Since there seems to be a number of different questions about Wilson's
numbers, Budiansky's credibility, my motivations, etc. etc. etc., I'm going
to take the time to type in the entire section from Budiansky that
discusses Wilson (since it doesn't appear that anyone is going to take the
time to read the book and judge for themselves). Warning, this post is
pure quote: hit the delete key at any time.
**********
from Budiansky, Stephen. 1995. Nature's Keepers: The New Science of Nature
Management. New York: Free Press.
"Indeed, the most well-publicized effort of late to apply basic
ecological theory to conservation and ecosystem management is, upon closer
inspection, little more than a bomb thrower's polemic masquerading as
science. The effort in question is principally that of one man, Harvard
biologist Edward O. Wilson, a relentless popularizer of the theory that 'we
are in the midst of one of the great extinction spasms of geological
history.' [7] Most people may be forgiven for being unaware that it is
only a theory; such are Wilson's credentials and so effective has been his
publicity campaign that the belief that fifty thousand or more species a
year are being driven to extinction has attained the stature of received
wisdom. Such statistics are cited repeatedly as verified fact not only in
environmental groups' fund-raising literature but in newspaper op-ed pieces
and in the utterances of government officials. Vice President Al Gore
regularly invokes the specter of mass extinction in his environmental
speeches, warning, for example, that 'more than half of all the creatures
God put on this planet may disappear within the lifetimes of people now
living.' [8]
"The fact that the actual, observed rate of extinction is not fifty
thousand species per year but one species a year thus comes as a surprise
to many. And the fact that Wilson uses different figures in different
contexts--low numbers in scientific journals, high numbers in his popular
books and magazine articles, and the highest numbers in newspaper
interviews and other carefully orchestrated media events--and, finally, the
fact that his conclusions are usually accompanied by calls for an immediate
halt to economic development are uncomfortably suggestive that something
closer to politics than to science may be at work here. [9]
"For those who get beyond the impressively alarming numbers, Wilson
does offer a theoretical justification for his predicted extinction rates.
The predictions are based on what is rather grandly termed the theory of
island biogeography. This 'theory,' however, consists of but a single
equation that attempts to relate the number of species found on an island
with the size of an island. The version of the equation that Wilson uses
(there is more than one) is S=CA superscript z, where S stands for the
number of species, A stands for the area, and both C and z are constants
chosen arbitrarily to try to make the formula come out right. So to
calculate the number of species found in an area A, one raises the area to
the zth power and multiplies the result by C. Ecologists who have studied
the distribution of species on ocean islands have found that they can
usually get a pretty good fit to the observed data by setting z in the
range of 0.2 to 0.4. What that translates to in practice is that an island
ten times as big typically has twice as many species.
"Claims by a few researchers that the factors C and z have some
literal biological significance have not been widely accepted; most agree
that they are, to put it simply, fudge factors. Unlike Newton's second
law, for example, which expresses a precise, immutable law of nature--force
equals mass times acceleration--this so-called species-area relation is
what scientists call an empirical formula. It is not derived from any
fundamental principles. It is just a mathematical attempt to fit a curve
to a vast body of scattered experimental data points.
"Wilson's global-extinction calculations, however, are a huge leap
from this already shaky ground. He assumes that the species-area formula
is not merely a rough generalization of scattered data, but a fundamental
law--a rigid connection between area and the number of species that can be
found therein. What is true for an island is true for the world. Cut the
area of, say, the world's tropical rain forests, and species will vanish
unwaveringly according to this mechanical relationship. This is how Wilson
derives his global extinction-rate figures. This is also how Wilson
derives so many different figures. In an article in the sober scientific
journal Science, using what he calls a 'conservative estimate' of the
number of species currently found in the rain forests and the lowest value
of z, he calculates 4,000 species a year are lost. In his popular book,
The Diversity of Life, again using 'the lowest z value permissible,' he
obtains the 'maximally optimistic conclusion' that 27,000 species are lost
each year. In interviews with journalists the number magically grows to
50,000 or even 100,000. [10] Actually, there is no magic about it--the
'theory' is so loose and sloppy that one can derive almost any answer one
wants, according to the assumptions one chooses to start with.
"This is but a caricature of mathematical ecology, one that does
little justice to the rigorous and sophisticated work that has been done in
recent years on population dynamics, ecosystem stability, extinction
probability, and predator-prey interactions. Although Wilson insists that
the species-area relationship has been verified by 'hundreds of independent
studies,' the literature is replete with criticisms of the entire concept.
