Reading practice 12
As night falls on Hawaii's Big Island, a low, jarring sound begins. It is a faint murmur at first, but as the darkness deepens, the sound grows louder, rending the stillness of the evening. These deep cries, from male E. coqui frogs, are met with lower, guttural croaks from their prospective1 mates; during this time, the sound for which the coqui is named (ko-KEE) fills the air. This sound has become the theme song of a growing environmental problem: invasive species' threat to ecological biodiversity.
Native to Puerto Rico, the small tree frogs—measuring about five millimeters long—probably2 arrived in Hawaii as passengers aboard potted plants imported from the Caribbean. Once coquis explored their new environment, they found an abundance of food, including insects, tiny spiders, and mites. In addition, they faced little ecological competition, as there are no other amphibians native to the islands, nor are there the snakes, tarantulas, or other Caribbean hunters that usually serve to keep the coqui population in check.3
The way the coqui hatch also gives the coqui an advantage in Hawaii's ecosystem. Frogs usually hatch into tadpoles, which require a consistent and substantial amount of water to survive.4 By contrast, the coqui emerges from the egg as a tiny but fully formed frog, which allows it to thrive in saturated moss, the dampened plastic that importers wrap around plants, or even a drop of water on a plant leaf. Moreover, young coquis don't begin to emit their signature calls until they are about a year old; consequently, avian predators are unable to locate the tiny frogs by sound.
Perhaps the coqui's most noteworthy feature is its extremely loud calling song. To a listener one to two feet away, a single coqui can produce a mating call up to 100 decibels. The unusual volume of the frog's call is compounded by two other factors. First, coquis congregate closely on relatively small parcels of land; one recent survey found 400 adult frogs in one 20-by-20-meter plot. This degree of concentration amplifies the sound the frogs make. Second, coquis tend to overlap their calls, with a single coqui seeking to fill gaps in other frogs' songs with its own effort to attract a mate. As a result, coquis create a "wall of sound" that is even more pronounced because Hawaii boasts few other night-calling species.5 For these reasons, human residents of Hawaii tend to regard coquis as nuisances, polluting the air with their incessant noise6
Conservationists worry about other ramifications of the coqui's invasion of the Hawaiian ecosystem. One problem is that while the coqui receives the bulk of residents' attention because of its nocturnal serenades, another, quieter genus of the frog—the greenhouse frog—represents an equal7 threat to the biodiversity of the island. As voracious insectivores, coquis and greenhouse frogs are threatening the survival of arthropods8 (invertebrate animals with jointed legs, including insects, scorpions, crustaceans, and spiders), whose populations are already close to extirpation due to other foreign predators. Ornithologists fear that depleting the insect population could result in serious consequences for Hawaii's food web, especially considering that the birds native to the islands are also insectivores.
Symbiotic interactions between the coqui and other invasive species pose another ecological threat. The presence of coquis could permit the flourishing of other so-called "dissonant" species, such as non-native snakes that prey upon the frogs. Herpetologists have speculated that nematodes and other types of vertebrate parasites can be transported with coquis and can infect indigenous fauna. Furthermore, many ecologists believe the proliferation of these frogs will further homogenize the island's biota.
Debate persists about how best to reduce or even eradicate the population of coquis and their cousins in Hawaii. Hand-capturing9 the tiny frogs is probably the most environmentally sensitive way to remove them from their habitat, but their sheer number renders this approach inefficient. The maximum concentration of pesticides that would not damage fauna or flora has not been potent enough to kill the frogs. Seeking a more creative solution, scientists have had some success treating the frogs with caffeine citrate, a drug typically prescribed to treat breathing and metabolic abnormalities in humans. Caffeine citrate can penetrate the coqui's moist skin, and the drug's high acidity essentially poisons the animal and inactivates its nervous systems. From a biodiversity standpoint, this technique has the added benefit of posing almost no danger to plants, which lack a nervous system, or to insects, which have an impenetrable, hard exoskeleton.
Even if new techniques finally exterminate the coqui, experts are skeptical that the invader's current effects on the 1,000 acres of Hawaii's ecosystem can be reversed. This patch of land is not expansive in comparison to Hawaii's total 4.1 million acres, yet it is an indication of potential widespread disaster: since the habitat and its native residents have thus far been able to adjust to the presence of coquis, eliminating the frogs could yield unintended and far-reaching consequences to the biodiversity of the habitat beyond arthropods. For now, scientists are likely to continue the delicate balancing act of limiting the coqui's population growth while preventing further damage to Hawaii's ecosystem.10
