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OCR for page 83
Colloquium
Declines of blames and biotas and the future
of evolution
David S. Woodruff*
Ecology, Behavior, and Evolution Section, Division of Biology, University of California at San Diego, La JolIa, CA 92093-0116
Although panel discussants disagreed whether the biodiversitY
crisis constitutes a mass extinction event, all agreed that current
extinction rates are 50-500 times background and are increasing
and that the consequences for the future evolution of life are
serious. In response to the on-going rapid decline of blames and
homogenization of biotas, the panelists predicted changes in
species geographic ranges, genetic risks of extinction, genetic
assimilation, natural selection, mutation rates, the shortening of
food chains, the increase in nutrient-enriched niches permitting the
ascendancy of microbes, and the differential survival of ecological
generalists. Rates of evolutionary processes will change in differ-
ent groups, and speciation in the larger vertebrates is essentially
over. Action taken over the next few decades will determine how
impoverished the biosphere will be in 1,000 years when many
species will suffer reduced evolvability and require interventionist
genetic and ecological management. Whether the biota will con-
tinue to provide the dependable ecological services humans take
for granted is less clear. The discussants offered recommendations,
including two of paramount importance (concerning human pop-
ulations and education), seven identifying specific scientific activ-
ities to better equip us for stewardship of the processes of
evolution, and one suggesting that such stewardship is now our
responsibility. The ultimate test of evolutionary biology as a
science is not whether it solves the riddles of the past but rather
whether it enables us to manage the future of the biosphere. Our
inability to make clearer predictions about the future of evolution
has serious consequences for both biodiversity and humanity.
The science of evolution, linked to the related sciences of
ecology, paleobiology, and genetics, seeks to explain the
history of life on earth. After about 150 years of formal inquiry,
we seem to be more than half way to accounting for the
development of blames and biotas, the biosphere, and ourselves.
We can now account for much of the past and present in terms
of genetics, ecology, and chance. However, the real measure of
a science's maturity is its ability to make sound predictions about
the future. Our discussion of the future of biomes and biotas,
even with one of the colloquium organizer's contributions (1-3)
as a guide, revealed that we are frankly unequal to this challenge
despite its urgency. Our inability to make clear predictions
(beyond sweeping generalizations) about the future of life on
earth has serious consequences for both biodiversity and the well
being of humanity. In the last 50 years, it has become widely
accepted that the eruption of the human population is causing
the extinction of much cherished biodiversity and is altering
biosphere-level processes that we depend on for $3-33 trillion
worth of environmental services annually (4, S). Our population
density is now >30 times that predicted for an omnivorous
mammal of our size, and it has been estimated that we usurp
>40% of the planet's gross terrestrial primary productivity to
our own ends (6, 7~. If our greatest achievement in the last
century was the collective understanding of what evolution and
its products, the biosphere, mean to our own survival, the
challenge of the present century is to develop a more predictive
www.pnas.org/cgi/doi/10.1 073/pnas.1 01093798
science of evolutionary ecology before it is too late to shape a
desirable future.
There is no doubt that the biodiversity crisis is real, and upon
us, and began roughly 30,000 years ago (8~. We speak with less
scientific assurance, however, about almost every one of the
widely quoted numbers describing its magnitude and signifi-
cance. Nevertheless, we live at a geological instant when global
rates of extinction are at an all time high for the last 65 million
years (My) and are increasing. Most extinctions go unrecog-
nized; thus, estimates of overall rates have high errors. Currently,
however, several million populations and 3,000-30,000 species
go extinct annually of a global total of >10 million species (9, 10~.
Probably at least 250,000 species went extinct in the last century,
and 10-20 times that many are expected to disappear this
century. Although we can identify the most threatened blames
and species in some groups tref. 11; see World Conservation
Union (2000) at http://www.redlist.org], we cannot make ac-
ceptably rigorous predictions about the consequences of these
extinctions for the future evolution of life or for the integrity of
the biosphere's environmental services that we still take for
granted.
