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Climate Change, Human Impacts, and the Resilience of Coral Reefs


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The diversity, frequency, and scale of human impacts on coral reefs are increasing to the extent that reefs are threatened globally. Projected increases in carbon dioxide and temperature over the next 50 years exceed the conditions under which coral reefs have flourished over the past half-million years. However, reefs will change rather than disappear entirely, with some species already showing far greater tolerance to climate change and coral bleaching than others. International integration of management strategies that support reef resilience need to be vigorously implemented, and complemented by strong policy decisions to reduce the rate of global warming.
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Climate Change, Human Impacts, and the
Resilience of Coral Reefs
T. P. Hughes,
* A. H. Baird,
D. R. Bellwood,
M. Card,
S. R. Connolly,
C. Folke,
R. Grosberg,
O. Hoegh-Guldberg,
J. B. C. Jackson,
J. Kleypas,
J. M. Lough,
P. Marshall,
M. Nystro¨m,
S. R. Palumbi,
J. M. Pandolfi,
B. Rosen,
J. Roughgarden
The diversity, frequency, and scale of human impacts on coral reefs are increasing to the
extent that reefs are threatened globally. Projected increases in carbon dioxide and
temperature over the next 50 years exceed the conditions under which coral reefs have
flourished over the past half-million years. However, reefs will change rather than
disappear entirely, with some species already showing far greater tolerance to climate
change and coral bleaching than others. International integration of management
strategies that support reef resilience need to be vigorously implemented, and comple-
mented by strong policy decisions to reduce the rate of global warming.
oral reefs are critically important for
the ecosystem goods and services
they provide to maritime tropical and
subtropical nations (1). Yet reefs are in
serious decline; an estimated 30% are al-
ready severely damaged, and close to 60%
may be lost by 2030 (2). There are no
pristine reefs left (34 ). Local successes
at protecting coral reefs over the past
30 years have failed to reverse regional-
scale declines, and global management of
reefs must undergo a radical change in
emphasis and implementation if it is to
make a real difference. Here, we review
current knowledge of the status of coral
reefs, the human threats to them now and in
the near future, and new directions for re-
search in support of management of these
vital natural resources.
Until recently, the direct and indirect
effects of overfishing and pollution from
agriculture and land development have
been the major drivers of massive and ac-
celerating decreases in abundance of coral
reef species, causing widespread changes in
reef ecosystems over the past two centuries
(35). With increased human populations
and improved storage and transport sys-
tems, the scale of human impacts on reefs
has grown exponentially. For example, mar-
kets for fishes and other natural resources
have become global, supplying demand for
reef resources far removed from their tropical
sources (6) (Fig. 1). On many reefs, reduced
stocks of herbivorous fishes and added nutri-
ents from land-based activities have caused
ecological shifts, from the original domi-
nance by corals to a preponderance of fleshy
seaweed (5, 7 ). Importantly, these changes to
reefs, which can often be managed success-
fully at a local scale, are compounded by the
more recent, superimposed impacts of global
climate change.
The link between increased greenhouse
gases, climate change, and regional-scale
bleaching of corals, considered dubious by
many reef researchers only 10 to 20 years
ago (8), is now incontrovertible (9, 10).
Moreover, future changes in ocean chemis-
try due to higher atmospheric carbon diox-
ide may cause weakening of coral skeletons
and reduce the accretion of reefs, especially
at higher latitudes (11). The frequency and
intensity of hurricanes (tropical cyclones,
typhoons) may also increase in some re-
gions, leading to a shorter time for recovery
between recurrences (10). The most press-
ing impact of climate change, however, is
episodes of coral bleaching and disease that
have already increased greatly in frequency
and magnitude over the past 30 years
(914 ).
Centre for Coral Reef Biodiversity, James Cook Uni-
versity, Townsville, Qld 4811, Australia.
tal Protection Agency, Old Quarantine Station, Cape
Pallarenda, Townsville, QLD 4810, Australia.
ment of Systems Ecology, Stockholm University, SE-
106 91 Stockholm, Sweden.
Center for Population
Biology, Division of Biological Sciences, Section of
Evolution and Ecology, 1 Shields Avenue, University of
California, Davis, CA 95616, USA.
Centre for Marine
Studies, University of Queensland, St Lucia, QLD
4070, Australia.
Scripps Institution of Oceanography,
University of California San Diego, La Jolla, CA 92093,
Smithsonian Tropical Research Institute, Box
2070, Balboa, Republic of Panama.
National Center
for Atmospheric Research, Post Office Box 3000,
Boulder, CO 80307, USA.
Australian Institute of Ma-
rine Sciences, PMB #3, Townsville, QLD 4810, Austra-
Great Barrier Reef Marine Park Authority, Post
Office Box 1379, Townsville QLD 4810, Australia.
