Richard B. Aronson

One-third of reef-building corals face elevated extinction risk from climate change and local impacts

The conservation status of 845 zooxanthellate reef-building coral species was assessed by using International Union for Conservation of Nature Red List Criteria. Of the 704 species that could be assigned conservation status, 32.8% are in categories with elevated risk of extinction. Declines in abundance are associated with bleaching and diseases driven by elevated sea surface temperatures, with extinction risk further exacerbated by local-scale anthropogenic disturbances. The proportion of corals threatened with extinction has increased dramatically in recent decades and exceeds that of most terrestrial groups. The Caribbean has the largest proportion of corals in high extinction risk categories, whereas the Coral Triangle (western Pacific) has the highest proportion of species in all categories of elevated extinction risk. Our results emphasize the widespread plight of coral reefs and the urgent need to enact conservation measures.

White-band disease and the changing face of Caribbean coral reefs

In recent decades, the cover of fleshy macroalgae has increased and coral cover has decreased on most Caribbean reefs. Coral mortality precipitated this transition, and the accumulation of macroalgal biomass has been enhanced by decreased herbivory and increased nutrient input. Populations of Acropora palmata (elkhorn coral) and A. cervicornis (staghorn coral), two of the most important framework-building species, have died throughout the Caribbean, substantially reducing coral cover and providing substratum for algal growth. Hurricanes have devastated local populations of Acropora spp. over the past 20–25 years, but white-band disease, a putative bacterial syndrome specific to the genus Acropora, has been a more significant source of mortality over large areas of the Caribbean region.Paleontological data suggest that the regional Acropora kill is without precedent in the late Holocene. In Belize, A. cervicornis was the primary ecological and geological constituent of reefs in the central shelf lagoon until the mid-1980s. After constructing reef framework for thousands of years, A. cervicornis was virtually eliminated from the area over a ten-year period. Evidence from other parts of the Caribbean supports the hypothesis of continuous Holocene accumulation and recent mass mortality of Acropora spp. Prospects are poor for the rapid recovery of A. cervicornis, because its reproductive strategy emphasizes asexual fragmentation at the expense of dispersive sexual reproduction. A. palmata also relies on fragmentation, but this species has a higher rate of sexual recruitment than A. cervicornis. If the Acropora spp. do not recover, macroalgae will continue to dominate Caribbean reefs, accompanied by increased abundances of brooding corals, particularly Agaricia spp. and Porites spp. The outbreak of white-band disease has been coincident with increased human activity, and the possibility of a causal connection should be further investigated.

One Hundred Questions of Importance to the Conservation of Global Biological Diversity

We identified 100 scientific questions that, if answered, would have the greatest impact on conservation practice and policy. Representatives from 21 international organizations, regional sections and working groups of the Society for Conservation Biology, and 12 academics, from all continents except Antarctica, compiled 2291 questions of relevance to conservation of biological diversity worldwide. The questions were gathered from 761 individuals through workshops, email requests, and discussions. Voting by email to short-list questions, followed by a 2-day workshop, was used to derive the final list of 100 questions. Most of the final questions were derived through a process of modification and combination as the workshop progressed. The questions are divided into 12 sections: ecosystem functions and services, climate change, technological change, protected areas, ecosystem management and restoration, terrestrial ecosystems, marine ecosystems, freshwater ecosystems, species management, organizational systems and processes, societal context and change, and impacts of conservation interventions. We anticipate that these questions will help identify new directions for researchers and assist funders in directing funds.

The rising tide of ocean diseases: unsolved problems and research priorities

New studies have detected a rising number of reports of diseases in marine organisms such as corals, molluscs, turtles, mammals, and echinoderms over the past three decades. Despite the increasing disease load, microbiological, molecular, and theoretical tools for managing disease in the worlds oceans are under-developed. Review of the new developments in the study of these diseases identifies five major unsolved problems and priorities for future research: (1) detecting origins and reservoirs for marine diseases and tracing the flow of some new pathogens from land to sea; (2) documenting the longevity and host range of infectious stages; (3) evaluating the effect of greater taxonomic diversity of marine relative to terrestrial hosts and pathogens; (4) pinpointing the facilitating role of anthropogenic agents as incubators and conveyors of marine pathogens; (5) adapting epidemiological models to analysis of marine disease.

Conservation, precaution, and Caribbean reefs

Some authors argue that overfishing is an important reason that reef corals have declined in recent decades. Their reasoning is that overfishing removes herbivores, releasing macroalgae to overgrow and kill the corals. The evidence suggests, however, that global climate change and emergent marine diseases make a far greater contribution to coral mortality, and that macroalgae generally grow on the exposed skeletal surfaces of corals that are already dead. Macroalgal dominance, therefore, is an effect rather than a cause of coral mortality. Marine protected areas (MPAs), which are usually established to protect stocks of reef fish, foster populations of herbivorous fish under at least some circumstances. Increased herbivory can reduce algal cover, potentially accelerating the recovery of coral populations inside MPAs; however, establishing MPAs will have only a limited impact on coral recovery unless policymakers confront the accelerating negative effects of the global-scale sources of coral mortality.

