Joseph Alper
Science
1998 May 22; 280: 1195-1196.In Israel's Negev desert, some microbes have developed a unique survival strategy: They pave the desert. Several species of bacteria and cyanobacteria secrete long-chain sugars that bind soil and sand into a black crust, which protects their damp colonies from the searing heat. But the microbes' labor benefits other species as well, according to Moshe Shachak and Bertrand Boeken of Ben-Gurion University in Sede Boker, Israel. After a downpour, the asphaltlike patches reduce water absorption by about 30%, increasing runoff, which pools in pits dug by desert porcupines and beetles. Windblown seeds germinate in the moist pits, giving rise to lush oases that can harbor dozens of species. "We see enormous effects ... by a host of tiny organisms," says Shachak.
"Ecosystem engineers" like these microbes have sparked a new approach to assessing how species interact with one another. Shachak, together with ecologist Clive Jones of the Institute of Ecosystem Studies in Millbrook, New York, and John Lawton of Imperial College's Centre for Population Biology in Silwood Park, U.K., have proposed a new concept of how ecosystem engineers, by shaping habitats to their own needs, alter the availability of energy--food, water, or sunlight--and thus dictate the fates of other species.
The concept has generated quite a stir among environmental scientists since it appeared in the journal Ecology last October. "Nobody had stepped back before and asked if this was a general phenomenon, then tried to put down some guiding principles," says David Tilman, an ecologist at the University of Minnesota, St. Paul. "This is one of those rare papers that gets you thinking in a new way." He and others think that after fine-tuning, the concept of ecosystem engineers may be ready to join an elite set of theories, such as natural selection and predator-prey theory, that help explain how species arise and interact.
Missing from ecology's theoretical underpinnings has been a way to account for how species, by altering habitats, perturb other species--even though, as Jones explains, "we've known for a long time that there are things species do to their physical environment that have enormous knock-on effects throughout the ecosystem." Then a few years ago, he and Lawton heard about the Negev story. The scientists soon grasped that ecosystem engineering was far more pervasive than humans erecting skyscrapers or beavers building dams. "Once we started looking in the literature and talking to people about this," says Jones, he and his colleagues realized "how important ecosystem engineers are at affecting species diversity, distribution, and survival."
The concept's guiding principle is that engineers indirectly control the flow of energy within an ecosystem. These species, the ecologists say, can have just as great an influence on an ecosystem as keystone species, or top predators. The concept holds that ecosystem engineers alter habitats through two overarching mechanisms. Autogenic engineers transform ecosystems by their own growth and are integral to the altered environment. Corals, for example, build reefs for their own needs that also serve countless other species. Although some species feed on coral, most, including brittle stars, anemones, and sponges, use reefs only for shelter. Similarly, trees create habitat for myriad species that live in and among tree crotches, where large branches diverge from trunks. Without coral reefs or trees, says Jones, associated species would perish.
The second class of organisms, allogenic engineers, alter the environment and then move on, leaving structures behind. Beavers, for instance, turn stream ecosystems into pond ecosystems by building dams that block stream flow. The pooling water drowns grasses and shrubs but provides marsh for herons and other species; crustaceans colonize debris from beaver dams. The Negev bacteria are also allogenic engineers.
The ecologists list six factors--including population density of an engineering species and the types of resources it controls--to help assess an engineer's importance to an ecosystem. The researchers hope that this framework can be used to make predictions about how, for instance, engineers that invade an ecosystem might alter it.
Researchers are already putting the concept to the test. Entomologist Bob Marquis and grad student John Lill of the University of Missouri, St. Louis, are studying how Pseudotelphusa caterpillars tie oak leaves together to form shelters. They have found that dozens of species--including spiders, weevils, and aphids--dwell in the shelters. By forcing researchers to look for those species that indirectly alter energy availability, the engineer concept "could help organize a great deal of what we're seeing in our experimental systems," says Marquis. Indeed, he says, it has prompted him and Lill to revise their research plan. Instead of merely observing engineers in action, says Marquis, "we are going to manipulate the leaves ourselves to quantify the effects of the leaf ties on the resulting ecosystems."
Others hope to put the concept to predictive use. Lawton and mathematician William Gurney of the University of Strathclyde in Glasgow, U.K., are trying to devise robust computer models that forecast how an engineer's activities could affect other species. "Experiments are now getting started," says Lawton, "but it will probably be a decade before we can really say what shape the models, and ultimately the theory, will take." Such models could someday be useful for protecting or restoring habitats. "It's hard to think about conserving ecosystems without considering the effects that engineers have on a system," says Shachak.
Experts agree that the nascent concept needs sharpening to help researchers home in on the engineers that, like keystone species, are crucial to an ecosystem's overall health. "At some level you could say that every organism is engineering its ecosystem and that this activity affects other organisms," says Alex Flecker, an ecologist at Cornell University in Ithaca, New York, who studies the ecosystem effects of fish that bulldoze sediments to find food in Andean streams. But "the important thing," Flecker says, is that the new concept has "organized the different types of engineering behavior we see in the field into a useful, testable framework."