Unwittingly, Earth becomes a lab

Every day we conduct unwitting experiments on our home in the universe. We clear land and dam waters, remove species and introduce new ones. Sometimes the effects are benign; sometimes they're dangerous. What's scary is that we don't have the extensive ecological research to be able to tell the difference.

Even small changes can trigger big effects, which no one foresees because the links aren't always apparent.

Sharks, for example, are gathering off Florida to head north for the spring. They scare beachgoers and, on extremely rare occasions, attack people. Nevertheless, these finned wanderers play a beneficial biological role. Overfishing them has set off a chain reaction that has put Caribbean coral reefs in danger.

Sharks keep down the population of lesser predators such as groupers, according to new research. Thin out the sharks and groupers thrive, eating parrot fish and other vegetarians that help keep the reef healthy. With fewer fish to eat the vegetation, algae take over the reef, weakening its ability to resist more direct human disturbance, according to Enric Sala with Scripps Institution of Oceanography in La Jolla, Calif.

Because such linkages are often complex, Dr. Sala, together with Jordi Bascompte and Carlos Melián at the Doñana Biological Station in Spain, built a comprehensive computer-based model of a Caribbean marine ecosystem. Their food web simulation covers 1,000 square kilometers (386 square miles) to a depth of 100 meters (109 yards) and includes 250 different species. Their work, reported last week in the Proceedings of the (United States) National Academy of Sciences, illustrates a general environmental problem. Thinning out or removing a native species - or introducing an alien species - in an ecosystem can be bad news. "Species are embedded in a complex network of relationships," Sala explains. It's hard to foresee what a disruption of any of those networks will cause.

A study of the impact of introducing foxes into the Aleutian archipelago published in Science last month underscores this point. Russian fur traders first brought in foxes in the 18th century so they could harvest their fur. More were introduced in the late 19th and early 20th century when the US government was encouraging fox farming. As foxes wiped out seabirds, lush grasslands turned into scrubby tundra. It was not obvious why this happened. US Fish and Wildlife Service biologist Vernon Byrd suspected a food-chain disruption. Field research carried out with colleagues, including Donald Croll from the University of California at Santa Cruz, showed that the nutrient-poor volcanic soils on the islands need fertilizer from bird droppings. Without the guano, the landscape degrades into tundra.

The effects of species disruption "spread throughout an ecosystem in unpredictable ways," says Dr. Croll.

In this case, there's hope. Bird populations and soil fertility are recovering as foxes are being eliminated. But you can't always count on recovery.

Fire was used as a landscaping tool when humans first entered Australia 50,000 years ago. This strategy probably turned a drought-tolerant ecosystem in the continent's interior into a desert that persists today, according to research by Gifford Miller at the University of Colorado in Boulder and his colleagues. "The systematic burning of the interior by the earliest colonizers differed enough from the natural fire cycle that key ecosystems may have been pushed past a threshold from which they could not recover," Dr. Miller explains.

That gives a chilling perspective to research by Bruce Wilkinson at the University of Michigan in Ann Arbor who finds that "humans are stripping soil from the surface of Earth far faster than nature can replace it."

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