Edition: U.S. / Global

ESSAY; Lost Rivets and Threads, and Ecosystems Pulled Apart

Published: July 4, 2000


In this formulation, an ecosystem is likened to an airplane: each act of environmental destruction -- the extinction of a species of plant or animal, for instance -- is akin to removing a rivet from the plane. At first, the losses make little difference, because there are lots of rivets. But if enough are removed or a few are taken away at crucial spots, the plane will crash.

The underlying assumption is that there are thresholds, or limits, beyond which increasing pressure suddenly precipitates a catastrophic ecosystem collapse. And from an environmentalist's point of view, catastrophe is a spur to urgent action.

But while it has recently become clear to many experts that the stability of ecosystems depends at least partly on the varieties of species they contain, it remains unclear at what point a collapse may take place when species are removed one by one.

For that matter, it is not altogether clear what would constitute an ecosystem collapse. Does it occur when a single species of weed has driven out all other plants and their animal dependents, for instance? Or does it require the obliteration of all life?

In the face of these difficulties, another metaphor is coming to the fore: that of the biological world as a rich, diverse tapestry. In this image, each act of environmental destruction is like pulling a thread from the tapestry.

''At first, the results are almost imperceptible,'' Carlos Davidson, a conservation biologist at the University of California at Davis, wrote recently in the journal BioScience. ''The function and beauty of the tapestry is slightly diminished with the removal of each thread. If too many threads are pulled -- especially if they are pulled from the same area -- the tapestry will begin to look worn and may tear locally.''

In this way of looking at the situation, there is no clear threshold of catastrophe, but rather a ''continuum of degradation,'' from ''a world rich in biodiversity to a threadbare remnant with fewer species, fewer natural places, less beauty, and reduced ecosystem services.'' And while there may be multiple rips and tears in the tapestry, any catastrophic collapses that might take place (like the crash of fishery) are relatively rare and local.

Yet even this metaphor seems incomplete. If the human impact on the rest of nature is as pervasive and encompassing as many scientists say, wear and tear by humans is not just fraying the tapestry. Rather, Homo sapiens is reweaving it into entirely new patterns, with a new mixture of colors and a new texture. And while the new pattern may please many an ordinary eye, it is markedly simpler, duller and less functional than the original.

The catalog of human influence on the natural world is now well known. Some ecologists say that nearly half the land surface of the earth has been transformed by human activity. Development, agriculture and logging continue to destroy and degrade natural habitats. Wetlands are filled in. Grasslands become farm fields. Forests become suburbs. Suppression of naturally occurring fires throws many kinds of ecosystems out of kilter, changing their species mixture and their very complexion. Wild species vanish wholesale at the local scale and are being driven to extinction globally at rates estimated at 100 to 1,000 times faster than the long-term ''background'' rate.

Innumerable hardy species of plants and animals are transplanted willy-nilly around the world, crowding out more fragile local varieties of life. Excess fertilizer use and burning of fossil fuels have more than doubled the amount of nitrogen in the environment. This huge shot of nature's universal fertilizer stimulates choking blooms of algae and aquatic vegetation and also enables plants that thrive in a high-nitrogen environment to expand and drive other species out of local ecosystems.

As a result, the natural world is becoming more simple, with fewer species interacting with one another, and more homogenized, as the same species populate ecosystems around the world, making them look more and more alike. ''We're simplifying the world on a mass scale, an unprecedented scale,'' said Dr. G. David Tilman, an ecologist at the University of Minnesota. At the same time, said Dr. Kevin S. McCann, a biologist at McGill University in Montreal, the world is moving toward ''biouniformity'' as a result of the mix-and-match of species interchanges taking place around the globe. The colors of nature's great tapestry, it seems, are bleeding into one another.

Dr. Tilman, who has carried out a number of pioneering studies about the relationship between reduced biological diversity and the stability of ecosystems, finds the tapestry analogy interesting: ''When you go to low diversity you go to very coarse weaves.''

But the analogy, he says, does not go far enough in addressing the changes in ecosystem workings that accompany reductions in biological variety and the consequent simplification of the landscape. Those changes, he said, are ''more dramatic'' than the tapestry metaphor allows.

The tapestry metaphor also assumes that nature is in a static state, says Dr. McCann, when in fact it is in constant dynamic flux. Tearing the fabric sends ''waves of dynamic change through an ecosystem, and the waves get bigger as biological diversity declines.''

As recently as a decade ago, the level of biodiversity was thought to be unimportant to ecosystem functioning. But now, Dr. Tilman and Dr. McCann wrote separately in a recent issue of the journal Nature, accumulating scientific evidence has made it clear that species richness is directly related to the stability of an ecosystem. In fact, Dr. Tilman wrote, diversity must now be added to the list of factors that generally shape ecosystems and govern their functioning, along with climate, soil type, moisture, fire and storm.

Diversity assures that in times of stress, as during a drought, the whole plant community (the foundation, matrix and support of animal life) will not be wiped out, and can spring back to normal once the stress is removed. If there are only one or two species, however, the entire system might crash, with disastrous consequences for animal and microbial life. Diversity also prevents one species from running amok and taking over the landscape.

The evidence so far suggests that as species are eliminated from an ecosystem, ''there is a tendency for a collapse,'' Dr. McCann said. What is not known, he said, is ''how far we have to go until we see a collapse.'' A collapse would not necessarily mean the conversion of an ecosystem to a single-species plant community, or monoculture. In fact, Dr. McCann said, the collapsed state may ''still be productive from a human perspective.'' But it would be a pale reflection of its former self, and more vulnerable to an outright if temporary crash in a drought or other severe disturbance.

If that is so, the rivet analogy may have some validity. But Mr. Davidson cautions that research in this area is still in its infancy, and that ''it is unclear that biodiversity loss will lead to ecosystem collapse.'' Therefore, he argues, the tapestry metaphor offers a more useful view of reality.

Even if there are no thresholds of collapse, he wrote in BioScience, destruction of the natural world and loss of biodiversity still matter:

''Rather, in calling for a stop to the destruction, it is the losses themselves that count, not a putative cliff that humans will fall off of somewhere down the road'' (yet another ecological metaphor).

It may be, as he suggests, that nature's complexities cannot be captured by simple analogies. To the extent they can, the tapestry-versus-rivets contest seems unlikely to be resolved until the insights of hard science catch up with those of the poet's imagination. And then some other metaphor altogether, undreamed today, may emerge triumphant.

Photo (Parker Fulton)