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biotic crisis
Colloquium
The biotic crisis and the future of evolution
Norman Myers*t and Andrew H. Knoll:
*Green College, University of Oxford, Oxford OX2 6HG, and Upper Meadow, Old Road, Oxford OX3 8SZ, United Kingdom; and "Department of
Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138
The biotic crisis overtaking our planet is likely to precipitate a major
extinction of species. That much is well known. Not so well known
but probably more significant in the long term is that the crisis will
surely disrupt and deplete certain basic processes of evolution,
with consequences likely to persist for millions of years. Distinctive
features of future evolution could include a homogenization of
biotas, a proliferation of opportunistic species, a pest-and-weed
ecology, an outburst of speciation among taxa that prosper in
human-dom~nated ecosystems, a decline of biodisparity, an end to
the speciation of large vertebrates' the depletion of "evolutionary
powerhouses" in the tropics, and unpredictable emergent novel-
ties. Despite this likelihood, we have only a rudimentary under-
standing of how we are altering the evolutionary future. As a
result of our ignorance, conservation policies fail to reflect long-
term evolutionary aspects of biodiversity loss.
Human activities have brought the Earth to the brink of biotic
crisis. Many biologists (e.g., refs. 1-5) consider that coming
decades will see the loss of large numbers of species. Fewer
scientists- witness the lack of professional papers addressing the
issue appear to have recognized that, in the longer term, these
extinctions will alter not only biological diversity but also the
evolutionary processes by which diversity is generated. Thus,
current and predicted environmental perturbations form a
double-edged sword that will slice into both the legacy and fu-
ture of evolution.
A simple consideration of time underscores the magnitude of
the challenge to scientists and public alike (ct ref. 6~. Episodes
of mass extinction documented in the geological record were
followed by protracted intervals of Diversification and ecolog-
ical reorganization; five million years can be considered a
broadly representative recovery time, although durations varied
from one extinction to another (7~. Suppose, too, that the
average number of people on Earth during the recovery period
is 2.5 billion (by contrast with the 6 billion today). Under these
conditions, the total number of people affected by what we do
(or do not do) during the next few decades will be in the order
of 500 trillion—10,000 times more people than have existed until
now. We are thus engaged in by far the largest "decision" ever
taken by one human community on the unconsulted behalf of
future societies.
The question of how current threats to biological diversity will
affect the future of evolution was first raised by one of us in the
mid-1980s (8~. It attracted virtually zero interest from fellow
biologists. Thirteen years later, he revisited the question, this
time with more detailed analysis, although still in exploratory
form (9~. This latter publication elicited attention from the
National Academy of Sciences, which undertook to sponsor a
Colloquium in March 20Q0. As a "scene setter" for Colloquium
participants, we drafted an overview account of topics to be
tackled, and that draft makes up the bulk of this paper. We hope
that it may serve the same purpose for readers of this special
section of PNAS.
The Core Concept
One of the first truisms absorbed by biologists is that evolution
is not predictable. We can no more predict the future compo-
www.pnas.org/cgi/doi j10. 1 073/pnas.091092498
sition of communities than some Ordovician ecologist could
have foreseen the Great Barrier Reef. However, despite our
inability to predict the products of evolution the trajectories of
future morphologies or the innovations of future physiolo-
gies we can make meaningful estimates about evolutionary
processes as they will be affected by the depletion of biological
diversity. We may have little basis for predicting what large
mammals might look like two million years from now, but much
better reason to suppose that there will be very few of them.
The evolutionary dimension to the current biotic crisis has
been vividly expressed by Michael Soule (10~: "Death is one
thing, an end to birth is something else." In other words,
impending extinctions will be far from the full final outcome of
current environmental disruption. At least as important will be
the alteration of evolutionary process, and for a period that is
difficult to estimate but must surely measure in millions of years.
First-Order Effects. There will be several first-order effects stem-
miIlg from the biotic crisis: (i) a major extinction of species within
the foreseeable future, estimated by some to remove between
one-third and two-thirds of all species now extant (1, 2, 5, 11~;
(i') a mega-mass extinction of populations, proportionately
greater than the mass extinction of species, within the foresee-
able future (12); (
ecological types that thrive in human-dominated ecosystems
(37, 38~?
