The Work of Nature
©1997 Yvonne Baskin, Natural History, 106(1):48-52
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 specified 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 or 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 problem 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.
A number of keystones have been found among organisms known as ecosystem engineers, creatures that physically shape the landscape by burrowing, pecking holes, felling trees, and other activities. These include hippopotamuses, which stomp trails and gouge out mud wallows; porcupines, which girdle and kill trees; and pocket gophers, badgers, aardvarks, rabbits, crabs, and ants, which tunnel and churn the soil.
Certainly none of these creatures can rival humans as earth shapers. Geomorphologist Roger Hooke, of the University of Minnesota, calculates that humans move forty billion tons of rock and earth worldwide each year-churning it up directly during mining operations and road and home construction and eroding it indirectly by plowing croplands and clear-cutting forests-a feat that surpasses the work of glaciers, winds, or even sea-floor volcanoes.
Yet there is a critical difference between our work and that of other species or physical forces: We may sculpt the earth more dramatically, but we seldom make it more ecologically complex. In contrast, natural physical disturbances, such as floods and fires, are critical to maintaining patchiness and diversity in the landscape. And as ecologists have begun to recognize, many animals (and plants) routinely foster a more heterogeneous and dynamic landscape than the forces of climate or geology-or human activities-alone could create. A world without elephants, prairie dogs, and living coral reefs would not be just an emptier version of the same scenery.
Consider, for instance, the work of a key architect of the boreal forest, the moose. In a classic northern forest succession, hardwood trees such as aspens and willows spring up after fires and blowdowns and are then overtaken and eventually shaded out by conifers such as spruces and balsam firs. The voracious and selective appetites of moose accelerate this process. Moose feed heavily on tender hardwood twigs and shoots-about thirty-five pounds a day, or roughly six tons a year, per moose suppressing hardwood growth enough to speed the takeover of conifers.
Their foraging also sets in motion a significant slowdown in the cycling of nitrogen, reducing the availability of this key plant nutrient. Less growth of deciduous hardwood trees means fewer quick-rotting, nitrogen-rich leaves wind up on the forest floor, which is increasingly carpeted with slow-rotting conifer needles (neither the microbes in a moose's gut nor those on the forest floor find conifer needles, filled with lignin and resin, easy to digest).
The moose's unwitting efforts are countered somewhat by another powerful landscape engineer, the beaver. Although beavers, too, use aspens and willows for food and building materials, they prefer to cut large-diameter trees, which encourages stumps and roots to sprout and generates dense stands of suckers and stump-sprouted hardwoods around beaver ponds.
When John Pastor, of the University of Minnesota, and his colleagues tested the soils at various sites on Isle Royale, Michigan, they found that the amount of nitrogen available to plants was highest in areas where beaver cutting encouraged hardwood sprouting and lowest where moose browsed heavily. Hardwoods require more nitrogen for growth than do conifers, so the decline in soil fertility in areas where moose browse reinforces the advance of the spruces and firs.
But spruces cannot dominate the boreal forest for long. Mature spruce forests with their thick carpet of dry needles and trees loaded with flammable resins and dead branches are more combustible than hardwood stands; they also burn hotter. So the more spruces there are, the greater the probability of forest fire. Eventually, fire, high winds, attacks of spruce bud worm, or even a greatly diminished nitrogen supply will combine to destroy expanses of spruces, allowing hardwoods to recolonize in the nutrient-rich ash and thus restarting the successional cycle.
As the forest goes, so go the moose. Although moose take shelter under the spruce trees,: they cannot eat them, so the end result of the moose's labors is a drastic crash in its own population. Other boreal and tundra species, such as lemmings, voles, hares, and lynxes, are also legendary for their boom-and-bust population cycles. Pastor and his colleagues believe the oscillations are part of a unique dynamic that makes northern ecosystems qualitatively different from others. They contend that human attempts to damp these swings by stabilizing moose numbers or suppressing fires will interfere with the integrity and functioning of these ecosystems.
