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CORAL
REEFS AND GLOBAL CHANGE:
ADAPTATION,
ACCLIMATION OR EXTINCTION? INITIAL REPORT OF A SYMPOSIUM AND
WORKSHOP
SYMPOSIUM AND
WORKSHOP OVERVIEW
INTRODUCTION
CORAL REEFS AND GLOBAL CHANGE:
ADAPTATION, ACCLIMATION OR EXTINCTION? was the theme of a
symposium and integrating workshop held in Boston, January
3-11, 1998, in conjunction with joint meetings of the Society
for Integrative and Comparative Biology (SICB), the
International Society for Reef Studies (ISRS), and the
Ecological Society of America (ESA). The focus of the
symposium and the subsequent workshop were reports of Working
Group 104 of the Scientific Committee on Oceanic Research (SCOR),
co-sponsored by the Land-Ocean Interactions in the Coastal
Zone (LOICZ) core project of the International Geosphere-Biosphere
Programme (IGBP) and with the support of the NOAA Coastal
Ocean Program. The reports of Working Group 104 members and
invited contributors formed the basis for discussions. This
working group has been studying the topic "Coral reef
responses to global change: the role of adaptation" for
nearly four years, and the symposium provided opportunities to
augment its findings with contributed and invited papers from
other experts, to benefit from public review and discussion,
and to integrate the output in a workshop. Proceedings will be
published in a forthcoming issue of American Zoologist.
Both the symposium and the
working group focused interactions within a highly
interdisciplinary group -- biogeochemists, geologists,
paleobiologists, climatologists, aquarists, geneticists, and
organismal, ecological, and evolutionary biologists -- on the
diverse lines of evidence concerning corals, reefs, and their
responses to environmental change. The variety of expertise,
the specific nature but global scale of the topic, and the
opportunity to develop conclusions over time all contributed
to the emergence of fundamentally new views of the nature and
functioning of 'coral reef systems' that will have major
implications for future research and management.
The conclusions below are based
on major points of the Symposium and contributed papers,
augmented by subsequent discussions and then integrated and
interpreted during a post-meeting workshop. Although
interdependent, they are presented under topical categories
for convenience.
CLIMATE AND
GLOBAL FORCING
One unique feature of the
Symposium was a review of the general status of global climate
change knowledge from a coral reef perspective, including
results of a major regional climate-change modeling effort
directed toward tropical marine environments. The focus of
symposium and working group was on the biotic effects of
climate, but review and analysis indicate the importance of
carbon-cycle feedback. Coral reefs depend on calcification for
production of the reef structure, but marine calcification is
a net source of atmospheric CO2, not a sink. The
effects of coral reef and other calcifying communities on the
global carbon cycle may be significant in the long term, but
are very minor compared to present anthropogenic CO2
emissions. However, direct effects of changes in atmospheric
CO2 on coral reef communities may be as great as or
greater than the effects of climate change. The key coral and
reef-related global climate and geochemistry points were:
- Despite the Kyoto protocol,
atmospheric CO2 will continue to increase
beyond 2100; it is projected to reach two times the
preindustrial level of about 270 ppm by the year 2070, and
approximately 700 ppm by 2100. These atmospheric changes
will cause significant changes in the carbon chemistry of
surface ocean water, especially decreases in pH and
carbonate ion concentration, which will reduce the calcium
carbonate saturation state.
- Translation of the expected
global warming (about 2 degrees Celsius by 2100) into
regional sea-surface temperature (SST) is difficult
because of uncertainty about the physical controls on
tropical SST, but probable outcomes include: (1) poleward
movement of the isotherms currently associated with coral
reef distributions; (2) some rise in mean SST within the
present tropical-subtropical latitude ranges; and (3) some
degradation of coral communities by local-to-regional
episodes of higher temperature within the present
warm-water zones.
- Projected rates of sea-level
rise (15-95 centimeters by 2100) are well within geologic
ranges and measurements of accretion rates for unstressed
reefs. However, interactions among decreased calcification
rates and other stresses (discussed below) may diminish
the ability of reefs to keep up with rising sea level.
- Widespread increases in
riverine flood frequency and magnitude will produce a
greater incidence of high turbidity, nutrient loading, and
other pollution episodes in coastal reef environments.
- There will be possibly
significant changes in ENSO and tropical cyclone
climatology. Tropical cyclones may increase 10-20% in
intensity by 2070, with a possible poleward extension of
storm tracks. Effects on reefs of these changes in
climatic stress, especially extreme events, will vary
regionally.
REEF
PERSISTENCE -- PAST, PRESENT, AND FUTURE
Four earlier glacial periods
over more than a billion years of earth history ended with
mass extinctions of reef organisms, prolonged periods without
reefs, and the eventual evolution of very different reef
assemblages. These events may provide very general models for
the potential effects of climate change on modern coral reefs.
However, such comparisons must be tempered by the long time
scales of these past events, and by understanding of the
characteristics of the scleractinian corals that are the
modern reef-builders. Within the Quaternary history of the
earth, contemporary levels of anthropogenic stress are unique,
and CO2 concentration, temperature and sea level
are all at or near past maxima -- and projected to rise still
further. The question of whether cumulative effects of human
impacts have the potential to accelerate major changes in such
processes, on global evolutionary as well as on local
ecological scales, was addressed by considering aspects of
coral reef history and science relevant to reef persistence.
