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OCR for page 31
Colloquium
The future of coral reefs
Nancy Knowlton*
Marine Biology Research Division 0202, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0202; and
Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Republic of Panama
Coral reefs, with their millions of species, have changed profoundly
because of the effects of people, and will continue to do so for the
foreseeable future. Reefs are subject to many of the same pro-
cesses that affect other human-dominated ecosystems, but some
special features merit emphasis: (/D Many dominant reef builders
spawn eggs and sperm into the water column, where fertilization
occurs. They are thus particularly vulnerable to Allee effects,
including potential extinction associated with chronic reproductive
failure. (i/D The corals likely to be most resistant to the effects of
habitat degradation are small, short-lived "weedy" corals that
have limited dispersal capabilities at the larval stage. Habitat
degradation, together with habitat fragmentation, will therefore
lead to the establishment of genetically isolated clusters of in-
breeding corals. (ii/9 Increases in average sea temperatures by as
little as 1°C, a likely result of global climate change, can cause coral
"bleaching" (the breakdown of coral-algal symbiosis), changes in
symbiont communities, and coral death. (iv) The activities of
people near reefs increase both fishing pressure and nutrient
inputs. In general, these processes favor more rapidly growing
competitors, often fleshy seaweeds, and may also result in explo-
sions of predator populations. (v) Combinations of stress appear to
be associated with threshold responses and ecological surprises,
including devastating pathogen outbreaks. (v/D The fossil record
suggests that corals as a group are more likely to suffer extinctions
than some of the groups that associate with them, whose habitat
requirements may be less stringent.
Coral reefs are often called the rainforests of the sea, although
calling rainforests the coral reefs of the land might be even
more appropriate (1~. As with rainforests, the importance of
coral reefs lies not so much in the diversity of the corals
themselves, but rather in the millions of species that live pri-
marily or exclusively in association with them. Veron (2), for
example, gives a minimum estimate of 835 species of reef-
building corals, and estimates for the biodiversity of reefs overall
range from 1-9 million (34.
Unfortunately, modern human civilization and coral reefs
make poor companions. Most activities of people (e.g., fishing,
deforestation, nutrient enrichment, burning of fossil fuels, and
use of toxic chemicals) either damage corals directly or damage
them indirectly by adversely modifying interactions with their
competitors, predators, pathogens, and mutualists. For example,
Edinger and colleagues (4) document losses in coral species
diversity ranging from 30-60% on reefs degraded by human
activities, with a 25% loss in generic diversity on two of these
reefs over just IS years. Thus, although concerted efforts to
protect reef habitats may slow their ongoing decline, it is difficult
to be optimistic about the health of reefs globally over the short
term in the context of increasing human populations and eco-
nomic growth (5~.
As the world changes with growing human domination, eco-
logical and evolutionary changes on coral reefs similar to those
outlined for terrestrial and other marine organisms and ecosys-
tems (e.g., refs. 116-118 and other articles in this colloquium)
are inevitable. A few factors do work in the favor of coral reefs.
www.pnas.org/cgi/cloi/10.1073/pnas.091092998
For example, the widely dispersing larvae and still large popu-
lation sizes of many important reef builders probably provide
some protection against extinction (6~. The diversity of coral reef
ecosystems may also make catastrophic invasions of exotic
species less likely, although invasibility may increase with dis-
turbance (7) and the degree to which diversity per se inhibits
invasions remains unclear (84. The fossil record clearly shows,
however, that marine species and ecosystems have their limits
(refs. 119 and 120 and other articles in this colloquium), and the
recent record indicates that these limits may be approached with
little warning (9, 10~. The broader ecological consequences of
reducing biodiversity (11) remain essentially uninvestigated for
coral reefs. Indeed, our understanding of even the basic physical
parameters of global change of relevance to reefs is inade-
quate (12~.
Below I focus on some of the peculiar features of corals and
other reef dwellers that are likely to affect their ecological and
evolutionary futures. Many of the examples are drawn from the
Caribbean, because change (and thus a possible glimpse of the
future) has been much greater there over the last several
decades. Nevertheless, the major points have applicability to
reefs worldwide.