The taxonomic course of the biodiversity crisis is reasonably
understood for terrestrial vertebrates and a few other groups
(114. In the last few centuries, we have lost one family of
mammals (Nesophontidae), half the birds of Hawaii, possibly the
most common bird in North America (the passenger pigeon),
and all of the moas a total of 1,139 documented plant and
animal species globally. Further, we have extirpated most of the
fish in the lakes of the northeastern United States and most of
the primates from the remaining forests of West Africa. The
situation in the oceans is poorly known but comparable or worse
(12~. If we step back 30,000 years, we have contributed to the
elimination of the megafauna of the Holarctic, Neotropics, and
Australian zoogeographic regions (70 species and 19 genera of
mammals in North America); these extinctions involve the
disappearance of several other families of mammals (13~. To-
day's taxon-specific global extinction rate estimates are 50-500
times background, and half the remaining vertebrates are at risk
of extinction, including most whales and primates. Already >30
species of mammals and birds survive only because of the
intensive care they receive in zoos and nature reserves. Taxon-
specific assessments of threat have been prepared by the Con-
servation Breeding Specialist Group of the World Conservation
Union (IUCN) for many groups ranging from palms to parrots
to Papilio, the swallowtail butterflies.
Lamentable as these expected species losses are, it has been
argued that even if we lose 90% of the species on the planet, we
This paper reports on a pane! discussion at the National Academy of Sciences colloquium,
"The Future of Evolution," held March 16-20, 2000, at the Arnold and Mabel Beckman
Center in Irvine, CA.
Abbreviation: My, million years.
*E-mail: dwoodruf~ucsd.edu.
PNAS 1 May 8, 2001 1 vol. 98 1 no. 10 1 5471-5476
OCR for page 84
may lose only 20% of the phylogenetic diversity (14~. This claim
can be made, because in most genera, there are several species,
and the survival of one, it is argued, may capture most of the
genetic variability of the whole Glade. Although this estimate is
controversial, it explains why some question whether we should
be saving rare species in species-rich clades. Is one tuatara worth
200 species of skinks? Are rare species treasures or dross (15)
from an evolutionary point of view? However, saving phyloge-
netic diversity is not currently the goal of global conservation
efforts, and science does not yet clearly indicate that it should be.
The ecological consequences of our destruction of biomes and
biotas are understood in broad generality as they impact human
well being locally and regionally. Less clear are the global
implications of habitat destruction, especially species-rich trop-
ical forests, wetlands, and coral reefs (16~. Predictions are
complicated further by the recent realization that human activ-
ities are altering climates globally. The exploration of Sala and
coworkers (17, 18) of the impact of various drivers of change
(and the interactions between these drivers) on global ecosys-
tems and biodiversity loss in the year 2100 illustrates both the
power and the current limitations of scientific inquiry at this level
of concern. Nevertheless, there is general agreement that the
biosphere will have fewer species and be subject to more weed,
pest, and disease outbreaks. Heretofore dependable nutrient
cycles may become less predictable as essential microbes suc-
cumb to anthropogenic toxins. The new blames will be more
easily disturbed and invaded, and will have an aesthetically
unappealing dullness. In considering these generalities, the
discussants agreed on one thing: evolution will continue as the
major driver or cause of biodiversity. Although we are ushering
in a period of geological time characterized by the homogeni-
zation of biotas (19, 20), dubbed the Homogecene at this
meeting, the basic processes causing evolution will continue.
Evolution is not over set back perhaps but by no means over.
In answer to the question, "Is the biodiversity crisis unprec-
edented?" there was also general agreement: no. There was,
however, surprising debate as to whether it warrants being called
a mass extinction event. Recall that a 1998 Harris (21) poll found
that 70% of biologists asked said they believed a mass extinction
was underway and accepted that 20% of all species will go extinct
in the next 30 years (22~. This issue arose when I asked the
discussants to continue the diversity line on a standard Sepkoski
plot of marine invertebrate families over the last 600 My to the
year 3000 to show a predicted 50% loss of species. Would one
expect the line to fall to 40% or to 60% of today's all time high
level? Jablonski and other discussants argued that it might only
drop 1% and therefore the biodiversity crisis is absolutely not a
mass extinction event.