Department of Biological Sciences, Stanford Univer-
sity, Hopkins Marine Station, Pacific Grove, CA 93950,
Department of Paleobiology, Smithsonian In-
stitution, Post Office Box 37012, National Museum of
Natural History, Washington, DC 20013, USA.
partment of Zoology, The Natural History Museum,
Cromwell Road, London SW7 5BD, UK.
of Biological Sciences, Stanford University, Stanford,
CA 94305, USA.
*To whom correspondence should be addressed. E-
Fig. 1. Map of the Indo-
Pacific Oceans showing
the scale of (A) the live-
fish trade, servicing res-
taurants in the burgeon-
ing cities of southeast
Asia, and (B) the aquari-
um tropical-fish trade
with major markets in
the United States and
Bleaching, Acclimation, and Adaptation
Regional-scale coral bleaching is strongly as-
sociated with elevated temperatures, particu-
larly during recurrent ENSO (El Ninˇo–
Southern Oscillation) events (8). Stressed,
overheated corals expel most of their pig-
mented microalgal endosymbionts, called
zooxanthellae, and become pale or white. If
thermal stress is severe and prolonged, most
of the corals on a reef may bleach, and many
may die. A popular model (9) shows an in-
variant bleaching “threshold” at 1°C above
mean summer maximum temperatures. This
threshold will be chronically exceeded as
temperatures rise over the next 50 years, lead-
ing to predictions of massive losses of all
corals (Fig. 2A). This model is based on two
simplifying assumptions: that all corals re-
spond identically to thermal stress, and that
corals and their symbionts have inadequate
phenotypic or genetic capabilities for adapt-
ing rapidly to changes in temperature. Below,
we challenge the conventional understanding
of these key issues.
Bleaching is conspicuously patchy (1517),
providing clear empirical evidence of the ab-
sence of a single bleaching threshold for all
locations, times, or species (contrary to the
conventional model depicted in Fig. 2A). Con-
sequently, bleached and unbleached corals are
often encountered side by side (Figs. 3A and
4B). The sources of this variation are poorly
understood and have been variously attributed
to extrinsic environmental patchiness (e.g. tem-
perature, light, turbulence), as well as intrinsic
differences (phenotypic and genetic) among
corals and their microalgal symbionts (1519).
Whatever the mechanisms, bleaching thresh-
olds are more realistically visualized as a broad
spectrum of responses (Fig. 2B). Furthermore,
bleaching susceptibilities may also change over
time as a result of phenotypic and genetic re-
sponses (Fig. 2C). In particular, substantial geo-
graphic variation in bleaching thresholds within
coral species provides circumstantial evidence
for ongoing evolution of temperature tolerance.
Average summer water temperatures
differ enormously within the geographic
boundaries of a typical coral species’
range. Based on our current knowledge of
taxonomy, the median latitudinal extent of
coral species in the Indo-Pacific is 56° (20),
with many species’ ranges straddling the
equator and extending to or beyond the
limits of reef growth (at 30°N and
30°S) where water temperatures are
much cooler (Fig. 3B). Similarly, the geo-
graphic extent of 35% of coral species in
the Arabian Gulf (where the mean summer
maximum is 36°C) (21) also includes Lord
Howe Island (24°C), the southernmost cor-
al reef in the Pacific Ocean (Fig. 3C).
Importantly, corals in the Arabian Gulf do
not bleach until they experience tempera-
tures that are extreme for that location, well
over 10°C higher than summer maxima in
cooler regions elsewhere in the same spe-
cies’ ranges, providing circumstantial evi-
dence of local adaptation. Furthermore, the
lower bleaching threshold in cooler loca-
tions implies that there is strong selection
for corals and their zooxanthellae to evolve
thresholds that are near, but not too far
beyond, the expected upper temperature at
that location. This pattern points to a po-
tential trade-off between the risk of mortal-
ity from extreme temperatures versus a
high cost of thermal protective mechanisms
(e.g., antioxidant enzymes, heat shock, or
photoprotective proteins and pigments).
An emerging area of research points to the
importance of genetic variation as a determi-
nant of bleaching responses in both corals
and zooxanthellae. Corals exhibit high levels
of genetic diversity, as expected for species
with large population sizes and prodigious
sexual reproduction (22). Similarly, zooxan-
thellae (Symbiodinium spp.) cluster into a
number of groups (based on cladistic analysis
of DNA sequences), with seven clades being
recognized so far, comprising many species
(19). This recent finding raises the issue of
current and future patterns in the distribution
and relative abundance of zooxanthellae
clades. A hypothesis that bleaching is “adap-
tive,” increasing coral fitness by facilitating
expulsion of susceptible zooxanthellae spe-
cies and uptake of more resistant ones (23),
has not been supported by observations on
the fate of bleached corals. Bleaching is more
accurately described as a stress response,
which is often followed by high mortality,
reduced growth rates, and lower fecundity
(16, 24). Although adult corals may acquire a
previously undetected clade under experi-
mental conditions (25, 26), a change in the
relative proportions of zooxanthellae as a re-
sult of bleaching, like similar rearrangements
of coral assemblages (Fig, 3B), does not nec-
essarily indicate that any evolutionary re-
sponse has occurred.