Proliferation of opportunistic species. reselected and generalist
species, often appearing as opportunistic species, may prolifer-
ate, especially if there is preferential elimination of K-selected
species that include natural controls of reselected populations
(32, 38~. Could this proliferation lead to what has been charac-
terized as a "pest and weed" ecology (39, 40~?
Depletion of "evolutionary powerhouses" in the tropics. Virtu-
ally every major group of vertebrates and many large categories
of invertebrates and plants originated in spacious zones with
warm, equable climates (41, 42~. In addition, tropical species
appear to have persisted for relatively brief periods of geologic
time, implying high rates of evolutionary turnover and episodes
of explosive speciation (21, 43, 44~. According to Jablonski (22),
the tropics have been "the engine of biodiversity" for at least 250
million years. Today, we face the prospect of severe depletion if
not virtual elimination of tropical forests, wetlands, estuaries,
coral reefs, and other biomes, with their exceptional biodiversity
and ecological complexity. Because some of these blames ap-
pear, in some senses at least, to have served in the past as
preeminent "powerhouses" of evolution (45, 46), their decline
could entail severe consequences for rediversification as the
biosphere emerges from environmental crisis.
Decline of biodisparity. Elimination of species is not the only
measure of an extinction event. There can be declines, as well,
in biodisparity, the biota's manifest morphological and physio-
logical variety (47-49~. Biodisparity impoverishment can be
assessed through the surrogate measure of loss of higher taxa or
guilds, and, over the past 2000 years, the preferential elimination
of species-poor genera has reduced biodisparity at rates even
greater than those of species loss (48~. Will the same pattern of
non-random culling persist in the future?
An end to speciation of large vertebrates. Even our largest
protected areas will prove far too small for further speciation of
elephants, rhinoceroses, apes, bears, and big cats, among other
large vertebrates (30, 50, 51~. What knock-on consequences and
ripple effects could there be for smaller species, indeed for biotas
as a whole given, for example, the depauperizing impacts of the
present-day decline of elephants (52~?
Emergent novelties. There may be many emergent novelties,
although these are especially difficult to predict. For instance,
there could be an explosive radiation within certain higher taxa,
notably small mammals and insects able to thrive in human-
dominated ecosystems. The question is not whether persistent
lineages can evolve in unexpected ways, but rather to what extent
the environmental constraints humans place on surviving pop-
ulations will channel innovations toward properties we associate
with pests.
Lessons from the Past?
The geological record is replete with extinction events, their
intensity ranging from the small and local to global mass
extinctions that shattered Earth's biological order. Inevitably,
extinctions were followed by rediversification, directed in the
case of the largest events by ecological reorganization. What can
we learn from paleobiology, other than the oft-quoted observa-
tion that recovery proceeds slowly in the wake of grand scale
biotic disruption (40, 53, 54~? Can we find generalities among
extinction episodes that can guide thinking about our own
future? Or, is it the differences among extinction events that
should command our attention? As David Jablonski (63) asks in
these proceedings, should we even focus on the five great mass
extinctions that capture most attention, or do the more numer-
ous, smaller events scattered throughout the geological record
provide closer analogs for the present?
The geologic record contains much evidence of bounce-back
processes (49, 54-59), but how far will these serve as analytic
5390 1 www.pnas.org/cgi/doi/10.1073/pnas.091092498
blueprints for what lies ahead? How can we estimate time frames
at issue? Should we anticipate a minimum period of several
million years "perhaps as much as 10 million (56~] before
evolution can reestablish anywhere near the biological configu-
rations and ecological circuitry existing before the current
crisis? Will some recovery processes operate in some sectors
of the biosphere, others in others, and with widely varying rates
(55, 58, 60~?
In some major extinctions, for example the Cretaceous-
Tertiary boundary event, environmental perturbation was swift
and sure, but also short-lived. Recovery began soon after
disruption. In the present biotic crisis, it is hard to envision a
scenario under which the factors that are driving the biosphere
toward grand scale biodiversity loss will be mitigated in the wake
of such loss. On the contrary, on any time scale we can envisage
(and any scenario that does not involve early mass mortality for
humankind), the situation becomes bad and then stays bad for
some time to come. Thus, on the time scale of the human species,
environmental disruption (or at least aspects of it) is permanent.