This difference of opinion is both important and potentially
dangerous. Mass extinction events are typically defined in terms
of their irreversible impact on large numbers of species in diverse
taxa on a global scale in a short period. In five previous events,
15-90% of the marine invertebrate species studied went extinct
(23, 24~. However, today, marine species account for only about
15% of biodiversity (25), and we are most concerned with losing
terrestrial species rather than higher level taxa. Thus, attempts
to show the magnitude of the current extinction event on a plot
of marine invertebrate families is inappropriate and dangerous
in that it belittles its significance. Unfortunately, the comparable
multitaxon plot of species numbers through time is not yet
available; when it is, we will be able to illustrate graphically the
probable impact of the current event in comparison with the
previous big five marine invertebrate mass extinctions. A hint of
what this impact might look like is provided by Alroy's studies of
North American mammal species through the last 98 My (26~.
The end-Pleistocene extinction rate of 32% is already as extreme
as any other during the previous 55 My but does not yet approach
the 76% rate observed at the Cretaceous-Tertiary boundary
5472 1 www.pnas.org/cgi/doi/10.1 073/pnas.101 093798
(27~. Regan et al.'s (28) contribution to the problems of esti-
mating global extinction rates and the use of fuzzy arithmetic to
consider multiple uncertainties appeared after this colloquium.
My personal opinion is that we are currently living in what will
eventually be recognized as a real mass extinction. If current
area-species curve-based projections are correct, we could lose
up to 50% of the planet's species in the next 1,000 years. Raup's
consideration of the number of species in genera, and of genera
in families, across phyla, shows that a 50% loss of species may
involve a 25% loss in genera and a 10% loss in families (29~.
Furthermore, extinctions do not occur at random in space and
in Glades. The losses will be higher in the tropics, because the
species/genera ratio changes with latitude (304. Purves et al. (11)
show clearly that the nonrandom phylogenetic losses of mammal
and of bird species since 1600 are already equivalent to the loss
of one monotypic phylum. The authors estimate that an addi-
tional 120 genera of mammals and birds are at risk over
expectations under random extinctions. Regardless of whether
such calculations qualify the current biodiversity crisis as a mass
extinction event, we all agreed that it would be inexcusable to let
it become one (or a worse one). To this end, we reached
conclusions that may be summarized here as: arm the scientists,
alert the public, and do anything to buy time.
Causes of the Decline in Biomes and Biota
The causes of the biodiversity crisis are well known and include
human impacts on habitats (habitat destruction, degradation,
fragmentation, and restructuring) and on organisms (overex-
ploitation, introduction of exotic competitors, predators and
parasites, and creating new pests) (8, 10, 31, 32~. Discussants
noted differences in geographic rates of habitat alteration and
destruction (largely complete in Europe and North America and
on-going in the tropics) and that such rates are unprecedented
in the tropics and subtropics in the Neogene. There was agree-
ment that community simplification (with loss of pollinators and
dispersers) and the regional homogenization of biotas, with
weedy opportunists replacing endemic specialists, are of serious
concern. The well recognized vulnerability of island biotas will
be exacerbated by our accelerated importation of parasites and
predators. The introgressive hybridization of cultivars and their
"wild" ancestors was noted as also requiring more attention,
because it can lead to the evolution of aggressive weeds and the
extinction of rare species (33~. Potential threats from transgenic
genetically modified organisms will require vigilance and careful
assessment (34~.
In coinciding with a period of rapid anthropogenic global
warming, the biodiversity crisis could not have come at a worse
time. The rate of warming is unusually fast but not without
precedent (35~. Further, most living species have experienced
global temperatures as warm as today's for <5% of the last 2-3
My (36~. Orbitally forced species range dynamics associated with
100,000-year Milankovich cycles have caused repeated changes
in the distributions of most temperate zone species (37, 38) and
caused ranges of some North American species to shrink pro-
gressively with successive cycles (13~. The ability of species to
respond to future climatic oscillations by range shifts will be
greatly reduced by our creation of an inhospitable matrix
between the remaining habitat patches. We can no longer expect
many terrestrial temperate zone species to shift naturally 1,000
km pole-ward at CO2 x2, when mean global temperatures are
predicted to be 5°C above today's. Increased nitrogen will also
have significant impacts on soils, plant productivity, and biodi-
versity (39~.