A major concern is that the accelerating
rate of environmental change could exceed
the evolutionary capacity of coral and
zooxanthellae species to adapt. A common
view is that corals are too long-lived to
evolve quickly, and that geographic differ-
ences in temperature tolerances have
evolved over much longer time frames than
the decadal scale of current changes in
climate. Although some corals are indeed
very long-lived, sexual maturity is reached
within 3 to 5 years and most species at all
depths rarely live longer than 20 years (27).
Nonetheless, highly skewed fecundity dis-
tributions (where a few very large, old
individuals swamp the gene pool), strongly
overlapping generations, and high levels of
asexual reproduction are common traits
that are likely to retard rapid evolution in
many coral species. Although mortality
rates from bleaching events are often very
high, we know virtually nothing about how
much selection this exerts or the heritability
of physiological traits in corals. Further-
more, adaptive evolution could be limited if
traits under selection are negatively genet-
ically correlated (28) or if gene flow is high
enough to preclude local adaptation. On the
other hand, high gene flow or connectivity
will promote resilience and recovery from
recurrent bleaching. The available evidence
indicates that rates of gene flow in corals
vary substantially among species (22, 29),
which implies that their differential ability
to migrate in response to climate change
and to adapt will result in further changes
to community structure beyond the imme-
diate effect of selective mortality caused by
severe bleaching. In contrast, subpopula-
tions on isolated islands or archipelagoes
(e.g., Hawaii and Bermuda) may represent
genetic outposts for virtually all coral reef
species, with little input from other, distant
Fig. 2. (A) A model showing a constant coral
bleaching threshold, which is likely to be chron-
ically exceeded in the future as oceanic tem-
peratures increase (9, 14). (B) An alternative
model that incorporates differences in bleach-
ing thresholds (e.g., among species, depth, and
locations), indicated by parallel lines. (C)A
more realistic scenario where changes in
thresholds also occur over time, attributable to
acclimation and evolution.
15 AUGUST 2003 VOL 301 SCIENCE www.sciencemag.org930
localities. If isolated reefs bleach, recovery
is likely to be far slower than in more
central, interconnected populations.
Lessons from the Past: The Geological
The geological record provides the only
source of data on long-term effects of climate
change on coral reef species and assemblages
(30, 31). Many extant species of corals ex-
tend backwards in time to the Pliocene [1.8 to
5.3 million years ago (Ma)], and most scler-
actinian genera originated in the Eocene to
Miocene (55.0 to 5.3 Ma) (32). Extant species
have dominated modern reefs for the past
half-million years, providing an invaluable
baseline long before human impacts began
(3, 4 ). New assessment of past climates has
revealed unexpectedly rapid shifts over de-
cades or less, especially at high latitudes,
with ice-age transitions being linked to abrupt
changes in the North Atlantic circulation
(33). Further rapid climatic changes may
have also occurred at lower latitudes in
warmer periods since the last glacial maxi-
mum (34). Consequently, there is now some
uncertainty about the speed of expected cli-
mate change relative to the past, although we
can be certain nonetheless that the projected
increases in carbon dioxide and temperature
over the next 50 years will substantially and
very rapidly exceed the conditions under
which coral reefs have flourished over the
past half-million years (10).
During the Pleistocene and Holocene,
many extant species of tropical and subtrop-
ical organisms underwent dramatic shifts in
geographic range in response to periods of
warming and cooling (35, 36). Some species
migrated faster than others, producing rapid
shifts in species composition, especially near
faunal boundaries (35). For corals, range
boundaries of extant coral species in the
warm Late Pleistocene extended up to 500
km further south along the western Australia
coastline (to 33°S) than they do today (37).
Closer to the center of their geographic range,
however, coral diversity and species presence
or absence in eastern Papua New Guinea
changed remarkably little during nine reef-
building intervals from 125 to 30 ka (31). On
a regional scale, these same species under-
went dramatic changes in distribution and
abundance as Quaternary glacial-interglacial
cycles caused sea level to repeatedly flood
and drain from continental shelves and oce-
anic islands (38). Many marine species ex-
hibit a genetic legacy of these range shifts,
local extinctions and expansions, and the
marked population fluctuations caused by
past climatic variation (29, 39, 40). Based on
this past history, we can expect regional and
global-scale disruption to coral reefs due to
climate change to accelerate markedly in
coming decades. Already, relative abundanc-
es of corals and of other organisms are chang-
ing rapidly in response to the filtering effect
of differential mortality (from bleaching and
other, more local human impacts) and differ-
ences in rates of recovery of species from
recurrent mortality events (16, 17, 41, 42).