Reproduction: Allee Effects, Inbreeding, and Hybridization
Sessile marine organisms, with very few exceptions, depend on
water to bring their gametes together; either eggs and sperm are
both released into the water column or eggs are fertilized
internally by sperm picked up from the water column. One
immediate consequence is that low gamete densities caused by
low population densities, asynchronous reproduction, or low
reproductive output per individual can lead to reproductive
failure (13, 14), a classic example of an Alice effect (154.
Corals themselves are commonly hermaphroditic and exhibit
two primary types of reproductive strategies: broadcasting and
brooding (16, 17~. Physically large and long-lived coral species,
the primary reef builders, are typically broadcasters that repro-
duce once or twice a year during an event known as mass
spawning. Buoyant bundles of eggs glued together with sperm
are released in approximate synchrony on just a few nights of the
year, when they float to the surface, break apart, and with luck
achieve fertilization. The small larvae of broadcasting species
typically disperse for at least 4 days (17~.
As with other outbred marine invertebrates (18), species in
this group do not generally self-fertilize successfully (17~. Thus,
eggs from one colony need to reach sperm from another for
reproduction to succeed. For broadcasting corals, we know very
little about critical gamete densities necessary for supporting
successful fertilization. The best study is that of Oliver and
Babcock (19), who showed that fertilization rates drop to low
levels by 3 h after peak spawning and on nights other than the
major night of spawning. This suggests that colonies spawning in
This paper was presented at the National Academy of Sciences colloquium, "The Future of
Evolution," held March 16-20, 2000, atthe Arnold and Mabel Beckman Center in Irvine, CA.
*E-mail: nknowlton~ucsd.edu.
PNAS 1 May 8, 2001 1 vol. 98 1 no. 10 1 5419-5425
OCR for page 32
temporal or spatial isolation (measured on the scale of a few
hours and tens or at most hundreds of meters) will often
experience nearly complete reproductive failure. Adverse envi-
ronmental conditions can also reduce fertilization rates inde-
pendent of gamete density (20, 21~. Moreover, nearby colonies
may release few gametes and thus be ineffective mating partners
if they are recovering from stressful conditions or damage
(21-24) or are small for whatever reason tsize rather than age
determines reproductive output (25, 26), so that fragments
typically have reduced fecundity or are infertile (22, 27~. Thus,
reef degradation may lead to sharply reduced reproduction, not
only because of lowered gamete production, but also because of
reduced rates of fertilization for those gametes that are released.
However, eggs failing to encounter nonspecific sperm will not
necessarily remain unfertilized, even when self-fertilization is not
feasible, because simultaneous or nearly simultaneous spawning
provides potential opportunities for interspecific hybridization
(2~. The extent to which this happens is a matter of debate,
because some species are clearly separated by either subtle
temporal differences in spawning time or gametic incompatibil-
ities (28~. However, these barriers are likely to become less
effective as opportunities for conspecific matings decrease. For
example, mechanisms to prevent selfing often decline in efficacy
after 4 h (29), and the same might happen with gametic barriers
to interspecific fertilization. Similarly, subtle temporal differ-
ences in spawning times between species whose gametes are
otherwise compatible (28) would be less effective if, for example,
unfertilized eggs from an early spawning species remained near
populations of later spawning species. Laboratory experiments
do clearly suggest that many species are capable of hybridization,
including species with very different morphologies (30, 31~.
However, the potential evolutionary impact of hybridization
between morphologically and ecologically distinct taxa is diffi-
cult to evaluate empirically, because of long generation times and
the difficulty of maintaining corals in captivity for determining
long-term survival and fertility of hybrids.
Brooding corals exhibit a very different reproductive strategy.
Only sperm are released, and fertilized eggs are retained within
the colony and released as swimming planula larvae. These
corals often reproduce on a lunar cycle for a number of months
per year (25), and the large larvae that are released probably do
not travel far, despite their physiological potential to do so
without feeding, thanks to the zooxanthellae that brooded larvae
contain (32~. For example, Carton and Olson (33) found that the
average swimming time for the larvae of the brooding coral Favia
fiagum was only about 4 min. Such limited dispersal suggests that
biparental inbreeding (mating between relatives) is not uncom-
mon in brooding corals (17~. Distances between potential mates
are probably even shorter than they are for broadcasting species,
perhaps as little as a few meters (34~. However, brooding species
are more likely to be able to self-fertilize (17, 35), probably as a
consequence of regular biparental inbreeding (184. Thus, one
likely consequence of habitat deterioration and destruction is an
increase in selfing as distances between fertile colonies increase.