Future of Evolutionary Processes
All predictions about the future of life on earth and about the
>10 million species and their various assemblages involve two
pivotal assumptions about a single species, our own. The first
Woodruff
OCR for page 85
assumption concerns human numbers and provides a simple
metric of the impact of our population. The second assumption
concerns our per capita consumption of natural resources, food,
and energy. Discussion of the future of evolution presupposes
the availability of acceptable 100-year and 1,000-year projections
for human populations. The 100-year prediction is reasonably
clear and leads to a consensus view of a warmer world with many
more species missing, with the survivors living in fragmented
habitats and losing genetic variability fast, and with "wilderness"
a largely historical state of nature. However, "reasonable clear"
is misleading, because the human population could reach as high
as 16 billion, or it could peak at 7.5 billion around 2040 and
return to 5.5 billion by 2100 (40~. Not surprisingly, the 1,000-year
projections for human numbers and behavior are too speculative
to print; however, it is already clear that we cannot expect a
return to a prebiodiversity crisis state of nature under even the
most favorable scenarios with reduced human impact. Recovery
from previous mass extinction events has taken 5-10 My (41, 42~.
Action taken in the next few decades will determine how
impoverished the biosphere will be in 1,000 years. By then, many
surviving "wild" species will require active maintenance by
wildlife managers using ecological and genetic methods yet to be
developed, in a world dominated by species commensal with
humans. Discussion focused on the origin of "commensals"-
from where do they come? from hot spots or disturbed areas?
from what clades? from what biomes? Under even the most
favorable speculations about the 1,000-year situation, there was
serious concern about the ability of biodiversity to "bounce
back" given the current prospects for tropical forests, wetlands,
and coral reefs.
The consequences for biotas over the next 100 years are easier
to predict.
Species Geographic Ranges. One of the lessons of paleobiology is
that a species geographic range is a good indicator of its
probability of surviving mass extinction events, ice ages, and
other major environmental changes (see refs. 13, 23, and 43~. Of
particular interest is the response of individual species to global
climate change and the probability that new species assemblages
will form, analogous to the "disharmonious" communities of the
Late Pleistocene. In the past, single species and interacting
species have moved rather than adapted to such change, but such
dispersal will no longer be possible. In future, terrestrial species
will have to adapt or their dispersal will have to be managed,
especially in plants and other low-vagility organisms. Ironically,
this realization comes just as progress is being made on one of
the great puzzles of the Modern Synthesis, the evolution of
species ranges (44), on how climate change leads to both local
adaptation in peripheral populations and range shifts (45~. Gene
flow is predicted to increase in commensal species and decrease
in natives as their ranges become fragmented. Spatial hetero-
geneity will therefore decrease in commensals and increase in
natives. Templeton (46) argues that range fragmentation will
lead to extinction and not speciation, because the individuals in
fragmented populations will not increase in numbers fast enough
for divergence to occur. Managers will have to move the
proverbial one individual per generation between remnant sub-
populations of metapopulations to counter genetic drift (47~.
The possibility that habitat fragmentation may actually increase
rather than decrease gene flow and population genetic variation,
as found recently in Acer (48), needs further examination.
Studies of probable adaptive responses of individual species to
global warming are in their infancy (e.g., ref. 49~.
Genetic Aspects of Risk Assessment. Although the ecological and
behavioral characteristics associated with high extinction risk are
reasonably well understood (but only in a few taxa), the popu-
lation genetic components of viability are also receiving atten-
Woodruff
tion (50~. Genetic drift is expected to decrease in the growing
populations of commensals and increase in the fragmented and
smaller populations of natives. Genetic risks that were largely
ignored in the last century will become dominant concerns in a
world of small, recently isolated populations with declining
genetic effective population sizes, Ne. Genetic erosion, the
decrease in population variation caused by random genetic drift
and inbreeding, is both a symptom and a cause of endangerment
of small isolated populations (51~. The phenomenon has been
long understood in terms of population genetic theory (47~;
however, the devastating early stages of the process in nature
have gone undocumented, because the changes are rapid sunder
standard models heterozygosity declines at 1/~2Ne) per gener-
ation; ref. 471 and difficult to monitor. Recently' a method for
monitoring genetic erosion based on non1nvas1ve genotypmg
using nuclear microsatellite variation has been introduced (52~.
