Real Biodiversity Crisis
S. Levin and Donald A. Levin
numbers are grim: Some 2,000 species of Pacific Island birds (about
15 percent of the world total) have gone extinct since human colonization.
Roughly 20 of the 297 known mussel and clam species and 40 of about
950 fishes have perished in North America in the past century. On
average, one extinction happens somewhere on earth every 20 minutes.
Ecologists estimate that half of all living bird and mammal species
will be gone within 200 or 300 years. Although crude and occasionally
controversial, such statistics illustrate the extent of the current
upheaval, which spans the globe and affects a broad array of plants
Species extinctions are, of course, perfectly natural. In the grand
drama of geologic time, paleontologists have seen countless species
enter and exit the stage. All species begin in some restricted setting
and then spread. Most subsequently undergo differentiation, and eventually
all species come to an end. The diversity of species at any point
in time is simply the result of these ongoing processes, which can
wax and wane in intensity. For the most part, the total number of
species inhabiting the Earth probably remains fairly static.
The current losses are, however, exceptional. Rates of extinction
appear now to be 100 to 1,000 times greater than background levels,
qualifying the present as an era of “mass extinction.” The globe has
experienced similar waves of destruction just five times in the past.
Devastating as they were, after each of these mass extinctions, biological
diversity ultimately recovered. The time it took varied among taxonomic
groups and also depended on just where the organisms lived. General
recovery probably required a few million years in each case. Taking
an optimistic view and assuming people will be around for a few million
years yet, we wanted to explore what our descendants’ world might
ecologists believe that species diversity will not rebound significantly
as the natural environment becomes permanently impoverished. Vast
tracts of wilderness, for example, may vanish from the earth in the
not-so-distant future. If ecological diversity is lost in this way,
some conservation biologists argue, speciation rates will be lowered
permanently. And because species diversity represents a dynamic equilibrium
between extinction and speciation, a lowered rate of speciation would
undoubtedly create a lower equilibrium in species number. Thus some
foresee a persistent speciation crisis and a plummeting number of
species as the main threat to biodiversity in the long term.
Although this argument may hold for, say, the bigger mammals, which
presumably require large areas for speciation, it is unlikely to apply
to the many terrestrial organisms that are immobile or have small
home ranges—in particular, plants and many invertebrates. Indeed,
there is ample reason to believe that such life forms may not suffer
in the long run. After all, fragmentation of their currently broad,
continuous ranges may actually promote speciation rather than retard
it. How so? Breaking up habitats will create populations that are
isolated from one another, reducing the level of gene flow. Further,
population sizes will be reduced, quickening the pace of genetic drift.
The combination of these two effects provides an ideal template for
Species numbers may ultimately rebound for a second reason as well:
because the disturbances people produce need not always decrease ecological
diversity. In many cases, human actions may merely alter the character
of habitats rather than eliminate them. That process might, in fact,
The study of island biogeography offers considerable support for this
notion. Islands tend to have depauperate biota relative to continents,
so ecological opportunity and thus rates of speciation are higher
on islands than on continents. This has been seen for Hawaiian plants
in the silversword family, for finches, for honeycreepers, for sparrows
and for fruit flies. The link between rates of speciation and
ecological opportunity is also apparent when one compares younger
islands with older islands in the same archipelago. Older islands
have more species than younger ones and so may offer less ecological
opportunity. Consequently, speciation rates for older islands are
lower than for younger ones. This relation has been clearly demonstrated
for Hawaiian plants: Speciation rates on the youngest island in the
chain, the big island of Hawaii, are 10 times greater than on the
oldest island, Kauai.
Just as with the emergence of a new volcanic island from the sea,
episodes of mass extinction offer particularly rich opportunities
for the surviving groups. For example, the mass extinction at the
end of the Cretaceous (65 million years ago) brought an end to the
dinosaurs, but a massive diversification of mammals took place soon
afterward. Similarly, with the dramatic decline of marine eurypterids
(large arthropods) about 410 million years ago, the first large marine
predators were lost. However, they were later surpassed in this role
by certain fish, notably the placoderms, a highly diverse group that
sported interlocking plates of armor. The placoderms underwent a spectacular
radiation during the Devonian (between 410 and 360 million years ago),
but at the end of this period all placoderms—large, small, marine
and freshwater—went extinct. They were replaced by the actinopterygians
(ray-finned fishes), which ultimately produced the teleosts, the dominant
group of modern fish.
is no reason to assume that basic evolutionary processes in the future
will differ substantially from those in the past. So we would expect
the total number of species ultimately to recover from the current
mass extinction, even if people continue to meddle with the environment
on a global scale. The alteration and fragmentation of existing habitats
ensures that any future radiation of mammals, for instance, will not
include large forms such as rhinoceroses, apes and big cats. But other
species may do quite well.