Under these circumstances (which may, to some degree, be
approximated by the persistent environmental discord after the
Permian-Triassic mass extinction), the prospects for rediversifi-
cation are limited.
Recovery Processes
How will ecosystems function in a world of diminished biodi-
versity? Does ecosystem function necessarily decay as diversity
declines, and if so, by how much and in what manner? Can
biodiversity and humans alike prosper in a world where most
biological diversity will be confined to relatively small parks and
reserves?
If biodiversity is indeed critical to ecosystem function, do we
know enough about the principles of evolution to intervene in
the recovery processes? To the extent that the answer to the first
part of this question is probably "yes" and the answer to the
second part is almost certainly "no," what would we need to learn
to attempt evolutionary interventions that will do more good
than harm?
More realistically, do we know enough to mitigate the loss of
biological diversity? As David Western writes in his colloquium
contribution, mitigating strategies will likely be carried out
predominantly in ecosystems dominated or influenced by hu-
mans and other species that thrive when humans are present.
How we think about our evolutionary future depends directly on
how successful we can hope to be in preserving biodiversity and
biodisparity.
Which taxa are likely to play prominent parts in recovery
processes? What "survivorship" traits (ecological, biogeo-
graphic, evolutionary) can we use to define those taxa that may
prove more successful in surviving current events? At the same
time, which taxa might tto cite Erwin's graphic phrase (55~] "win
the extinction but lose the recovery?" Might certain biotas
already be "stressed" by Pleistocene climatic oscillations, making
them more vulnerable to depletion (61, 62~? Or are they
"hardened" purged of their most vulnerable members by Pleis-
tocene events (63~?
Should we in fact speak of "recovery"? What is it that is
supposed to be recovering (the dinosaurs didn't)? Should we not
view the recovery phase as more like a transition to new and
novel departures of multiple sorts (55~? Plainly there is much
scope for pioneering research in response to the many questions
raised (54~. We need to consider planning priorities. What
research is most pressing? What is readily achievable? What is
already underway? What deserves most financial or institutional
support? What potential is there for interdisciplinary research,
for instance that which combines genetics and restoration ecol-
ogy, or paleontology and conservation biology?
Myers and Knoll
Conservation Responses
Should we be content simply to safeguard as much as we can of
the planetary stock of species? Or should we pay equal if not
greater attention to safeguarding evolutionary processes at risk
(cf. refs. 64-66~? Consider, for instance, biodisparity: to cite
Jablonski (49), "If we are concerned with avoiding the loss of
particular functional groups, or with maximizing the potential
source pool for evolutionary recovery, then biodisparity mea-
sures may provide a more appropriate assessment, beyond sheer
numbers of taxa, of how priorities should be set."
Following on from these considerations is the question of
whether we should seek to maintain the evolutionary status quo
by preserving precise phenotypes of particular species, or
whether we should prefer to maintain phylogenetic lines that will
enable evolutionary adaptations to persist, thereby leading to
new species (67, 68~. Is it sufficient for us to maintain, for
example, just the two elephant species we already have, or should
we try to keep open the evolutionary option of further elephant-
like species in the distant future?
This is an unusually significant question, with unusually sig-
nificant implications for conservation strategies. Elephants,
along with many other large mammals, are inclined to move
around a good deal, a trait that enables them to maintain gene
flow across large areas. As a result, their gene pools often tend
to be fairly uniform tan elephant in East Africa may not be so
different from one 4,000 km away in South Africa (68~. Re-
grettably the remaining populations of elephants, substantial
and extensive as they are, albeit fragmented and declining fast,
are probably already below the minimum numbers to keep open
the possibility of speciation (69~.
In marked contrast to elephants, with their slow breeding
rates, many insect species have immense breeding capacities and
rapid turnover rates. These latter attributes offer quick adapt-
ability to environmental shifts, whereupon genetic changes are
passed along promptly. These attributes not only leave many
insect species well suited to survive the environmental upheavals
of human activities, but they offer exceptional scope for specia-
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Myers and Knoll