Compared with the mass spawning species, brooding corals
have many of the attributes of weeds: they often grow to smaller
sizes, reproduce earlier, have shorter life spans, and are com-
petitively inferior to larger and more aggressive broadcasting
species (25, 26, 36~. Moreover, their ability to self-fertilize would
make them less vulnerable to Allee effects and allow them to
persist even at low densities. Brooding corals have come to
dominate many Caribbean reefs following disturbances of the
1980s and 1990s (37), and they also succeeded disproportionately
during the Oligocene-Miocene extinction event (38~. Neverthe-
less, brooding corals are very vulnerable to some forms of
disturbance, including high-temperature disruption of their sym-
biotic associations (39), to which I now turn.
5420 1 www.pnas.org/cgi/doi/10.1073/pnas.091092998
Coral-Algal Symbiosis
One of the striking features of coral reefs is the intimate
nutritional symbiosis between the coral animal and single celled
dinoflagellates, typically known as zooxanthellae. Corals pro-
vide excretion products to their algal guests which in turn
provide photosynthetic products to their coral hosts (40~. All
reef-building corals are obligately dependent on their zooxan-
thellae, which are probably responsible for the characteri-
stically high rates of calcification that reef-building corals
achieve (41~.
The ecological balance between corals and their algal part-
ners, and hence the success of corals as reef builders, is poten-
tially very sensitive to environmental conditions. This stems
from the fact that although we tend to think of mutualisms as
cooperative relationships, they are often better viewed as recip-
rocally selfish associations (42~. That is, other things being equal,
each partner tries to maximize its net gain from the association
by minimizing costs and maximizing benefits. This selfishness has
the potential to lead to ecological instability, because if stress
makes it difficult for one partner to provide its normal comple-
ment of benefits, the other partner may respond in the short term
by terminating the relationship, a strategy that is normally an
appropriate response to the ever present threat of cheaters
"partners that receive but do not return benefits (42~.
The implications of this are more than academic in the context
of the environmental changes to which reefs are subject today.
When corals are exposed to elevated temperatures or UV
radiation, for example, they "bleach"—that is, photosynthetic
pigments are drastically reduced, typically because of algal death
or expulsion (43~. Bleached corals can survive without their
normal complement of zooxanthellae for weeks or sometimes
months, but their growth and reproductive output are reduced
and eventually they die. Global warming is of particular concern,
because temperatures as little as 1°C over the normal seasonal
maximum can provoke substantial bleaching (44~. Coral bleach-
ing has increased dramatically over the last several decades (44),
and 1998 (with its unusually strong El Nino) was characterized
by massive bleaching on a worldwide scale (45~. In some areas,
reefs were decimated to unprecedented extents, based on the
fossil record (39), and climate models suggest that temperatures
sufficient to induce bleaching could become annual events
within a few decades (45~.
Nevertheless, it remains unclear whether coral reefs as we
know them will succumb to global warming, because coral-algal
symbioses do have some capacity to increase their ability to
withstand stresses such as high temperatures. For many years,
physiological acclimatization was viewed as the primary mech-
anism (46), and recent studies have shown, for example, that
acclimation to high light can provide some protection against
high temperature (47~. There is also renewed interest in evolu-
tionary responses with the realization that zooxanthellae have far
more genetic diversity than previously realized. Pioneering
studies by Trench (48) and Rowan (49) have shown that what was
once viewed as a single species living in association with multiple
invertebrate phyla, is in fact a diverse assemblage, now shown to
consist of at least four major clades (50, 51) whose genetic
differences are comparable to those exhibited between different
families or orders of free-living dinoflagellates (49~. Some coral
species host just one type of symbiont, whereas other corals host
multiple types, sometimes within individual colonies (49-51~.
This diversity is of particular significance with respect to global
change, because different types of zooxanthellae exhibit striking
differences in their susceptibility to bleaching (52) and their
ability to recolonize bleached hosts (53~. Thus, reefs may be able
to survive predicted increases in sea temperature and other
coming environmental changes by shifts in the kinds of zoox-
anthellae that are typically hosted by corals. Indeed, even
Knowlton
OCR for page 33
bleaching itself may be adaptive if it facilitates symbiont ex-
change (54~.