While the distinctions among
corals, coral communities, living coral reefs, and coral reef
systems (or ecosystems) are both real and important, these
categories are often confused. Their fates are intimately
interconnected, but they are not the same, and an uncritical
focus on 'coral reefs' may ignore vital aspects of the various
interacting components that form coral reef ecosystems.
- Many reefal species also
exist beyond the range of reefs and/or within non-reefal
communities, where they may play important roles in
preserving the resilience of reefal systems
- Both evidence and inference
suggest that coral reefs (as accreting coral communities
living on calcium carbonate accumulations 'of their own
construction') are not only difficult to define, but are
also more vulnerable and more episodic in both space and
time than are their component species and assemblages.
- There is empirical evidence,
at all scales and from a wide range of sources (geological
and biological; field, laboratory and aquarium;
theoretical and experimental), that corals as a group of
organisms possess a wide range of adaptive and acclimative
mechanisms, and that most of these mechanisms appear
relatively robust and/or resilient in the face of stress.
Coral community structure is dynamic on scales of years to
centuries, and this dynamism may provide long-term
resilience (or 'community adaptation') in response to
stresses, including global environmental change. However,
the unprecedented variety and magnitudes of chronic
anthropogenic alterations of modern reef environments may
undermine this resilience.
COMMUNITY AND
POPULATION DYNAMICS
Consideration of global and
large-scale regional distributions of reef organisms,
populations, coral reef systems, and environmental variables
yielded the following conclusions, some of which represent
substantially new or different perspectives on corals and
reefs. Some key points are described in terms of 'metapopulations',
which may be thought of as sets of spatially separated
sub-populations linked by dispersal, or more simply as
'populations of populations'.
- Distributions of reefs
(certainly) and coral communities (probably) are
correlated not only with sea-surface temperature, but also
with available light and with calcium carbonate saturation
state. Independent studies indicate that these additional
variables may be important controls over coral and reef
function and distribution. This is a significant change to
the traditional view that temperature is the primary
control on reef biogeography.
- Limits of distribution and
abundance of reefal species are not necessarily determined
by the same processes that determine limits for reefs and
reefal communities. Modern distribution patterns, as well
as the ability of reefal species to adjust distributions
in the face of climate change, can be modelled using
meta-population models in which patterns of abundance and
distribution are functions of local population dynamics.
These dynamics in turn determine persistence and
dispersal, which controls the origination of populations.
Both local population dynamics and patterns of dispersal
are sensitive to past and future climate change.
Ecological, genetic, and
species diversities reflect history, including
consequences of changes in patterns of dispersal within
metapopulations.
Patterns determined during
periods of intense disruption (e.g., low sea-level stands)
may persist over temporal scales much longer than needed
for their establishment, and may persist beyond subsequent
disruptions.
Regional patterns also
reflect persistent barriers to effective gene flow and
dispersal over larger temporal and spatial scales; these
barriers may constrain the development of reefs.
Regions with extreme
isolation of reef communities, restricted gene flow,
limited dispersal, and few refugia will be most vulnerable
to major changes and least likely to respond resiliently,
especially in geographically or ecologically marginal
areas.
RESPONSES AND
REACTIONS
Because coral reef 'systems'
interact with each other and with global climate across a wide
range of time and space scales, there are some fundamental
limits on predictability. However general predictions can be
made about the effects of global or large scale processes at
scales of years to centuries.
- Calcification of reef
corals, coralline algae, and coral-algal communities is
sensitive to the calcium carbonate saturation state of the
water. While this is consistent with studies of
foraminifera, geochemical expectations, and geological/paleontological
evidence, it has not previously been recognized as a major
factor in coral/reef biology. There was consensus that the
greatest global climate-related threat to corals and reef
systems is the very real possibility that increased CO2
will cause reduced rates of calcification.
Some, but not all,
participants felt that a rough estimate of the magnitude
of this effect was possible; and that an overall 10-20%
decrease in reef-related calcification may follow from the
projected doubling of preindustrial CO2 levels
within the coming century.
- Mechanistic understanding of
acclimitization and adaptation by corals is extremely
limited.
Understanding of
acclimative mechanisms is largely confined to
photosynthetic responses to environmental irradiance.
In contrast, little is
known about the cellular and biochemical pathways of
acclimatization/adaptation to temperature change and other
environmental variations, although recent studies of the
symbiotic associations that sustain corals reveal that
complexity and flexibility in host-symbiont partnerships
may provide important and rapid mechanisms of
acclimatization/adaptation to environmental change.
More importantly, because
the mechanisms of coral calcification and their
interactions with the photosynthesis of the symbiotic
algae are largely unknown, corals' abilities to
acclimatize or adapt to changes in seawater chemistry
resulting from global increases in atmospheric CO2
cannot be predicted.