There are two major differences, however,
between current climate-driven changes and
the recent past. First, because the oceans
today are already at a high sea-level stand, the
projected rise [0.1 to 0.9 m in the next 100
years (10)] will be very small compared with
sea-level changes during the Pleistocene.
Second, unlike the past, the response of reef-
dwelling species to projected climatic trends
will be profoundly influenced by people. As
outlined below, human impacts and the in-
creased fragmentation of coral reef habitat
have preconditioned reefs, undermining reef
resilience and making them much more sus-
ceptible to future climate change.
Managing Coral Reef Resilience
Clearly, the capacity of coral reef ecosystems to
continue to generate the valuable goods and
services (on which human welfare depends) has
to be better understood and more actively man-
aged. Sustaining this capacity requires im-
proved protection of coral reef resilience (43).
Marine protected areas (MPAs) are currently
the best management tool for conserving coral
reefs and many other marine systems (44, 45).
MPAs range from ineffective paper parks, to
multiple-use areas with varying degrees of
protection, to marine reserves, or no-take
areas (NTAs). NTAs provide the most ef-
fective protection for extractive activities
such as fishing, affording a spatial refuge
for a portion of the stock from which larvae
and adults can disperse to adjoining ex-
ploited areas (44, 45).
NTAs, when properly supported and po-
liced, are effective in preserving fish stocks
because they change human behavior. They
do not, however, prevent or hold back warm
water, or stop bleaching. For example, in
1998, the biggest and most destructive
bleaching event to date killed an estimated
16% of the worlds corals, including reefs in
the western Pacific, Australia, and Indian
Ocean that are widely regarded as the best
managed and most pristinein the world (2).
Fig. 3. (A) Differential bleaching responses of nine species of corals in Raiatea, French Polynesia,
during May 2002. (a, Acropora anthocercis;b,A. retusa;c,Montipora tuberculosa;d,Pocillopora
verrucosa;e,M. caliculata;f,Leptastrea transversa;g,P. eydouxi;h,P. meandrina;i,L. bewickensis;
j, Porites lobata;k,L. purpurea.(B) Latitudinal extents of Indo-Pacific reef corals, measured from the
northern- and southernmost point in their range. (C) Geographic range boundaries of 24 species of
Indo-Pacific corals found in the Persian Gulf and at Lord Howe Island off Australia, where average
maximum summer temperatures differ by 12°C. The coloring shows temperatures in the Southern
Hemisphere summer of 1997/1978, when unprecedented mass bleaching occurred (2).
If NTAs do not provide a refuge from bleach-
ing, then how can they help protect coral
reefs from climate change? Overfishing, par-
ticularly of herbivorous parrotfish and sur-
geonfish, affects more than just the size of
harvestable stocksit alters the entire dy-
namics of a reef (35, 46 ). Reduced her-
bivory from overfishing, increased levels of
disease, and excess nutrients can impair the
resilience of corals and prevent their recovery
following acute-disturbance events like cy-
clones or bleaching, leading instead to a
phase shift to algal-dominated reefs (Fig. 4, D
to F). Resilience is also eroded by chronic
human impacts that cause persistently elevat-
ed rates of mortality and reduced recruitment
of larvae (7, 12, 41, 43).
Although climate change is by definition a
global issue, local conservation efforts can
greatly help in maintaining and enhancing
resilience and in limiting the longer-term
damage from bleaching and related human
impacts. Managing coral reef resilience
through a network of NTAs, integrated with
management of surrounding areas, is clearly
essential to any workable solution. This re-
quires a strong focus on reducing pollution,
protecting food webs, and managing key
functional groups (such as reef constructors,
herbivores, and bioeroders) as insurance for
sustainability (7, 46).
NTAs also act to spread risk, whereby
areas that escape damage can act as sources
of larvae to aid recovery of nearby affected
areas (47 ). This highly desirable property of
NTAs raises the issue of how close they need
to be to promote connectivitythe migration
of larvae and/or adultsbetween them (44,
45). Critically, coral reef organisms, includ-
ing different species of corals, vary greatly in
their larval biology and potential for dispersal
(22, 29). The clear implication is that NTAs
must be substantially more numerous and
closer together than they are currently to
protect species with limited dispersal capabil-
ities. Furthermore, isolated reefs that are
largely self-seeding are unlikely to be pro-
tected by distant NTAs, and therefore will be
much less resilient to climate change.
