New York Times (pg. F4)
July 4, 2000


By William K. Stevens

In trying to illuminate what humans are doing to the natural
environment, scientists and conservationists over the years
have come up with a number of descriptive images. One of the
best known is the metaphor of the rivets.

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

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

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.

Copyright 2000 The New York Times Company