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From locusts to crabs, species numbers may rise and fall at random, study suggests


From sudden plagues of locusts to mysterious declines in sought-after creatures like the Dungeness crab, the booms and busts of nature have puzzled researchers. A new study suggests that scientists may sometimes have difficulty finding an environmental cause simply because there is not one.

In the study published recently in the journal Science, researchers at the University of California at Davis found evidence that many animals, even when they are unperturbed by unusual weather or any other alterations in their environment, can undergo wildly unpredictable changes in their numbers.

Using a very simple computer model inspired by the life cycle of the Dungeness crab, the researchers found that instability and change are the rule for these animals rather than the exception, that their population numbers whirl along through time, never settling down, even after tens of thousands of generations. The results, they say, suggest that nature is more unpredictable and unstable -- and difficult to study -- than researchers had guessed.

"It's an important finding," said Dr. Robert May, the Royal Society Research Professor at Oxford University, who is considered a pioneer in introducing chaos theory to ecology. "It's an important insight clarifying the magnitude of the job ahead of us."

Dr. Simon Levin, the Moffett Professor of Biology at Princeton University and director of the Princeton Environmental Institute, called the results fascinating. Because the model was based on "one of the simplest equations you could write down, I wouldn't have expected such complex behavior."

Researchers found that knowing how these populations change over a few or even a few hundred generations gives no insight into the populations' past or future behavior.

This presents a thorny problem for field ecologists and natural resource managers. Aiming to understand and often to control population fluctuations in the wild, these scientists may be at a loss, confined to just a few summers to do their work.

Inspired by the biology of the Dungeness crab, the researchers modeled a world in which one might expect simplicity if ever it were to be found. Along a theoretical coast, adults produce young, which disperse from one of hundreds of subpopulations to others, where the young form new groups of adults, which produce young the next year, and so on.

When there are too many or too few adults in one subpopulation, that group produces fewer juveniles for the next generation. But the environment never changes.

Despite the model's simplicity, over time the total number of individuals along the coastline fluctuated wildly.

Most disturbing of all, total population numbers could remain steady for thousands of generations, then without warning suddenly boom or crash. They could even cycle nicely up and down, then revert to chaotic behavior -- and back and forth -- over as many as 20,000 generations.

Because no environmental changes are allowed in the model, the only causes for the increases and decreases are the internal dynamics of the population, like migration or competition for food or space.

"We were very surprised by what came out," said Dr. Alan Hastings, a professor and the chairman of the division of environmental studies at the University of California at Davis who wrote the paper with Kevin Higgins, a graduate student. And with environmental perturbations added, Dr. Hastings predicted, the chaotic behavior would last even longer.

Though inspired by the biology of the Dungeness crab, whose young can disperse widely along a coast, researchers said these dynamics could be expected of any quickly reproducing animal with a sedentary life phase and a dispersing life phase, including many marine creatures, insects and even some small mammals such as mice or voles.

"Of course from a pest insect point of view, it's a real problem," said Dr. William Murdoch, a population ecologist at the University of California at Santa Barbara. "If this is really what real populations are like, it presents a big difficulty in analyzing and predicting how they're going to behave. It makes things even harder than they were before."

Dr. Louis W. Botsford, a professor of wildlife and fisheries biology at the Davis campus, said that while it remains unclear how to translate the findings into management practice, the study is useful since it provides a new potential explanation for the mysterious booms and busts to which many marine creatures are subject.

For example, he said, in the late 1950s, the crab population in central California declined from a catch of 12 million pounds a year to less than 1 million pounds. On the other hand, in the last few years, lobster catches in Maine have been about 50 percent higher than usual for no obvious reason, reaching a high for the century in 1990.

Researchers said they have tended not to focus on populations' unpredictable and transient behavior but on their anticipated end-point behavior, that time when populations become more stable, reaching some equilibrium. But the new study suggests that for some animals, it can take so long to reach any kind of stability that there is no point in worrying about the end state.

The study is the latest in a growing body of work that is forcing ecologists to turn away from their comforting equilibrium models of the world. Nature, the data seem to insist, is in a state of constant flux and turmoil.

"The important message," said Dr. Peter Turchin, a research ecologist at the Southern Forest Experiment Station in Pineville, La., "is that the transient behavior becomes the end point, so to speak. So instead of focusing on long-term equilibrium behaviors, we should pay a lot of attention to how they get to that point. That may be more important."

Some biologists cautioned that the new study, while a useful tool for understanding how populations might behave under some conditions, does not mean that all populations are doomed to instability. How much of a role these dynamics play in the real world, they said, remains to be seen.

In any case, the news that nature is more unpredictable and unstable than researchers had thought is not necessarily all bad.

"The recognition that systems are not at or very close to equilibrium certainly complicates the world view of some people who are trying to manage systems," Dr. Levin said. "On the other hand, I think it's a hopeful fact for people managing for biodiversity maintenance."

He explained that it is just this kind of instability and transience that keeps species that might dominate under stable conditions from being able to do so, allowing many more species to persist.

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