Research and Management Challenges
Coral reefs are highly productive hotspots of
biodiversity that support social and economic
development. Their protection, therefore, is a
socioeconomic imperative, as well as an en-
vironmental one. Global warming, coupled
with preexisting human impacts, is a grave
threat that has already caused substantial
damage. However, the available evidence in-
dicates that, at a global scale, reefs will un-
dergo major changes in response to climate
change rather than disappear entirely.
There is, nonetheless, great uncertainty
whether the present economic and social ca-
pacity of coral reefs can be maintained. To
limit the damage, emerging management
strategies based on greatly expanded net-
works of NTAs, coupled with stronger pro-
tection of adjacent habitats, need to be vigor-
ously implemented. NTAs are unlikely to
prevent mortality of corals from bleaching,
but they will facilitate a partial recovery of
reefs that are reconfigured and populated by a
subset of resistant species and genotypes.
NTAs are not a panacea; their implementa-
tion needs to be complemented by heightened
protection of adjacent areas and by strong
international policy decisions to reduce the
rate of global warming.
Research in support of reef management
urgently needs to increase the scale of experi-
ments, sampling, and modeling to match the
scale of impacts and key biological processes
(e.g., dispersal, bleaching, and overfishing) and
go beyond the current emphasis on routine
monitoring and mapping. Indeed, most coral
reef research is parochial and short-term, and
provides little insight into global or longer-term
changes. For example, current knowledge of
biogeographic-scale patterns on reefs is based
on species presence or absence at local sites and
pays scant attention to temporal, regional, or
global patterns of relative abundance or func-
tional attributes of species (48) that could be
exploited for management of resilience. Simi-
larly, studies of intergenerational (genetic) re-
sponses to climate change (28) are urgently
needed for reef organisms, particularly corals
and zooxanthellae. Another crucial area for fu-
ture work is genetic dissection of population
structure and modeling of connectivity, which
could incorporate many of the unusual life-
history traits of clonal organisms, selection co-
efficients based on mortality from bleaching,
and experimental measurements of heritabili-
ties. Emerging research on marine reserves and
how they work to protect harvested stocks and
Fig. 4. (A) Aquarium fish, such as Anampses lennardi from northwest Australia, are often
endemic species and susceptible to overharvesting. (B) A bleached colony of Acropora nasuta
(bottom), and unbleached Pocillopora meandrina (top), showing contrasting responses to
thermal stress. (C) A bleached monospecific stand of the staghorn coral Acropora formosa.(D)
Parrotfishes, such as Scarus ferrugineus, are important herbivores. (E) Parrotfish grazing-scars.
(F) Macroalgae (top) and overgrowth of corals (bottom) are promoted by overfishing of
herbivores and degraded water quality.
15 AUGUST 2003 VOL 301 SCIENCE www.sciencemag.org932
spread risk (44, 45) also needs to be expanded
and applied specifically to the tropics. These
approaches must be integrated with socioeco-
nomic aspects of coral reef resilience, incorpo-
rating adaptive management systems that oper-
ate locally, regionally, and globally.
International integration and scaling-up of
reef management is an urgent priority (2).
Ecological modeling studies indicate that, de-
pending on the level of exploitation outside
NTAs, at least 30% of the worlds coral reefs
should be NTAs to ensure long-term protec-
tion and maximum sustainable yield of ex-
ploited stocks (49, 50). Yet, even in affluent
countries, such as the United States and Aus-
tralia, less than 5% of reefs today are NTAs.
Wealthy countries have an obligation to take
the lead in increasing the proportion of reefs
that are NTAs, while simultaneously control-
ling greenhouse-gas emissions.
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51. We thank James Cook University and The Queens-
land Government for funding a meeting of the au-
thors, and A. Green, N. Knowlton, and B. Willis for
providing comments on a draft. This is contribution
No. 078 of the Centre for Coral Reef Biodiversity at
James Cook University.
... Further, thermal stress has degraded corals' color rapidly, which has affected the corals through bleaching effects. Because of these environmental changes, nearly 30% of coral-reefs have suffered, which may increase to 60% by 2030 [6][7][8]. ...
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Coral-reefs are a significant species in marine life, which are affected by multiple diseases due to the stress and variation in heat under the impact of the ocean. The autonomous monitoring and detection of coral health are crucial for researchers to protect it at an early stage. The detection of coral diseases is a difficult task due to the inadequate coral-reef datasets. Therefore, we have developed a coral-reef benchmark dataset and proposed a Multi-scale Attention Feature Fusion Network (MAFFN) as a neck part of the YOLOv5′s network, called “MAFFN_YOLOv5”. The MAFFN_YOLOv5 model outperforms the state-of-the-art object detectors, such as YOLOv5, YOLOX, and YOLOR, by improving the detection accuracy to 8.64%, 3.78%, and 18.05%, respectively, based on the mean average precision (mAP@.5), and 7.8%, 3.72%, and 17.87%, respectively, based on the mAP@.5:.95. Consequently, we have tested a hardware-based deep neural network for the detection of coral-reef health.