Experimental bleaching does appear to result in novel asso-
ciations (51, 53) but we know almost nothing about the long-term
consequences of such shifts in symbiont associations. Symbionts
that initially colonize bleached colonies are probably rapidly
growing opportunistic genotypes (53) that may not be ideal
partners from the coral's perspective. These may subsequently be
replaced by stress-resistant mutualists (53), but again we do not
know how these associations perform as mutualisms relative to
those that were established before bleaching. Thus, although
opportunistic or stress-resistant symbionts may provide protec-
tion against outright mortality (51), the consequences for growth
rates or reproductive output are unclear.
Global warming is not the only aspect of global change with
which corals and their symbionts must contend. Many aspects of
the oceans' biogeochemistry are changing in response to human
activities (55~. Rising levels of carbon dioxide that underlie much
of global warming may be detrimental in their own right to
corals. Coral reef growth depends on the net accumulation of
calcium carbonate, which is affected by the saturation state of
calcium carbonate in surface waters. Kleypas and colleagues (56)
argue that by 2100, increased levels of carbon dioxide might
cause calcification to decrease by 17-35% relative to preindus-
trial levels. Such a decrease could result in weaker coral skele-
tons, reduced growth rates, increased susceptibility to erosion,
and perhaps even a reduction in the ability of higher latitudes (a
potential refuge from higher temperatures) to sustain reef
growth (57~.
Increased nutrients of the types associated with changing
land-use patterns (58) may also directly harm corals. High
nutrient levels can result in reduced rates of growth and calci-
fication (59), as well as decreases in reproduction (60), probably
because of their impact on the symbiotic association between
corals and zooxanthellae (40, 59~. Nevertheless, experimental
manipulations of nutrient levels on corals often yield slight and
sometimes unexpected results, particularly when done in the
field (61~. Thus, it remains unclear whether current levels of
eutrophication are having a major, direct effect on the health of
corals. However, nutrients may also have a variety of indirect
effects that are discussed in later sections.
Emergent Diseases
.
Dinoflagellates are not the only important microbes on reefs.
Although poorly known, pathogens probably greatly outnumber
mutualists, and they are capable of completely transforming reef
communities through their effects on ecologically dominant
organisms. The most spectacular example of disease in the
oceans is the decimation of the once abundant sea urchin
Dicrd`ema antillarum throughout the tropical western Atlantic.
Between 1983 and 1984, more than 95% of these urchins died
because of a still uncharacterized pathogen that swept through
the entirety of the urchin's geographic range with the exception
of the eastern Atlantic (624. Nearly two decades later, recovery
is still limited (63), and in many places densities remain ex-
tremely low, despite the relatively short generation times and
high fecundity of Diadema (64~. Although reasons for the
failure of Diadema to recover may be complex, the ef-
fect of low density on fertilization rates is probably a major
contributor (64~.
Reef-building corals themselves appear tO ne Increasingly
affected by disease (65~. The ecological effects of coral patho-
gens are likely to be especially severe because rates of mortality
can be very high [up to 2 cm of coral tissue daily (664i, whereas
coral growth and recruitment rates are typically intrinsically low
te.g., annual growth rates of ~1 cm per year in many massive
corals (67~. The Caribbean, once again, provides particularly
troubling examples (68~. Before 1980, shallow-water reefs
Knowlton
. · · .
throughout the region were dominated by the genusAcropora. By
1990, most stands of Acropora were reduced to scattered small
patches by an unknown pathogen. Recovery has been slight (65),
despite the relatively high growth rates that characterize the
genus. Recovery from diseases of massive corals is likely to be
especially prolonged because growth rates in these corals are far
slower (67~. Diseases that attack Montastraea (69) are perhaps
the most threatening to Caribbean reefs, because of the domi-
nance of this coral as a reef builder throughout the region.
The direct evolutionary impact of disease on coral reefs
remains unclear. Lessios (62) found no evidence for reduced
genetic variation in Diadema following catastrophic mortality,
perhaps because even 95% mortality does not result in small
enough populations sizes in formerly abundant organisms. Al-
ternatively, the bottleneck in population size may not have
persisted long enough at the time of the study for genetic effects
to accumulate. The ecological impacts of diseases on reefs are
already substantial, however, via the direct effects of coral
pathogens on coral abundance and the indirect effects of the
demise of a dominant herbivore on seaweeds (discussed below).
