chytrid is already present in Hawaii, and the coqui populations there appear to tolerate it without mass die-offs, which makes its utility as a biocontrol agent questionable at best.
The primary chemical tool that has seen widespread use is citric acid, sprayed on vegetation at concentrations high enough to kill frogs on contact. It works, but it is labor-intensive, needs repeated application, and is only practical for small, defined areas like nurseries, parks, and individual properties. Hand-capture and habitat modification, including removing leaf litter and trimming low vegetation to make the environment less hospitable, are used in combination. None of these approaches has come close to eradicating coqui from any large area. On the Big Island, the consensus among researchers and managers is that island-wide eradication is no longer feasible. The realistic goal has shifted to containment and local suppression in high-priority areas, particularly near ports and nurseries that could ship frogs to other islands.
The Other Invasive Frog You Have Not Heard Of
The coqui gets almost all the public attention, but it is not the only Caribbean frog that has colonized Hawaii. The greenhouse frog (Eleutherodactylus planirostris), a Cuban species, has spread across the Big Island just as widely. Surveys comparing the two species found that their site occupancy estimates were nearly identical, about 0.35 for the greenhouse frog and 0.31 for the coqui, suggesting the greenhouse frog is at least as widespread.15Biological Invasions. Detection probabilities of two introduced frogs in Hawaii: implications for assessing non-native species distributions The greenhouse frog is smaller and has a much quieter call, which means it flies under the radar of both residents and management agencies. Yet its ecological impacts, as a leaf-litter insectivore in forests without native amphibians, could be comparable to the coqui’s. Because resources for invasive species management are finite, the greenhouse frog represents a blind spot: a species causing similar ecological disruption with almost none of the monitoring or control effort.
Cane Toads in the Mix
Hawaii’s invasive amphibian situation extends beyond Caribbean frogs. Cane toads (Rhinella marina) were deliberately introduced in the 1930s in a misguided attempt to control sugarcane beetles and have been a fixture of lowland habitats ever since. Unlike the coqui, which is a forest-interior species, cane toads occupy a range of environments from landscaped golf courses to forested sites. A recent analysis of stomach contents from 93 cane toads found over 1,800 prey items spanning 42 different categories, with toads in forested areas eating a wider variety of prey than those in landscaped environments.16Food Webs. Trophic ecology of the invasive cane toad across contrasting environments in Hawai’i The dietary overlap between cane toad populations in different habitats was only about 49%, meaning they shift their diet substantially depending on what is available. Cane toads are toxic at all life stages, which poses an additional threat: any native or non-native predator that tries to eat one risks poisoning. Together with the coqui and the greenhouse frog, cane toads round out a trio of invasive amphibians occupying different niches across the Hawaiian landscape, each causing distinct problems in ecosystems that evolved amphibian-free.
Why Genetic Poverty Has Not Slowed Them Down
One of the persistent puzzles of the coqui invasion is how a founding population with dramatically reduced genetic diversity has managed to thrive. Genetic analyses found that allelic richness and gene diversity were significantly lower in Hawaiian populations than in Puerto Rico, and the frogs display fewer color pattern variations as well, two versus three to five per population back home.17PubMed. Strong founder effects and low genetic diversity in introduced populations of Coqui frogs In conservation biology, low genetic diversity is usually a warning sign for a population’s long-term viability. But for invasive species, genetic poverty often does not matter as much as you might expect. The coqui’s ecological advantages in Hawaii, no specialist predators, fewer parasites, abundant food, a mild climate, appear to far outweigh whatever costs come with reduced genetic variation. The frogs’ ability to acclimate physiologically to colder temperatures and exploit new microhabitats suggests that behavioral and physiological flexibility can compensate for a narrow gene pool, at least under current conditions.