... In ecology, biodiversity has been the subject of extensive study, and has been shown to affect the stability, health, ecosystem processes, and overall performance of ecosystems (Naeem et al., 1994;Loreau et al., 2002;Tilman et al., 2006). Moreover, the implementation of new technologies (Mora et al., 2011) and the rapid changes that many ecosystems are experiencing (Hughes et al., 2003;Hughes et al., 2007) emphasize the need to further examine biodiversity worldwide, especially in the tropics (Paulay, 2003). Macroalgae are an integral component of tropical reefs and contribute significantly to the biodiversity of tropical reef ecosystems in the Pacific (Vroom, 2011). ...
... The last IPCC report (2019) estimated that up to 99% of corals and reefs may disappear with +2°C of global warming before the end of the century if nothing is done to both reduce considerably CO 2 emissions into the atmosphere and local disturbances (see also Perry et al., 2014;Schönberg et al., 2017;Eyre et al., 2018;Tribollet et al., 2019). Nevertheless, to better predict the fate of coral reefs there is still a crucial need to understand the long-term dynamics of reef bioerosion processes, especially that of biogenic dissolution of carbonates by microboring flora as it is one of the main processes of reef dissolution (Schönberg et al., 2017;Tribollet et al., 2019) and the ability of corals to adapt and to be resilient to changes owing to their microbiome (Hughes et al., 2003;Ainsworth et al., 2017;McManus et al., 2021). To date, only a few bioerosion studies focused on the effects of hypersedimentation, eutrophication and ocean acidification and warming on microboring communities colonizing dead carbonate substrates (mostly dead corals) over short periods, i.e. over a few months or years (Carreiro-Silva et al., 2005;Tribollet, 2008a;Tribollet et al., 2009;Reyes-Nivia et al., 2013;Grange et al., 2015;Enochs et al., 2016;Tribollet et al., 2019). ...
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Coral reefs are increasingly in jeopardy due to global changes affecting both reef accretion and bioerosion processes. Bioerosion processes dynamics in dead reef carbonates under various environmental conditions are relatively well understood but only over a short-term limiting projections of coral reef evolution by 2100. It is thus essential to monitor and understand bioerosion processes over the long term. Here we studied the assemblage of traces of microborers in a coral core of a massive Diploastrea sp. from Mayotte, allowing us to explore the variability of its specific composition, distribution, and abundance between 1964 and 2018. Observations of microborer traces were realized under a scanning electron microscope (SEM). The area of coral skeleton sections colonized by microborers (a proxy of their abundance) was estimated based on an innovative machine learning approach. This new method with 93% accuracy allowed analyzing rapidly more than a thousand SEM images. Our results showed an important shift in the trace assemblage composition that occurred in 1985, and a loss of 90% of microborer traces over the last five decades. Our data also showed a strong positive correlation between microborer trace abundance and the coral bulk density, this latter being particularly affected by the interannual variation of temperature and cumulative insolation. Although various combined environmental factors certainly had direct and/or indirect effects on microboring species before and after the breakpoint in 1985, we suggest that rising sea surface temperature, rainfall, and the loss of light over time were the main factors driving the observed trace assemblage change and decline in microborer abundance. In addition, the interannual variability of sea surface temperature and instantaneous maximum wind speed appeared to influence greatly the occurrence of green bands. We thus stress the importance to study more coral cores to confirm the decadal trends observed in the Diploastrea sp. from Mayotte and to better identify the main factors influencing microboring communities, as the decrease of their abundance in living massive stress tolerant corals may have important consequences on their resilience.
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Coral reefs worldwide are at risk due to climate change. Coral bleaching is becoming increasingly common and corals that survive bleaching events can suffer from temporary reproductive failure for several years. While water temperature is a key driver in causing coral bleaching, other environmental factors are involved, such as solar radiation. We investigated the individual and combined effects of temperature, photosynthetically active radiation (PAR), and ultraviolet radiation (UVR) on the spawning patterns and reproductive physiology of the Hawaiian mushroom coral Lobactis scutaria, using long-term experiments in aquaria. We examined effects on spawning timing, fertilisation success, and gamete physiology. Both warmer temperatures and filtering UVR altered the timing of spawning. Warmer temperatures caused a drop in fertilisation success. Warmer temperatures and higher PAR both negatively affected sperm and egg physiology. These results are concerning for the mushroom coral L. scutaria and similar reproductive data are urgently needed to predict future reproductive trends in other species. Nonetheless, thermal stress from global climate change will need to be adequately addressed to ensure the survival of reef-building corals in their natural environment throughout the next century and beyond. Until then, reproduction is likely to be increasingly impaired in a growing number of coral species.