One major review of, in fact, a hundred published data sets of species-area
relations found that only half of the variation in the number of species
encountered from one island to another could be accounted for by
differences in area according to the formula Wilson uses. The same review
concluded that the tendency of z values to fall in the range of 0.2 to 0.4
was probably nothing more than a mathematical coincidence. The entire
'theory,' the authors concluded, may be nothing more than a 'sampling
phenomenon,' totally lacking in predictive power. [11] The wider you
search, the more species you will find. This is about as profound an
insight as noting that one is likely to encounter more different colors of
automobiles the farther one travels from home. A recent paper by two
distinguished conservation biologists concluded: 'The species-area curve
(in a mainland situation) is nothing more than a self-evident fact: that as
one enlarges an area, it comes eventually to encompass the geographical
ranges of more species. The danger comes when this is extrapolated
backwards, and it is assumed that by reducing the size of the forest, it
will lose species to the same gradient.' [12] A formula that roughly holds
for isolated islands may have nothing whatever to say when it comes to
predicting what will happen on millions of square miles of mainland.
"An apparently unintended, but nonetheless telling, parody of the
entire concept appeared two decades earlier in McKay's Bees, a novel by
Thomas McMahon, a Harvard University engineering professor who studies
biomechanics:
" 'He made a game of elaborating the species of beetles to be found in a
limited space. In Edward and Jiffy's garden alone, he found thirty-seven
species. Within a range of a hundred feet of the hotel in any direction,
he found more than fifty. If he extended his circle to a half-mile radius,
his number of beetle species exceeded two hundred. . . . If fifty species
could be found in an acre, and two hundred could be found in a square mile,
Sewall estimated that half a million different species of beetle were
lurking in the bushes within a day's ride of the hotel.' [13]
"One reason the species-area curve tends to exaggerate so wildly is
that it ignores the textures of habitats. Species are not distributed
randomly but in patches. Reducing all of a forest's multiplicity of
habitats to a single mathematical variable representing 'area' is a
biological absurdity. For one thing, a little bit of conservation in the
right place may be much more important than the loss of thousands of square
miles of forest elsewhere. The species-area curve also ignores
conservation measures already taken, which have, for example, preserved
habitat crucial to 95 percent of the tropical birds in Africa. And it
ignores the possibility that at least some species can survive in secondary
forests that regrow on deforested land.
"(It also is contradicted by actual experience. Almost ninety
percent of the Atlantic coastal forests of Brazil have been cleared in the
last five hundred years; the species-area equation would predict a loss of
half of all species. Instead, a survey by Brazilian zoologists found that
not a single known species could be declared extinct. Several birds and
six butterflies believed twenty years ago to be extinct have been
rediscovered. The reason for their persistence, Edward O. Wilson
notwithstanding, appears to be simple and unsurprising: Many of the rarest
species are rare because they are restricted to tiny microhabitats that
occur in widely dispersed, isolated patches across the landscape. Even
with the loss of 90 percent of total forest area, representative samples of
all of these microhabitats survive in the remaining twenty thousand square
miles of parks, reserves, and remote areas. 'An insect fauna already
accustomed to appreciable isolation and disturbance in nature, on both long
and short time scales, has met with savoir faire the challenge of human
occupation . . . essentially all the original species seem to be
persisting,' concludes one recent biologist who has the advantage of
actually living and working in Brazil.) [14]
"But perhaps the greatest failing of island biogeography as a basis
for ecological guidance is that it is virtually insight-free. One square
mile is indistinguishable from any other square mile as far as the
species-area formula is concerned. The theory offers no practical or
realistic guidance for a conservation planner or ecosystem manager at all.
It is nothing more than a restatement of the most extreme preservationist
sloganeering in scientific-sounding language. The only lesson that one can
draw from the species-area relation is 'save everything.' And indeed that
is precisely how Wilson and other scientist-advocates have tried to use the
results--to elevate a quixotic crusade against economic development to the
status of a scientific imperative.
"Meanwhile, back in the real world, more practical conservation
organizations and governments would like to know which areas to focus their
conservation efforts on--because, in the real world, most of us know that
we cannot save everything. Park managers need to know how to effectively
maintain disturbance processes and how to compensate for the disruption of
migration routes, for the loss of key species, and for the invasion of
exotics so that the species in areas already 'saved' will actually have a
chance to survive.
" 'It is sad,' one prominent conservation biologist, Daniel
Simberloff, has concluded, 'that the unwarranted focus on island
biogeography theory has detracted from the main task of refuge planners,
determining what habitats are important and how to maintain them.' [15]"
(Budiansky pp. 164-169)
****************
Notes (p. 262):
[7] Wilson, Diversity of Life , 280.