These ecological changes appear to be without precedent over at
least the last several thousand years, based on examination of the
extensive Caribbean fossil reef record (70-72~.
As with any recent change, evaluating the role of anthropo-
genic effects is a challenge. This is particularly true for marine
diseases, because there is almost no baseline information on
earlier disease prevalence and even the pathogens responsible
are largely unknown (65, 68~. Harvell and colleagues (68) suggest
that apparent increases in the incidence of disease in marine
ecosystems generally could be at least in part the consequence
of global climate change, and they note that terrestrial activities
of man appear to have introduced at least one pathogenic agent
to coral reefs via run-off. It has long been recognized that stress
can make corals vulnerable even to normally benign microbial
associates (73), and thus, disease seems likely to be a major
player on reefs of the future. The ability of corals to respond
evolutionarily to the threat of pathogens is probably fairly
limited, given the enormous difference in generation times
between corals and their microbial enemies.
Shifting Ecological Balances: Competitors and Predators
of Corals
Pathogens are not the only biological enemies of corals; sub-
stantial mortality is also associated with overgrowth by compet-
itors and the feeding of predators. Evidence for increases in
these sources of mortality in recent decades is accumulating,
thereby suggesting that corals are currently waging a losing battle
on this front as well
The most important competitors of corals today on most reefs
are seaweeds (74~. There is general agreement that the com-
petitive balance between corals and macroalgae is shaped pri-
marily by the magnitude of herbivory and nutrient availability,
but their relative importance and how they interact continues to
be the subject of debate (75-77~. Small-scale experiments sug-
gest, however, that herbivory is often likely to be much more
important than nutrients in limiting algal growth (78~.
The histories of Kaneohe Bay, Hawaii (79) and Discovery Bay,
Jamaica (37) illustrate many of the relevant issues on a broader
scale. Concerns about eutrophication related to the explosion of
the green bubble alga Dictyosphaeria cavernosa led to the
diversion of sewage from Kaneohe Bay beginning in 1977, and
the opportunity to monitor the response of the reef community
to this major, albeit uncontrolled, experiment (79~. By 1983, algal
abundance had dropped to 25% of peak levels and coral
abundance had increased. Since then, however, algal cover has
again increased and coral recovery has slowed or even been
reversed. In the case of Discovery Bay (37), the uncontrolled
experiment was the Caribbean-wide die-off of the herbivorous
PNAS I May 8, 2001 I vo~. 98 | no. 10 1 5421
OCR for page 34
sea urchin D. antillarum described above, which at any one site
occurred over the course of only a few days (62~. In Discovery
Bay (which was in the process of recovering from a major
hurricane several years earlier) dead substrates were quickly
colonized by small ephemeral algae, but these were replaced
over several years by larger, long-lived species capable of over-
growing living coral. The result has been the decline of coral
cover from 52% to To, and the increase in algal cover from 4%
to 92% (37~. Similar changes, albeit somewhat different in timing
and extent, have occurred elsewhere (62~. The general consensus
is that the die-off of such an important herbivore, particularly in
the context of low abundance of herbivorous fishes due to
overfishing, was the primary cause of the shift from a coral-
dominated to an algal-dominated reef (37, 75, 77~.
These events suggest that herbivory is often the more impor-
tant regulator of competition between algae and corals, although
eutrophication can also shift the balance toward algal over-
growth, particularly when it is extreme (as in Kaneohe Bay).
Algae are not the only competitors of corals that could be
affected by eutrophication, however; nutrient enrichment and
consequent increases in bacterial populations might also facili-
tate the success of filter-feeders that have few natural predators
and are capable of overgrowing corals. The ascidian Trididem-
num solidum, which increased on reefs of Cura~cao by 900%
between 1978 and 1993 (80), may be a case in point, although no
data showing the cause of the increase exist. In any case, because
the effects of both decreased herbivory and increased eutrophi-
cation are likely to be augmented in the future, the future of
corals will almost certainly include increases in mortality from
competitors.