With a high species diversity of coral reefs, Wuzhizhou (WZZ) is a tropical marine ranch built by China. In the present study, we investigated the structure and function of coral reef food webs on WZZ in Sanya to understand how climate change and human activities affected coral reef ecosystems. Using carbon and nitrogen stable isotopes and the Bayesian mixture models, we assessed the major organic matter pathways and causes of seasonal variation in coral reef fauna and isotope values to delineate ecosystem function. Macroalgae and benthic microalgae were the primary food sources feeding this food web, while there were also some notable seasonal variations. Nutrient changes caused by temperature, upwelling, and tropical cyclones might be the primary reasons for the changes in consumer community composition and carbon or nitrogen isotopes. Seasonal environmental changes did not affect the stability of the nutrient structure of the coral reef food web on WZZ.
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Silkworms have limited ability to regulate their body temperature; therefore, environmental changes, such as global warming, can adversely affect their viability. Polyamines have shown protection to various organisms against heat stress. This study evaluated the qualitative and quantitative changes in heat-stressed Bombyx mori larvae polyamines. Fifth instar Bombyx mori larvae were divided into two groups; control group, reared at room temperature, i.e., 28 ± 2 °C, and the heat shock group, exposed to 40 °C. Dansylation of the whole worm polyamines and subsequent thin-layer chromatography revealed the presence of components with the same Rf value as dansyl–putrescine, spermidine, and spermine. The dansyl–putrescine, spermidine, and spermine polyamines were identified by mass spectrometric analyses. After heat shock, the thin-layer chromatography of the whole-larvae tissue extracts showed qualitative and quantitative changes in dansylated polyamines. A new polyamine, caldopentamine, was identified, which showed elevated levels in heat-stressed larvae. This polyamine could play a role in helping the larvae tolerate various stress, including thermal stress. No significant changes in silk fiber’s economic and mechanical properties were observed in our study. This study indicated that PA, caldopentamine, supplementation could improve heat-stress tolerance in Bombyx mori.
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Sea-level fluctuations continually alter the distribution and nature of shallow-water environments, although not all habitats are equally affected. Shallow-water habitats on coral reefs around oceanic islands can be divided into markedly different inner-and outer-reef systems. During regressions, the former are stranded while the latter persist. Previously I showed that numerous species are restricted to inner-reef habitats; I predicted that these would undergo local extinction across most of the central Pacific Ocean during regressions, and would expand back into the region during high sea stands. An examination of the fossil record of bivalves on Niue and other central Pacific islands provides support for both of these hypotheses, and shows that the range of some inner-reef specialists can vary substantially among high sea stands. Despite such unstable ranges, limited data do not indicate higher global extinction rates for inner-reef specialists. Sea-level fluctuations can provide vicariant opportunities for speciation, but also impede the potential for geographic differentiation of populations of inner-reef specialist taxa, because the lifespan of insular populations is often limited to the duration of single high sea stands.
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One of the most intriguing questions in community ecology remains unanswered: Are ecological communities open assemblages with each species reacting individually to environmental change, or are they integrated units consisting of multispecies assemblages acting in concert? I address this question for marine organisms by examining the taxonomic composition and diversity of Indo-Pacific reef coral communities that have undergone repeated global change between 125 and 30 Ka (thousand years before present). Investigation of community constancy through time relies on two critical questions: (1) Are there significant differences in taxonomic composition among communities from different times? and if not, (2) Are the observed patterns in temporal similarity significantly different from expected patterns resulting from a random sampling of the available within-habitat species pool? Constancy in taxonomic composition and species richness of Pleistocene reef coral assemblages is maintained through a 95-k.y. interval in the raised reef terraces of the Huon Peninsula, Papua New Guinea. Fossil reef coral assemblages show limited membership in species composition despite repeated exposure to marked fluctuations in sea level (up to 120 m) and sea-surface temperatures (up to 6°). During the 95-k.y. interval, the reefs experienced nine cycles of perturbation and subsequent reassembly with similar species composition. Spatial differences in reef coral species composition were greater among the three study sites than among reefs of different ages. Thus local environmental parameters associated with riverine and terrestrial sources had a greater influence on reef coral composition than global climate and sea level changes. The ecological dynamics of reef communities from Papua New Guinea are in marked contrast to those of Quaternary terrestrial and level bottom marine communities which appear to show unlimited community membership on both larger and smaller time scales. Differences in community assembly among ecosystems mean either that coral reefs are fundamentally different or that different ecological patterns and processes are occurring at different temporal scales.
