by Whit Gibbons

January 30, 2005

More and more plants and animals today are found in regions beyond their natural range. Betsie Rothermel, research ecologist at the Savannah River Ecology Laboratory, provides an intriguing perspective of an environmental dilemma resulting from animal introductions. The following is Betsie's account of her winter holiday in Maui, as seen through the eyes of an ecologist.

"Every time I visit Hawaii, I am treated to an outstanding nature experience. In past visits, I swam with wild dolphins and snorkeled with sea turtles. This time I watched humpback whales and interrupted some spawning toads. Allow me to explain. . . .

"Amphibians and nonmarine reptiles do not occur naturally in Hawaii. Whereas some insects, birds, plants, and even a mammal (a bat) drifted or were blown to the islands and diversified into new forms, amphibians and reptiles (herps) simply never crossed the vast expanse of ocean to colonize this isolated chain of islands. But because of accidental or deliberate introductions by people, more than two dozen nonnative species of herps now live in the Hawaiian Islands.

"The ‘Iao (pronounced ee-ow) Valley is a steep-sided, mist-shrouded ravine in the heart of the West Maui Mountains. The road into ‘Iao dead-ends in a park with trails to a scenic lookout and patches of taro (a vegetable introduced to Hawaii by Polynesian settlers more than a thousand years ago) growing in pools of water diverted from the stream. Busloads of tourists come here daily. While circling one of the pools, something strange caught my eye--long, gelatinous strings enclosing tiny black beads, criss-crossing the bottom of the pool and entwined in clumps of vegetation. Toad eggs! My eyes settled on a large shape in the water. Aha! A giant toad. In fact, not just one toad, but a huge female with her mate clasped securely to her back, sitting motionless on the bottom of the pool.

"An adult giant toad is an impressive sight. Picture your garden-variety southern toad on steroids. Giant toads can grow up to nine inches long and weigh up to three pounds. Giant toads (also known as cane toads) were introduced to Hawaii by sugarcane growers in the 1930s, in hopes they would control insect pests, an approach with questionable effectiveness. These two were the largest specimens I had ever seen, and my excitement soon attracted a small crowd. While taking photos, I explained how giant toads have managed to invade many areas outside their native range in tropical America. This one female would lay up to 30,000 eggs, which would soon hatch into tiny black tadpoles that would feed voraciously on algae, grow quickly, and metamorphose into toadlets smaller than a dime. I pointed out the paratoid glands on their head, which secrete a milky, toxic substance if the animal is attacked. Even the eggs and tadpoles are toxic to predators. Combine productivity and invincibility with a flexible diet, and you have the formula for a very successful invader.

"As the excitement over the toads was dying down, someone spotted a small, dark lizard on a downed branch. I concluded it was a metallic skink, a native to Australia. Then we saw another, better-camouflaged lizard sitting only inches away. I recognized this one instantly--a green anole, the species found in the southeastern United States. Thus within 20 steps, I found toads from South America, a skink from Australia, and a lizard from the U.S. mainland, none of which belonged on this volcano in the middle of the ocean.

"Ecologists, and especially herpetologists, in Hawaii must be plagued by mixed emotions--admiration for these amazing animals, tempered by apprehension about their potential effects on the fragile island ecosystems and endemic fauna. The question is, how harmful are these 'invaders'? Abundant amphibians might compete with birds or invertebrates for insect prey. Introduced reptiles undoubtedly prey on native wildlife. Combined with habitat loss and other factors, this could drive more species to extinction, something Hawaii does not need; the islands have already lost more than half of their native birds since colonization by humans. I left ‘Iao thinking about the potential downside of being a herpetologist in Hawaii, a job that might require eradicating one’s favorite animals."

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