[8] Address to the Opening Session of the United Nations Commission on
Sustainable Development, June 14, 1993.
[9] Heywood and Stuart, "Species Extinctions in Tropical Forests," 93-100,
summarizes the field evidence for mass extinctions. The authors note that
documented extinction rates of birds and mammals have increased from one
every four years to one per year; if other organisms show a like propensity
to extinction, the total extinction rate would be at most about two
thousand per year. "Despite extensive enquiries," they conclude, "we have
been unable to obtain conclusive evidence that massive extinctions have
taken place in recent times."
[10] Ehrlich and Wilson, "Biodiversity Studies," 759; Wilson, "Toward
Renewed Reverence for Life," 72; Wilson, Diversity of Life , 276, 280;
"Species Loss: Crisis or False Alarm?" New York Times, 20 August 1991;
Mann, "Extinction: Are Ecologists Crying Wolf?" 737.
[11] Edward O. Wilson, "Before Skies Become Entirely Barren of Birds; Mass
Extinctions Grow," Letters, New York Times , 25 May 1993; Connor and McCoy,
"Statistics and Biology of the Species-Area Relationship," 801-3. Connor
and McCoy observed that in a "completely unrelated discipline," the same
formula has been used to relate brain weight with body weight of various
organisms, and z values consistently fall within the 0.2 and 0.4 range,
too. Apparently, any data set characterized by a wide range in the
independent variable (area, in the case of the species-area curve) and a
small range in the dependent variable (species) that fits a power
relationship of the form S = CA superscript z fairly well has this
property. They conclude that the species-area connection may be a
"correlation . . . without a functional relationship."
[12] Heywood and Stuart, "Species Extinctions in Tropical Forests," 105.
See also Simberloff, "Do Species-Area Curves Predict Extinction?" 75-79,
and Budiansky, "Extinction or Miscalculation?"
[13] Quoted in Simberloff, "Do Species-Area Curves Predict Extinction?" 75.
When asked, McMahon said he was in fact unaware of island-biogeographical
theory when he invented this scene for his novel.
[14] Brown, "Conservation of Neotropical Environments," 380; see also Brown
and Brown, "Habitat Alterariton and Species Loss in Brazilian Forests,"
126-28.
[15] Simberloff quoted in Heywood and Stuart, "Species Extinctions in
Tropical Forests," 104. The politics surrounding extinction predictions
have become rather ugly within the field of conservation biology. Perhaps
because alarming statistics have proved so successful a public relations
tool, many ecologists find themselves reluctant to criticize them too
loudly. Heywood, in an interview with me for an article on the subject
that appeared in U.S. News & World Report ("The Doomsday Myths," 13
December 1993), stated that biologists "are much more skeptical in private"
about the very high extinction figures used by Wilson but "are very
cautious about expressing their views [publicly] because they don't want to
be seen rocking the boat. This is the fear it might damage 'the cause.' "
Sources (Bibliography pp. 273-296):
Brown, Keith S., Jr. "Conservation of Neotropical Environments: Insects as
Indicators." In The Conservation of Insects and Their Habitats, edited by
N. M. Collins and J. A. Thomas. R.E.S. Symposium XV. London: Academic
Press, 1991.
Brown, Keith S., Jr., and G. G. Brown. "Habitat Alteration and Species
Loss in Brazilian Forests." In Tropical Forests and Species Extinction,
edited by T. C. Whitmore and J. A. Sayer. London: Chapman & Hall, 1992.
Budiansky, Stephen. "Extinction or Miscalculation?" Nature 370 (1994): 105.
Connor, Edward F., and Earl D. McCoy. "The Statistics and Biology of the
Species-Area Relationship." American Naturalist 113 (1979): 791-833.
Ehrlich, Paul R., and Edward O. Wilson. "Biodiversity Studies: Science and
Policy." Science 253 (1991): 758-62.
Heywood, Vernon H., and S. N. Stuart. "Species Extinctions in Tropical
Forests." In Tropical Forests and Species Extinction, edited by T. C.
Whitmore and J. A. Sayer. London: Chapman & Hall, 1992.
Mann, Charles C. "Extinction: Are Ecologists Crying Wolf?" Science 253
(1991): 736-738.
Simberloff, Daniel. "Do Species-Area Curves Predict Extinction in
Fragmented Forest?" In Tropical Forests and Species Extinction, edited by
T. C. Whitmore and J. A. Sayer. London: Chapman & Hall, 1992.
Wilson, Edward O. The Diversity of Life. Cambridge: Harvard University
Press, 1991.
_______. "Toward Renewed Reverence for Life." Technology Review,
November-December 1992, pp. 72-73.
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