Consider the differences in future prospects for primates and rodents.
Primates have both high rates of speciation and high rates of extinction.
Human activities will likely increase their rates of extinction but
may well reduce their opportunities for speciation. In contrast, people
may only marginally increase the rates of extinction for rodents,
while perhaps promoting their speciation somewhat. The rate of speciation
for rodents currently exceeds their rate of extinction by far. Thus,
the future may bring a decline in primate variety and an increase
in the kinds of rodents roaming about.
Such shifts could be even more dramatic at higher taxonomic levels.
Groups with short generation times, small home ranges and great dispersal
capabilities—many insects, for example—will clearly be at an evolutionary
advantage in a world full of human disturbance and unstable, patchy
habitats. (Interestingly, in the mass extinction at the end of the
Cretaceous, there is no evidence of any major change in insect faunas.)
So people’s activities today affect more than just the tally of species
that may go extinct soon: Our actions will also influence the diversity
of species that can evolve and persist for millions of years to come.
Conservation efforts, therefore, should aim to do more than stem the
near-term loss of species. Resources should be directed to ensuring
that there will be a rebound in the diversity of plant and animal
species, not just in their numbers.
How might one do that? One response to the current crisis is the preservation
of biodiversity “hotspots,” areas with exceptionally large numbers
of endemic species. But a narrow focus on saving the greatest number
of species possible risks losing important biological attributes.
To minimize this problem, higher-level taxa, such as families, should
be used in defining biodiversity hotspots and setting conservation
Consider, for example, tropical plants. Ghillean Prance at the Royal
Botanical Gardens, Kew, has noted that Malesia—the tropical region
running from peninsular Malaysia to Papua New Guinea and the Solomon
Islands—contains fewer plant species but more plant families than
the entire neotropics. Because the number of families probably provides
a better measure of the diversity of characteristics and evolutionary
potential than does the number of species, preserving the flora of
Malesia should be of considerable concern to conservationists interested
in maintaining a high degree of biodiversity over the long haul.
After the current spasm of extinction ends, the number of species
may well return to past levels, but there will surely be fewer higher-level
taxonomic groups than today. Such a wholesale shift in earth’s biota
will impoverish the planet for many millions of years to come. In
our view, this is the real threat to biodiversity—not a decline in
species per se, but a long-term erosion in the variety of biological
characteristics and functions that grace the natural world.
G. L., S. M. Case, A. C. Wilson and J. L. Patton. 1977. Rapid speciation
and chromosomal evolution in mammals. Proceedings of the National
Academy of Sciences of the United States of America 74:3942–3946.
Hallam, A, and P. B. Wignall. 1997. Mass Extinctions and Their
Aftermath. Oxford: Oxford University Press.
Levin, D. A., and A. C. Wilson. 1976. Rates of evolution in seed plants:
Net increase in diversity of chromosome numbers and species numbers
through time. Proceedings of the National Academy of Sciences of
the United States of America 73:2086–2090.
Levin, D. A. 2000. The Origin, Expansion, and Demise of Plant Species.
New York: Oxford University Press.
Rosenzweig, M. L. 2001. Loss of speciation rate will impoverish future
diversity. Proceedings of the National Academy of Sciences of the
United States of America 98:5404–5410.
S. Levin is a biologist with the National Marine Fisheries Service.
He has studied, among other topics in zoology, the evolutionary dynamics
of salmon. Donald A. Levin, Phillip’s father and a professor of biology
at the University of Texas, Austin, is similarly qualified to comment
on the vagaries of gene flow through generations. Address for Phillip
Levin: National Marine Fisheries Service, 2725 Montlake Boulevard
East, Seattle, WA 98112-2097. Internet: email@example.com