Copyright 2001 Washington Post
December 3, 2001
By Guy Gugliotta
Washington Post Staff Writer

The world's smallest reptile can sit on a dime and lives on a tiny, scrub-covered Caribbean island that larger predatory species -- including humans -- either couldn't find or didn't want.

This little gecko lizard, Sphaerodactylus ariasae, suggests that good things come in small packages. The Caribbean islands, together about the size of Oregon, are also home to the Lesser Antillean threadsnake and the bee hummingbird -- the world's smallest snake and bird, and the Cuban coqui, the smallest frog in North America.

Islands have been renowned for unique ecosystems since Charles Darwin visited the Galapagos, but they also provide lessons in the intricate relationship between habitat and the ability of animals, both large and small, to survive amid humankind's intrusions.

"Islands get colonized by species that float over on logs or get carried in a bird's feathers," said biologist Blair Hedges of Pennsylvania State University. "Some groups never make it, but the groups that do, have more room to spread out. They don't have competition."

Hedges and University of Puerto Rico biologist Richard Thomas found the tiny gecko in "leaf litter" surrounding a limestone sinkhole on Isla Beata, a 16.3-square-mile islet off the Dominican Republic. Their findings, which are being announced today, are reported in the December issue of the Caribbean Journal of Science.

The gecko is dark brown, measures six-tenths of an inch and is easily recognizable as a lizard. "You have to get down on your hands and knees and root through the forest floor to look for a sudden movement," Hedges said. "They're very dark and very fast."

Hedges said he and Thomas found eight lizards on Isla Beata and in nearby Jaragua National Park, on the Dominican mainland, but traces soon vanished as the searchers moved inland.

"It's a national park, but there are people living there," Hedges said. "They have to eat, so they cut trees to plant cassava and beans, and to make charcoal. One family can wipe out acres and acres."

Because of dangers like these, ecologists regard the Caribbean as one of 25 threatened biodiversity "hot spots," areas making up only 1.4 percent of the Earth's surface but where 44 percent of all plant species and 35 percent of all vertebrate land species can be found.

Besides the Caribbean, hot spots include the islands of Sri Lanka, Polynesia-Micronesia, Madagascar, New Zealand, Indonesia and the Philippines. They also include mainland sites such as Panama's Darien Gap, the Mediterranean coast (including all of Italy and Greece) and the Burmese peninsula.

Threats can be deliberate -- from poachers or slash-and-burn farmers -- but can also come from almost incidental human encroachments on fragile habitat. "The areas that survive are the areas farthest away from roads," Hedges said.

Yet species endure in the Caribbean, even though only about 10 percent of the region's virgin forest remains. Isla Beata has a long history as a pre-Columbian indigenous settlement, a colonial-era pirate hideout, a salt mine that closed in 1960 and a campsite for today's local fishermen.

Scientists are quick to point out that the isolation of island living does not ensure species prosperity. "It may be true that as we create an island world, we may create new species," said Columbia University biologist Stuart L. Pimm, an expert in species extinction and survival. "But we may have to wait a few million years for them to evolve."

Also, "islandizing" mainland habitat by accidentally framing chunks of wilderness with highways or hedgerows is not likely to preserve biodiversity. "Water is not a threat to a real island," Pimm said. "But as we create forest fragments, the areas around them become weedy, invasive and bring in bad species."

Tiny animals like Hedges's gecko may be able to survive in shrinking habitat, but large animals suffer. "On average, big species become extinct a lot faster than small species," Pimm said.

This phenomenon -- that bigger species need bigger land masses to survive -- is the theme of a new study describing the largest land vertebrates at 25 oceanic locations and five continents over the last 65,000 years.

The study, published last week in the Proceedings of the National Academy of Sciences, found that animal species grew bigger on larger land masses, that plant-eaters were approximately 10 times as big as carnivores in the same area, and that cold-blooded animals, with lower metabolism, were much bigger than warm-blooded creatures, who needed more to eat.

The study examined habitats ranging from the 29-acre Galapagos islet of Plaza Sur, where the six-pound Galapagos land iguana is king, to the 21 million-square-mile Eurasian land mass, home of the now-extinct, 12,000-pound woolly mammoth.

"Roughly it takes between a thousand [individuals] or a few thousand of anything to survive," said Jared Diamond, a UCLA Medical School physiologist and the study's co-author.

Since land requirements are greater for big animals, those are "the most threatened, because they evolved to occupy the full size of their habitat." Humans, increasingly, are filling their space.

Diamond said he and his co-authors chose 65,000 years as the study's cutoff point because that is when modern humans reached Australia. The largest species on this biggest of islands were marsupials, mammals who nurse offspring in an external pouch, and the biggest was the 2,500-pound diprotodon. By 40,000 years ago, however, it had been hunted to extinction. The biggest marsupial in Australia today is the 190-pound red kangaroo.

A similar fate befell the giants of the Americas after humans crossed the land bridge from Asia between 13,000 and 20,000 years ago. The Columbian mammoth, at 13,000 pounds, the sabretooth tiger, at 860 pounds, and the 950-pound American lion are all extinct.

By contrast, today's African lion and elephant, considerably smaller than their departed American cousins, are still the biggest animals that continent ever produced over the last 65,000 years. They survive, Diamond suggested, because they watched modern humans evolve over several million years.

"Animals had a chance to get more suspicious as humans got more sophisticated," Diamond said. "Unfortunately for the mammoths, mastodons and sloths in America, the first humans they encountered were professional hunters." Error: Unable to read footer file.