Some of the hardest workers to appreciate, or even identify, live in the soil, yet changes in the underground community can alter the landscape as dramatically as shifts in the work of moose or beavers.
A comparison of two of the many clear-cuts that scar Washington's Olympic Peninsula illustrates the power of the soil community. Douglas fir seedlings replanted in one of these clear-cuts already tower over most of the researchers who monitor their progress. In another clearing just across the road, only a few scraggly, three-foot-high trees dot an expanse of weeds and stumps. Yet the two plots were logged and replanted only a year apart, in 1987 and 1988. One plot represents a forest comeback; the other, a struggling failure.
One key difference is evident. The fungal mats that once covered nearly a third of the forest floor, often looking like campfire ash or shredded orange peel, have disappeared from the failed clearing. The mats are formed by truffle-bearing root symbionts, known as mycorrhizal fungi, on which virtually all conifers rely. Indeed, 90 percent of all plant species form mutually beneficial relationships with these fungi, which can either grow around plant roots like a sheath or else penetrate the cells of the roots. The fungi tap into the plant's sugar stocks for energy. In exchange, roots use the hyphal filaments of the fungi to extend their own reach, absorbing more phosphorus and, in some cases, other nutrients and water as well.
The absence of mycorrhizae is a telltale sign that the subsoil landscape has been as severely altered as the treeless clearing above. In laboratory culture dishes and under microscopes, the evidence clearly shows that loggers destroyed something more than trees on the failed plot. -Within the first year after the clear-cut, Elaine Ingham, of Oregon State University, and her fellow researchers found that 90 percent of the fungi in the sop had vanished; bacteria and nematodes also dropped dramatically; springtails and mites had grown scarce; and a large pulse of nitrogen, apparently released by the decay of all those dead organisms, had leached into the groundwater. Just why the soil community suffered so much more in one clearing than in the other is hard to know-perhaps heavy equipment compacted the soil too much, debris fires burned too hot, or fumigant or herbicide applications prior to replanting proved too harsh.)
Without these tiny organisms, this patch of soil has forgotten how to support a forest. Three-fourths of the fir seedlings planted died in the first year, more than 90 percent within five years. Today, the site hosts a bacteria-dominated soil community more typical of grasslands than of conifer forests. (Once bacteria begin to dominate, they slowly turn the soil alkaline, rendering it more suitable for grass or crops than trees.)
In a healthy ecosystem, plants are not just takers but also givers. They shelter the soil from the harsh actions of wind, sun, and rain and supply it with litter and detritus, replenishing its carbon and nutrient stocks. In addition, anywhere from 10 to more than 50 percent of the sugar that plants make in their leaves through photosynthesis eventually moves from their roots and from mycorrhizal hyphae out into the soil. There it feeds the microbes and other organisms that convert the plants' detritus back into reusable nutrients. Organisms dependent on direct subsidies from plants-particularly symbionts such as mycorrhizae and nitrogen-fixing bacteria-are seldom equipped to survive without them. Unless the plant community rebounds after a clear-cut, a fire, or other disturbance, the balance will shift, favoring microbes that live on rotting organic matter. And once the soil-plant partnership deteriorates, restoring the original plant community may not be possible.
Farmers in southwestern Australia have also learned the hard way what can happen when you disrupt the synergy between native plants and their environment. When would-be farmers settled this semiarid frontier at the turn of the century, they certainly must have expected water problems, but never the kind their grandchildren face today. The ancient expanse of roving sand plains and uplands was then covered by a mosaic of heath scrub and eucalyptus woodlands, one of the richest plant communities outside the Tropics. Today 90 percent of it has been converted to wheat fields and sheep pastures, and after less than a century of cultivation, more than half the arable land in the region suffers, ironically, from too much water: waterlogging, flooding, erosion, and salt "scalding."