Lack of knowledge about the
ranges of reproductive behavior and their environmental
relationships limits understanding not only of dispersal
and recruitment, but also of whether hybridization and
somatic mutation are critical adaptive mechanisms.
- Sensitivity to saturation
state, in conjunction with other influences operating over
various scales, means that global climate-related change
will apply significant stresses to coral reef systems.
This finding DOES NOT alter
the assessment that the primary threat to reefs and coral
communities is local and regional anthropogenic stresses,
often of a chronic nature.
This finding DOES
substantially alter the previously common view that the
effects of global climate change are negligible, or indeed
potentially positive, compared to local threats. Instead,
global-scale changes are expected to significantly and
progressively increase the vulnerability of many reefs to
both acute and chronic local stresses.
- Resilience and/or robustness
of modern assemblages may be "lost" as
environmental changes exceed the adaptive and acclimative
capacities established under previous rates and ranges of
disturbance.
Globally, the issue of
greatest concern is atmospheric CO2, which is
projected to exceed the estimated maxima for the
Quaternary period during the coming century. This will
both cause and interact with temperatures that also
approach the maxima experienced by reef organisms over the
past several million years.
Local and regional
anthropogenic stresses are unprecedented in distribution,
magnitude, rate of change, and, in some cases, nature.
These will interact synergistically not only with each
other, but also with global climate-related factors.
IMPLICATIONS
These findings and
observations, resulting from focused interdisciplinary review
and interpretation of many lines of evidence addressing corals
and reefs, provide perspectives different from those obtained
from discipline-based or local studies. This picture of coral
reef ecosystem responses to the effects of global increases in
CO2 is fundamentally different from that of
terrestrial ecosystems, for which it is widely accepted that
increased primary productivity is advantageous. In contrast,
the dominant global trend for coral reefs, a reduction in
calcification, is fundamentally unfavorable for coral reef
systems.
Widespread observations of
intrinsic resilience and robustness in corals and reefs
suggest that they need not necessarily disappear as a result
of accumulating stresses. However, the recognition that global
factors are likely to increase reef vulnerability to currently
dominant anthropogenic stresses adds urgency, as well as new
perspectives, to the need to develop new management,
protection, and conservation measures on relevant spatial and
temporal scales. Long-standing lack of knowledge about the
mechanisms of calcification, the nature of symbioses, the
physiology of acclimatization, reproductive biology (ranging
from taxonomic and geographic inventories of behavior and
success to mechanisms of adaptation), the nature and extent of
biodiversity, and the long-term ecological structures and
dynamics of coral reef communities hinder our ability to make
decisions and useful predictions that address the issues
raised by our rapidly developing understanding of large-scale
processes. These uncertainties point toward research needs
that will concurrently address both fundamental and applied
problems.
Individual corals, communities,
and living reefs are controlled by fundamental interactions
among many environmental variables and biotic responses at
local scales; this limits detailed or quantitative predictions
in most situations. In addition, living reef communities are
products of complex and dynamic interactions at all scales --
from dynamic, multicomponent symbiotic variations on scales of
days or weeks, to gene flow involving metapopulations at
millennial time scales. These are significant external factors
determining the responses of individual reefs and communities
to local conditions, and their probabilities of future change.
CONCLUSIONS
The Working Group's findings
indicate the need for significant revision of our research,
assessment, and management approaches to coral reef problems.
Key issues include:
- Calcium carbonate saturation
state is potentially the most important control on
calcification by reef organisms and communities.
Increasing atmospheric CO2 levels therefore
represent a global and increasing threat to coral reef
systems. This significantly changes our assessment of the
importance of global climate change issues in contributing
to reef system stresses.
- Coral reef systems are the
results of interacting processes and constraints operating
at a variety of time and space scales, whereas the study,
management, and protection of reef ecosystems are
intrinsically local and regional enterprises operating on
human time scales. Effective local action must be informed
by global understanding of controlling responses and
interactions at a variety of scales -- simply reducing or
mitigating local stresses may not be adequate to insure
reef sustainability in the presence of large-scale
constraints or systemic deterioration, and in the absence
of mechanistic understanding.
- Corals, and to some extent
reef communities, possess diverse and impressive arrays of
acclimative and adaptive mechanisms -- reproductive
strategies, flexible symbiotic relationships,
physiological acclimatization, habitat tolerance, and a
range of community interactions. However, inadequate
understanding of these phenomena, and of the critically
important calcification mechanisms, severely limits our
ability to predict and manage the future of reef systems.
- Local anthropogenic threats
remain the most immediate threat to coral reef systems,
but responses to local stresses are now seen as being
substantially conditioned by global environmental changes
and by biological patterns and distributions established
over much larger time and space scales. Consideration of
the biological effects of reduced calcification and other
stresses indicates that other environmental changes
affecting tissue growth and metabolism, calcification, or
demography (reproduction, recruitment, survival) of corals
are all likely to reduce further the capacity of reef
communities to maintain themselves, and increase the
probabilities that thresholds to major disruptions -- such
as ecosystem collapse or organism extinctions -- will be
exceeded.
For
further information contact:
Dr. Terry Done, AIMS
phone: +61 (07) 47534344
fax: +61 (07) 47725852
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