The same anthropogenic factors that can affect the compet-
itors of corals eutrophication and overfishing have also been
implicated in some of the spectacular explosions in predators of
corals (corallivores) witnessed over the last few decades. The
crown-of-thorns starfish, Acanthaster planci, is the most infa-
mous of these (81, 82), but explosions of predatory snails,
particularly in the genus Drupella, have also been noted (83~. At
least in the case of Acanthaster, the extent of outbreaks appears
to be unprecedented, because the size structure of corals pre-
ceding the earliest documented outbreaks could not have existed
if current magnitudes and frequencies of outbreaks were a
long-term feature of reefs (84~. Outbreaks also appear to be
shifting in nature from episodic to chronic (82~. As with the
competitors of corals, fishing out of predators on corallivores
(top-down control) may play a larger role than the enhancement
of survivorship of corallivores in the larval stage by eutrophi-
cation (bottom-up control; ref. 82~. However, not all studies
support the importance of the former (85) or the lack of
importance of the latter (86~. The factors contributing to
Drupella outbreaks are even less well understood (83~.
Sea Level, Storms, and Bioerosion
One of the most frequently discussed consequences of global
climate change is rising sea level. Past rises in sea level have often
been associated with global increases in reef development (87),
but rapid sea level rise can also result in the drowning of reefs
if it is too rapid, because of the light dependency of coral-algal
symbiosis and declining light levels with increasing depth. Reef
drowning is of potential concern because projected rates of
future sea level rise come close to estimates of past sustained
rates of reef accretion before the onset of anthropogenic effects
(12~. Although recruitment of newly submerged areas could
keep many species from going extinct, the three-dimensional
complexity of a true reef, on which other organisms depend,
would be lost.
Net vertical reef accretion is a balance between growth and
destruction (88), so that any of the features discussed previously
that slow coral growth have the potential to contribute to reef
5422 1 www.pnas.org/cgi/doi/10.1073/pnas.091092998
drowning. However future global change is also likely to affect
processes on the other side of the equation in particular, reef
destruction via storms and the activities of organisms that bore
into or scrape the surfaces of calcium carbonate skeletons
(bioeroders). Both bioerosion and storminess have been pro-
jected to increase in response to anthropogenic global change,
the former because of the association between eutrophication
and the nutrition of bioeroders (88), and the latter because of the
association between high temperatures and cyclonic storms (12~.
The likely extent of these changes remains subject to debate, but
either could result in a slowdown of reef accretion (88~. Such a
slowdown increases the probability that reefs will not be able to
keep up with rising sea levels.
Increased bioerosion and storminess will probably have spe-
cific evolutionary consequences in addition to their general
effects on reef growth. Fragmentation can facilitate production
and spread of asexual propagules, but it is often costly for the
organisms involved because many fragments die and survivors
have lowered fecundity (22, 27~. Thus, increased bioerosion and
storminess should favor strong skeletons or the ability to prop-
agate effectively by fragmentation. On the other hand, rising sea
level should lead to selection for rapid vertical growth. Some of
these selective effects may operate within species, but most will
probably favor some species at the expense of others. However,
because coral species with very dense skeletons are often slowly
growing, and fragmentation results in lowered three-
dimensionality, reefs of the future may find themselves caught
evolutionarily between the proverbial rock and a hard place.
Threshold Effects, Multiple Stable States, and Metapopulations
Reef biologists who have watched coral cover decline from 50%
to 5~o over the course of their careers are understandably
distressed by the state of reefs today and their prospects for the
future. But even more alarming than the magnitude of the
decline has been its speed and the fact that few scientists saw it
coming. For example, by 1980 the reefs of Discovery Bay,
Jamaica had been studied for decades and overfished for cen-
turies (89~. Nevertheless, the implications of extreme overfishing
for resiliency of these reefs to subsequent disturbances was not
appreciated until recovery failed.
Several common attributes of biological systems make pre-
diction difficult. The first are threshold effects or breakpoints
(90~. These quintessentially nonlinear relationships are common,
but nevertheless often surprising: when the thermostat is turned
up one notch, people tend to expect one notch's worth of
additional heat, not a house in flames. Responses to single
variables can behave in this fashion; for example, calcification
may remain constant over a range of saturation states, but then
drop abruptly below some threshold value (56~. Allee effects are
classic threshold phenomena, because populations increase
above a minimum population size but decrease below it (15~.