Why did the water cycle change so dramatically as the eucalypts, banksias, hakeas, and saltbush were replaced by wheat fields? Part of the answer, ecologists have found, lies in what plants do with the rain that falls on their leaves. Eucalypts, it turns out, route 25 percent of the rain they capture to "stemflow," channeling it down branches, stems, and trunks and into the soil at the base of the bole. There, it gets drawn up by the roots and used for growth or transpired back to the atmosphere through stomata] pores in the leaves. Australian acacias divert 40 percent to stemflow. Communities of evergreen banksia shrubs and grasstrees, topped with yuccalike arrays of spiked leaves, can capture and redirect up to 70 percent of the rainfall that reaches them.
Another factor that makes Australia's native vegetation successful is a diversity of root systems, which exploit moisture from every level of the soil. Finally, the native plants are mostly perennial and often evergreen, using water year-round. Over eons, this community has evolved mechanisms for capturing, drawing in, and breathing out virtually all the moisture that falls throughout the seasons.
Contrast those talents with the meager abilities of wheat and pasture crops. Crops are annuals, using water only during the growing season. Even at the height of their growth, these uniformly shallow-rooted plants cannot make use of all the rain. Some of sit pools and runs off the soggy ground, carrying away topsoil; some filters deep into the plowed earth beyond the reach of crop roots, slowly raising the water table and floating up salt deposits.
Today, the diverse talents of plants are finally gaining recognition. Current strategies for restoring these Australian fields to health and sustainable productivity include replanting buffer belts of thirsty, salt-tolerant native trees and shrubs. And in South Africa, one of the earliest economic analysis ever to put a dollar value on the work of nature recently focused on detrimental shifts in the water cycle that might follow the loss of native plant communities.
Brian van Wilgen, of the CSIR Division of Forest Science and Technology; Richard Cowling, of the University of Cape Town; and Chris Burgers, of Cape Nature Conservation, tackled a complex question: Can the ongoing expense of keeping alien trees from taking over the treeless native fynbos (shrubland) communities in the mountainous watersheds of the Cape region be justified economically in a nation faced with an overwhelming backlog of human needs?
The basic problem is that proteas and other shrubs of the fynbos-with 8,574 species, perhaps the most biologically diverse of the earth's six recognized plant kingdoms-are being crowded out in many places by dense groves of Mediterranean pines, Australian acacias, and other exotic trees and shrubs imported long ago by European settlers. The tree invasion has a recognized economic cost: trees use more water than shrubs and so reduce water supplies in the region. But removing them from the slopes is expensive, requiring a continuing regimen of hand pulling and chopping, combined with well-timed burns. Does the amount of water saved balance the expense of keeping the fynbos system intact and functioning?
The ecologists found that it does: An 80 percent replacement of shrubs by trees would mean a 30 percent loss in water supply. Even with ongoing expenses of tree control, the unit cost of water from a protected fynbos watershed would be 14 percent lower.
Losses of native diversity can not only create costly changes in water or nutrient cycling, fire regimes, rainfall, or soil vitality but can also threaten human health. A number of emerging or resurgent diseases worldwide-encephalitis, malaria, hemorrhagic fever, Lyme disease, snailborne schistosomiasis-are linked to landuse changes that eliminate natural controls on disease agents and also bring them into increasing contact with people. Throughout the Tropics, for example, artificial water impoundments, such as dams, irrigation works, and rice paddies, provide extra breeding sites for mosquitoes, aquatic snails, and other disease vectors. More than 30,000 farmers and field workers contract Japanese encephalitis each season in the flooded rice fields of Asia. In the Argentine pampas, conversion of rangelands to maize fields has led to the spread of a formerly rare mouse that is both host and vector of Argentine hemorrhagic fever. More than 20,000 people have been infected since the virus was identified in 1958, and a third of these have died.
Realizations like these are not conclusions but starting points for a new field of study. Fortunately, a growing number of ecologists are committed to the complex, long-term studies needed to understand the vital work of plants, animals, and microbes in sustaining healthy ecological processes. Their task is enormous, and the stakes are high. As the Odums realized, these processes are the life-support systems for spaceship Earth, and we degrade them at our peril.
Adapted from The Work of Nature: How the Diversity of Life Sustains Us (Island Press, 1996).
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