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Despite extensive research into the coral bleaching phenomena there are very few data which examine the population biology of affected species. These data are required in order to predict the capacity of corals to respond to environmental change. We monitored individual colonies of 4 common coral species for 8 mo following historically high sea-surface temperatures on the Great Barrier Reef in 1998 to compare their response to, and recovery from, thermal stress and to examine the effect of bleaching on growth and reproduction in 2 Acropora species. Platygyra daedalea and P. lobata colonies took longer to bleach, longer to recover and longer to die. In contrast, Acropora hyacinthus and A. millepora colonies bleached quickly and most had either recovered, or died, within 14 wk of the initial reports of bleaching. Whole colony mortality was high in A. hyacinth us (88%) and A. millepora (32%) and partial mortality rare. In contrast, most colonies of P. daedalea and P. lobata lost some tissue and few whole colonies died. The mean proportion of tissue lost per colony was 43 6.6% and 11 +/- 1.1% respectively. Consequently, observed hierarchies of species susceptibility will depend critically on the time since the onset of stress and must consider both whole and partial colony mortality. Colony mortality was highly dependent on visual estimates of the severity of bleaching but independent of size. Growth rates of Acropora colonies were highly variable and largely independent of the severity of bleaching. A. hyacinthus was more susceptible to bleaching than A. millepora with 45% of surviving colonies gravid compared to 88%. High whole-colony mortality combined with a reduction in the reproductive output of surviving Acropora suggests that recovery to former levels of abundance is likely to be slow.
This article identifies ecological goods and services of coral reef ecosystems, with special emphasis on how they are generated. Goods are divided into renewable resources and reef mining. Ecological services are classified into physical structure services, biotic services, biogeochemical services, information services, and social/cultural services. A review of economic valuation studies reveals that only a few of the goods and services of reefs have been captured. We synthesize current understanding of the relationships between ecological services and functional groups of species and biological communities of coral reefs in different regions of the world. The consequences of human impacts on coral reefs are also discussed, including loss of resilience, or buffer capacity. Such loss may impair the capacity for recovery of coral reefs and as a consequence the quality and quantity of their delivery of ecological goods and services. Conserving the capacity of reefs to generate essential services requires that they are managed as components of a larger seascape-landscape of which human activities are seen as integrated parts.
Genetic analyses of marine population structure often find only slight geo- graphic differentiation in species with high dispersal potential. Interpreting the significance of this slight genetic signal has been difficult because even mild genetic structure implies very limited demographic exchange between populations, but slight differentiation could also be due to sampling error. Examination of genetic isolation by distance, in which close populations are more similar than distant ones, has the potential to increase confidence in the significance of slight genetic differentiation. Simulations of one-dimensional stepping stone populations with particular larval dispersal regimes shows that isolation by distance is most obvious when comparing populations separated by 2-5 times the mean larval dispersal distance. Available data on fish and invertebrates can be calibrated with this simulation approach and suggest mean dispersal distances of 25-150 km. Design of marine reserve systems requires an understanding of larval transport in and out of reserves, whether reserves will be self-seeding, whether they will accumulate recruits from surrounding exploited areas, and whether reserve networks can exchange recruits. Direct measurements of mean larval dispersal are needed to understand connectivity in a reserve system, but such measurements are extremely difficult. Genetic patterns of isolation by distance have the potential to add to direct measurement of larval dispersal distance and can help set the appropriate geographic scales on which marine reserve systems will function well.
Composition and zonation of coral reef communities are unstable on the scale of human lifetimes, but stable on average over thousands to hundreds of thousands of years. Traditional small-scale ecological studies can miss community patterns which are obscured by the noise of short-term change. Tests for stability require monitoring of populations over many generations—millenia for long-lived corals and forest trees. The only recourse is the fossil record.
We compare and contrast the design of networks of marine reserves for two different, commonly stated goals: (1) maintaining high yield in fisheries and (2) conserving biodiversity, in an idealized setting using simple models. The models describe larval dis- persal over a system of evenly spaced reserves of equal size, assuming sedentary adults. We initially demonstrate that, since populations in reserve systems can be sustained either by covering a minimal fraction of the coast with small reserves or by covering a smaller fraction of the coast with few large reserves, cost considerations dictate that the conservation goal would be best met by reserves as large as practically possible. In contrast, the fisheries goal of maximizing yield requires maximizing larval export outside of reserves, which we show means that reserves should be as small as practically possible. Meeting the fisheries goal is ultimately more costly because it suggests a larger area of the coastline should be in reserves, but it also improves on conservation goals by enhancing sustainability for species dispersing longer distances.