When two variables interact synergistically, threshold re-
sponses and ecological surprises are probably even more likely
(91, 92~. For example, neither sedimentation nor high nutrient
levels are good for corals, but their combined effect is far worse,
because fine muds then aggregate into a smothering marine
"snow" (93~. Similarly, the collapse of reef ecosystems along the
north coast of Jamaica seems to have resulted from the syner-
gistic interaction of overfishing and disease (374. Unfortunately,
our understanding of how multiple stressors interact remains
limited (94, 95~.
Also coupled with threshold dynamics is the concept of
multiple stable states (9, 90~. The existence of multiple stable
states implies that two different ecological communities can be
stable under the same conditions, with history determining
which community is present at any particular point in time.
Multiple stable points are linked with threshold effects because
it is often the case that the position of the breakpoint depends
Know~ton
OCR for page 35
on the direction in which the community is moving. For example,
a switch from coral dominance to algal dominance might occur
at specific levels of eutrophication and herbivory, but much
lower nutrient levels or higher levels of herbivory might be
required to shift the system back to its original coral-dominated
state. This pattern of response is well known for lakes (96), and
there is no reason in principle that it might not apply to coral
reefs (9~.
Switches from coral dominance to algal dominance are dra-
matic, but they are not the only cause for concern. As on land,
potential coral reef habitat is being eliminated and fragmented
with the spread of destructive processes both in the sea and on
the shore. Models considering this aspect of changing spatial
structure provide disturbing insights. For example, in a simple
two-species model, habitat destruction (removal of suitable
patches) can result in the extinction of a competitive dominant
that disperses poorly at the expense of a competitively inferior
species that disperses well, even when remaining patches un-
dergo no intrinsic changes themselves (97~. More elaborate
versions of this idea predict a certain percentage of inevitable
extinctions over time associated with a certain fraction of habitat
loss a so-called "extinction debt" again affecting competi-
tively dominant species first and rising sharply as habitat loss
increases (98~. Estimates of 40% reef habitat loss through
irreparable damage over the next several decades (5) are sober-
ing in this context, because the models imply that competitively
dominant corals, which are often major reef builders, may not be
able to persist even in areas not strongly impacted by the
activities of people.
Applications of these models to specific coral reef situations are
limited and require careful consideration of how model concepts
and terms relate to the biology of reef organisms. Stone (99, 100)
analyzed a Red Sea reef flat and concluded that numbers of species
extinctions associated with habitat reduction would be especially
catastrophic because competitively dominant corals were already
rare. However, this result reflects the fact that reef flats are
regularly disturbed and, thus, always dominated by weedy corals. In
contrast, the major reef builders on Caribbean reefs are (or were
until recently) competitively dominant species, either via aggressive
interactions or their ability to overtop their neighbors, whereas
weedy corals are typically small understory forms. Here, the
number of species extinctions might be smaller, but the ecological
impact larger. Records of sea level changes in the fossil record are
particularly interesting in this context. Pandolfi (101) has shown that
habitat loss of 90% associated with a marked drop in sea level about
18,000 years ago resulted in the rapid extinction of two coral species
(which were, as predicted by the models, competitive dominants).
The models (97, 98), as they have been applied to reefs to date
(99, 100), assume that each patch is occupied by a single species.
In this sense, the analyses describe the dynamics on single reefs,
with patches being de facto the spaces occupied by individual
colonies. Metapopulation models in the strict sense describe
patches surrounded by uninhabitable area, with rates of colo-
nization between populations being slower than the dynamics
within populations (ref. 102, and papers in this colloquium). This
structure is more appropriate for describing regional dynamics-
for example, the many reefs of Caribbean islands and banks
separated by uninhabitable deep water that is only occasionally
crossed by propagules (103~. Marine organisms with limited
dispersal abilities that occasionally disperse long distances by
rafting (104) almost certainly meet these assumptions, but even
organisms once assumed to be too widely dispersing to conform
to metapopulation models have recently been shown to recruit
to parental populations to a surprising extent (105-107~. We do
not currently have nearly enough information to parameterize a
regional metapopulation model for coral reefs in a quantitatively
useful way (103~. It is worth noting, however, that at this spatial
scale the true weeds might not be corals at all, because broad-
Knowlton
casting corals can potentially travel but rarely successfully re-
quit, whereas brooders regularly recruit, but not at long dis-
tances. The weeds would instead be the rapidly colonizing
ephemeral algae seen on reefs after hurricanes and other major
disturbances.