Our studies showed that, although genetic erosion accompanied
habitat fragmentation and demographic collapse in some spe-
cies, the process apparently can begin before detectable demo-
graphic decline of local populations of other species (524. This
finding is important, because genetic studies of threatened
populations usually are performed only after demographic stud-
ies indicate that there is a problem. In the future, managers will
have to survey both demography and genetics, and their inter-
action, to assess a fragmented population's viability. In addition
to genetic drift, inbreeding can also threaten a fragmented
population's viability (53), and again, recent application of
molecular genetic assays provides a clear demonstration of its
impact on extinction in nature (544. The implication of these
observations is that wildlife managers will increasingly have to
intervene; nature can no longer be left alone to function, because
our actions have doomed countless isolated populations to slow
genetic decline and extirpation.
Genetic Assimilation. The threats of genetic swamping of rare
species by common congeners are seen as increasing (33~.
Molecular genetic methods now permit the detection of earlier
incidents of genetic assimilation that have extirpated or exter-
minated one of the hybridizing taxa. The assimilated taxon
remains as a phantom in the gene pool of the surviving species
whose variability is enhanced by the interaction. Whether this
increased variability increases its evolvability is not known, but
it may. This issue is relevant to the more frequently confronted
circumstance involving threatened polytypic species and super-
species: is it preferable to save a single "generic" taxon or several
separate subspecies? Existing theory does not give a clear
general answer.
Natural Selection. As a bold generalization, selection pressures on
commensals are predicted to increase, largely as a result of
artificial selection. Similar increases in selection pressures on
populations of natives are also expected, but largely through the
agent of natural selection. There was general agreement that
selection intensities will increase because of environmental
changes. Tilman (39) discusses selection for dispersal, compet-
itive ability, and plasticity. The relationship between community
simplification, disturbance and invadability, and selection pres-
sures on small populations needs more attention. Selection at the
ecosystem level (55) was not discussed, but it is predicted that the
proportion of reselected species will increase and that the
number of pest species will probably double.
Mutation Rates. Mutation rates may rise as a result of increases in
background mutagen concentrations, increases in UV-B caused
by ozone depletion by N2O and chlorofluorocarbons, and locally
significant nuclear waste storage. Lande (56) has argued that,
even without any increase in mutation rates, the viability of many
Copulations will become increasingly compromised. The rate of
PNAS 1 May 8, 2001 1 vol. 98 1 no. 10 1 5473
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OCR for page 86
5474 1 www.pnas.org/cgi/doi/10.1073/pnas.101093798
equate. Below this size, we will have to engineer evolvability to
ensure long-term viability. Bioneering, the interventionist ge-
netic and ecological management of species, communities, and
ecosystems in a postnatural world, is poised to become a growth
industry. It is not the control of nature that we should seek but
rather a deeper appreciation of the natural dynamics of these
complex systems and a willingness to work with rather than
against these dynamics (664. Although many of the above
predictions are frankly speculative, there was general agreement
that the biosphere in the year 2100 will be less predictable and
that events then will unfold at rates traditionally labeled as
"unpleasant surprises." Myers' (67) precautionary principle and
Wilson's (68) admonition about the one thing (loss of biodiver-
sity) our descendants are least likely to forgive us for are basic
maxims guiding our response to the biodiversity crisis. Evolu-
tionary processes will continue but with results that are increas-
ingly difficult to predict.
Recommendations
The discussants identified 10 recommendations for policy, re-
search and education. These include two of paramount impor-
tance, seven identifying specific activities to better equip us for
the stewardship of the processes of evolution, and one suggesting
that such stewardship is now our responsibility.
