NY Times, July 4, 2000
ESSAY
Lost Rivets and Threads, and Ecosystems Pulled Apart
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 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.
Louis Proyect
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