Broader Consequences for Biodiversity
The extent to which degraded reefs and other habitats can
support the associated diversity of healthy coral reef habitats-
the current homes for the myriad crustaceans, worms, mollusks,
bryozoans, and other groups that are found on reefs—is un-
known. Given that reef associates have many times the diversity
of the corals themselves, several issues are relevant: (i) Are reefs
as ecosystems especially vulnerable to environmental change or
slow to recover? (ii) Are reef dwellers less vulnerable than corals
themselves? The fossil record of past extinctions provides the
only real data for evaluating these questions.
It is often stated that reef ecosystems are both more vulnerable
to extinction and slower to recover, but rigorous analyses are
surprisingly limited (87~. Past extinction events appear to have
had a diverse suite of causes, not surprisingly, because global
change in any direction from the status quo is likely to accelerate
extinction; this probably explains why there is no strong bias
against tropical ecosystems overall. Within the tropics, however,
it does appear that shallow-water, low-nutrient carbonate plat-
forms have been more vulnerable than other tropical environ-
ments, although there is no strong evidence that such ecosystems
recover more slowly (87~. There is also a limited amount of
evidence suggesting that photosymbiotic organisms might have
been more vulnerable to extinction than nonphotosymbiotic
organisms, both at the end of the Cretaceous (87) and during the
more minor Oligocene-Miocene extinction (108~. Corals were
also more vulnerable to the changes associated with the rise of
the Isthmus of Panama; there are no living examples of closely
related sister species among the zooxanthellate scleractinian
corals on the two sides of the Isthmus, whereas such sister taxa
are common in other groups (109~.
One possible explanation for this pattern relates to differences
in distributions between the groups. Many organisms character-
istic of coral reefs are not restricted to reefs (87~. This is true of
corals themselves, of course, which can be found growing as
scattered colonies without creating the three-dimensional struc-
ture and complexity that the term reef implies. However,
extrareef distributions are likely to be even more characteristic
of other groups of reef-dwellers, at a variety of taxonomic levels.
In fishes, for example, all families considered typical of reefs
have ranges that extend outside the boundaries of reefs (110,
111~. Similarly in bryozoans, more than 75% of reef-associated
species of the Caribbean are also found in nonreefal settings
(112~. Should this be a general pattern, which seems likely, then
even elimination of most coral reef habitats would probably not
result in the extinction of a comparable proportion of coral reef
builders and dwellers (87~. No taxonomically comprehensive
analysis of obligate versus facultative reef associates exists, but
the above suggests that even the loss of all true reefs would leave
many facultative reef associates as survivors, and thus many of
the deeper branches of the tree of life intact (113~.
The most relevant guides to the future are, of course, past
extinctions associated with changes similar to those projected for
the coming century. Unfortunately, we have little to guide us in
this regard. Mass extinctions have been intensively studied, but
they are remote in time and, hence, involve organisms whose
phylogenetic affinities are distant from the organisms whose
responses we wish to predict. Moreover, despite the alarming
nature of the ongoing anthropogenic extinctions, they do not
begin to approach the severity of these cataclysmic events (ref.
119 and other articles in this colloquium); if they do, Homo
sapiens will have a lot more to worry about than the future of
PNAS I May 8, 2001 1 vol. 98 1 no. 10 1 5423
OCR for page 36
coral reefs. The most relevant comparisons are with the Oligo-
cene-Miocene and Pliocene-Pleistocene extinctions (38, 108,
114), but both of these events involved cooling episodes rather
than global warming. The combination of nutrification, global
warming, and loss of top members of the food chain (not to speak
of novel, introduced chemicals) is unprecedented over the last 65
million years. Thus, it is perhaps not surprising that many of the
reef organisms that persisted and thrived during the most recent
biological upheavals are those that are suffering the most now
(114~. Who the winners will be this time around is impossible to
predict, but we may not be that happy with the outcome.
The Camel's Last Straw?
In the face of so many unknowns, qualitative analogies can
provide an important complement to quantitative analyses. For
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PNAS 1 May 8, 2001 1 vol. 98 1 no. 10 1 5425
Representative terms from entire chapter:
coral reef