1. Promote efforts to reduce human population growth and
resource use, because conservation goals cannot be achieved
without addressing human needs and aspirations.
2. Promote the teaching of ecology and evolutionary biology in
the educational process at all levels.
3. Promote efforts to complete a rapid inventory of the
planet's biota, including Species 2000 and the Global
Biodiversity Information Facility, to provide these foun-
dational data in 20 years rather than 600 years, at present
rates of activity (69-71~. We also need to establish the true
evolutionarily significant units in the few hundred species
we select for intensive management and protection, of the
> 104 species that will need interventionist management by
2100 (72, 73~.
4. Promote research on landscape- and on seascape-level
processes so as to improve fundamental species level
conservation.
5. Foster research on the predictive use of the fossil record. If
the past has taught us anything, it is that evolution is a
hierarchical process (74) that cannot be predicted beyond
some crude generalizations. Paleobiology promises to give
us the perspective to assess and react to the biodiversity crisis
scientifically.
6. Promote research on the relationship between genetic vari-
ability and population viability and ultimately evolvability
(50, 53~. Most evolutionary and conservation biologists
assume that increasing genetic variance always enhances the
probability of population survival and evolution, but this
assumption is not generally true (75~. In constant and
unpredictable environments, genetic variance reduces pop-
ulation mean fitness. In predictable, highly variable envi-
ronments, genetic variance may be essential for adaptive
evolution and population persistence. Because almost all
predictions point to natural populations losing genetic vari-
ability, we may need to reexamine Fisher's Fundamental
Theorem in the light of advances in understanding of the
genetics of quantitative and quasineutral trait evolution.
Also the possible conversion of nonadditive genetic variance
to additive variance in small populations leading to increased
variance in fitness needs more study, as does the issue of
genetic load, which takes time to evolve and is still difficult
to detect experimentally (76~. If most new variation so
Woodruff
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produced is deleterious, or mildly deleterious, then perhaps
these concerns can be set aside in the short term.
7. Promote research on genetic control of pests and their
vectors to diminish their importance in disturbed ecosystems
and improve the human condition (e.g., ref. 77~.
8. Promote the development of a global system of nature
reserves especially in the tropics. The current IUCN goal of
10% national set-asides to represent each biome and the
latest proposals to focus efforts on Biodiversity hot spots (a
Global 200 and a Global 25, among others) all deserve
encouragement, because they will save blames and biotas
more effectively than single species conservation efforts
(78-80~. The arguments for greater cooperation between
the various stakeholders (academic, nongovernmental, gov-
ernmental, and local communities) and for better science in
setting global priorities should be heeded (81, 82), but
scholarly debates among ourselves about the weaknesses of
any one proposal are counterproductive if they delay action.
Solutions offered by scientists will almost always be com-
promised in their application to the real world by reasonable
human rights concerns, and furthermore, the sooner we
move beyond parks and reserves in our planning, the better
(83, 84~.
9. Promote political, legal, and regulatory changes to redesign
and recommission existing protected areas so that they may
better conserve their native biotas in the face of climate
change, edge effects, and increased demand for sustainable
use by local people and recreational use.
10. Finally, some of us advocate a shift from saving things, the
products of evolution, to saving the underlying process,
evolution itself (46, 72, 85~. Facilitating this process will
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ultimately provide us with the most cost-effective solution to
the general problem of conserving nature. The human
predicament requires that we accept responsibility for this
process and its products. Like it or not, evolutionary biolo-
gists have to recognize that the ultimate test of their science
is not their ability to solve the riddles of the past and the
origin of species, but rather to manage their viability and
prevent their premature extinction, to manage the bio-
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doing fundamental research and the other doing applied
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most difficult problems ever tackled by science difficult
because of their complexity and because many cannot be
approached with the reductionist methods that served us
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I thank the numerous participants in the colloquium's formal and
informal discussions. This essay constitutes my personal attempt, 8
months after the meeting, to highlight their contributions and concerns.
Our discussions were unrecorded at the time; thus, errors and omissions
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Representative terms from entire chapter:
mass extinction
