The work of nature

How many species can an ecosystem lose before its vital processes begin to falter? As global changes in land use, atmospheric gases, and climate threaten to accelerate extinction rates, ecologists around the world are asking a crucial question: Can we spot the species most vital to protect?

Thirty years ago, probing the pattern of life among the intertidal rocks along the coast of Washington State, ecologist Robert Paine found that one species of seastar (starfish) preyed so skillfully on mussels that it effectively kept these aggressive creatures from monopolizing space on the rocks. When Paine removed the seastars from sections of the shoreline, the mussels began to multiply, crowding out limpets, barnacles, and other marine organisms from the rock surfaces. The total number of species living on the rocks dropped by half.

Paine's work was among the earliest to suggest that a single species might hold the key to both the diversity and the stability of its community. Since then, other researchers have also shown that, ecologically, not all species are created equal. The most abundant organisms, the ones that dominate space and resources or define the very character of a community, usually also contribute most to controlling the lushness of plant growth, the fertility of the soil, and other processes. Often these species are highly visible: redwood trees define a redwood forest; kelp, a kelp bed. Sometimes, however, highly influential species are less conspicuous; they may even be rare. For these unexpectedly powerful creatures, from fig trees to seastars, Paine coined the label "keystone."

Like the keystone in an arch--the wedge-shaped stone at the pinnacle that stabilizes the span--these organisms hold a community together. Their power is disproportionate, and their removal creates ripple effects that can not only change the terms of life for all others in a community but also alter the nature and vitality of ecological processes.

Despite their importance, our knowledge of keystone species worldwide is still limited. Until recently, relatively few researchers attempted to identify specific microbes, plants, and animals that play vital roles in sustaining our "life support systems," as the pioneering ecologists--and brothers--Eugene P. and Howard T. Odum called the self-renewing, life-sustaining natural processes that make our planet uniquely habitable. Over the past three decades, a growing number of ecologists have studied these large-scale processes that generate and revitalize soils, refresh the air, cleanse the waters, moderate regional weather, pollinate crops, and keep most potential pests and disease agents in check. But few have addressed whether eliminating this creature or that might alter ecological processes and thus degrade living conditions on the earth.

As global changes in land use, atmospheric gases, and climate threaten to accelerate the loss of species, the question of who is doing what has taken on increasing importance. With a sense of urgency, ecologists around the world have been assessing what is known about the role of biological diversity in sustaining our ecological life supports. And they are asking some crucial questions: What else do we lose when we lose species? How great a loss can any given ecosystem suffer before vital processes begin to falter? Can we spot the species most vital to protect?

Ecologists have a long way to go before they can spot keystone species from general principles rather than through painstaking observations such as Robert Paine's. And their task is made more difficult by increasing evidence that a creature's ecological importance is not fixed. The seastar, for instance, is not a keystone in areas where sand routinely washes over the rocky shoreline, burying mussels and keeping their population in check.

Like physical setting, historical events may affect a creature's status. Take the case of two islands along the west coast of South Africa. Lobsters abound in the waters around Malgas Island, where they are keystone predators, holding down populations of mussels and most whelks. Nearby Marcus Island looked much like Malgas until the 1960s, when for unknown reasons, its lobsters disappeared. Amos Barkai, of the University of Cape Town, and Christopher McQuaid, of Rhodes University, wondered why lobsters hadn't recolonized. They got a rather unexpected answer when they took a thousand lobsters from Malgas and turned them loose at Marcus. The whelks immediately turned the tables on the returning, would-be keystones, attaching themselves by the hundreds to each lobster, weighing down and devouring the much larger creatures. Within a week, not a single live lobster could be found. Freed from the lobsters' dominance, the whelks had burgeoned to invincible numbers that prevented a shift back to the old order.

Although predators, such as lobsters and seastars, still dominate the keystone roster, the elite list now includes creatures that exert their power in any number of ways, from controlling the supply of key resources to altering the flow of water across the landscape. They include snails in Israel's Negev Desert, which grind rock into soil as they feed; fig trees in the rain forests of Amazonia, which supply critical food resources in times of scarcity; elephants, which prevent shrubs and trees from dominating the African savanna; and tsetse flies, which carry sleeping sickness and so keep humans, cattle, and many wild mammal populations low in infested areas.