Climate flickers and range shifts of reef corals

Staghorn coral (Acropora cervicornis) and elkhorn coral (Acropora palmata), are important reef builders in the Caribbean. In the early to middle Holocene (10 000–6000 years ago), when sea temperatures were warmer than today, Acropora-dominated reefs were common along the east coast of Florida as far north as Palm Beach County. The fossil record shows that the northern limits of these two cold-sensitive species subsequently contracted to Biscayne Bay, south of Miami, apparently as a result of climatic cooling. This response of the Acropora species to climate provides a context for interpreting recent shifts in their geographic distribution. Despite recent disease-induced mass mortalities throughout the Caribbean and western Atlantic, the two species are now re-expanding their ranges northward along the Florida Peninsula and into the northern Gulf of Mexico, coincident with increasing sea temperatures. In the face of continued global warming, the northernmost limit of this range expansion will ultimately be...

Stasis, biological disturbance, and community structure of a Holocene coral reef

Disturbances have drastically altered Caribbean coral reefs over the past two decades. Acropora cervicornis (staghorn coral), which predominated at intermediate depths (5-25 m) from the 1950s through the 1970s, has virtually disappeared from most reef environments. Other coral spe- cies have declined as well, and the cover of macroalgae has increased. In apparent contrast, fossil reef sequences suggest that the species composition and zonation of coral assemblages did not change during the Pleistocene and Holocene. One interpretation of these observations is that coral species persisted on Caribbean reefs for hundreds of thousands of years as components of tightly integrated communities, and that a rare or unique combination of disturbances led to the syn- chronous decline of A. cervicornis and other corals throughout the region. The hypotheses of (1) community integration and (2) a unique, recent community transition, were tested by ecological and paleoecological observations in the shelf lagoon of the Belizean Barrier Reef. The reef growing along the flanks of Channel Cay, a lagoonal shoal, was monitored by point counts along transects over a ten-year period (1986-95). This reef was covered primarily by A. cer- vicornis at 3-15 m depth until the late 1980s. After 1986, A. cervicornis experienced a mass mortality from White Band Disease, an epizootic of presumed bacterial origin. The cover of A. cervicornis dropped from -70% in 1986 to nearly 0% in 1993. Agaricia spp. (lettuce corals) responded oppor- tunistically to the availability of free space in the form of A. cervicornis skeletal rubble. Agaricia, which had been a minor constituent of the sessile biota (10% cover in 1986), replaced A. cervicornis as the most common occupant of space on the reef (56% cover in 1995). The percent cover of other coral species and macroalgae remained low throughout the ten-year period. Similar changes were observed on other reefs over an area of at least 250 km2. The Acropora-to-Agaricia transition left a clear signature in the sedimentary record. Trenches dug into the reef at Channel Cay revealed the accretion of a layer of Agaricia rubble with a mean thick- ness of 22 cm in the decade after 1986. Due to the unconsolidated, uncompacted nature of the reef sediments, evidence of previous Acropora-to-Agaricia transitions should have been visible in the fossil record as vertical accumulations of A. cervicornis branches interrupted by layers of imbricated Agaricia rubble. Coring studies at Channel Cay revealed that no other Agaricia layers were deposited during at least the past 3800 years; the recent transition was unique on a time scale of millennia. This result supports the contention that excursions from the Acropora-dominated situation are un- usual in the history of Channel Cay and nearby reefs. However, the dynamics of the transition do not support the community integration hypothesis for the Channel Cay reef, indicating instead that different coral taxa in this assemblage responded differently, or not at all, to a large-scale biotic disturbance. The community transition also underscores the potential for biological factors in gen- eral, and disease in particular, to alter the composition of ecological communities and their sedi- mentary remains.

Florida's mystery coral-killer identified

An unusual coral disease appeared on the Florida Reef Tract in June 1995. It was distinct in its microbiology, its pattern of tissue degradation, the species susceptible to it, and its regional distribution. Symptoms included a sharp line between healthy and diseased tissue, as occurs with other coral diseases, but the pathogen responsible for the new outbreak seemed more virulent, affected a wider variety of species, and destroyed tissue much more rapidly than these other ‘line’ or ‘band’ diseases. We have identified the pathogen responsible for this new disease as a new species of Sphingomonas.

THE EXPANDING SCALE OF SPECIES TURNOVER EVENTS ON CORAL REEFS IN BELIZE

Beginning in the late 1980s, white-band disease nearly eliminated the stag- horn coral Acropora cervicornis from reefs in the central shelf lagoon of Belize. The lettuce coral Agaricia tenuifolia replaced Acropora cervicornis in the early 1990s, but anomalously high water temperatures in 1998 caused severe bleaching and catastrophic mortality of Agaricia tenuifolia. The short-lived transition in dominance from Acropora cervicornis to Agaricia tenuifolia left an unambiguous signature in the fossil record of these uncemented lagoonal reefs. Analysis of 38 cores, extracted from 22 sampling stations in a 375-km2 area of the central lagoon, showed that Acropora cervicornis dominated continuously for at least 3000 years prior to the recent events. Agaricia tenuifolia occasionally grew in small patches, but no coral-to-coral replacement sequence occurred over the entire area until the late 1980s. Within a decade, the scale of species turnover increased from tens of square meters or less to hundreds of square kilometers or more. This unprecedented increase in the scale of turnover events is rooted in the accelerating pace of ecological change on coral reefs at the regional level.

PHASE SHIFTS, ALTERNATIVE STATES, AND THE UNPRECEDENTED CONVERGENCE OF TWO REEF SYSTEMS

Initial conditions can generate differences in the biotic composition of spatially disjunct communities, but intense, large-scale perturbations have the potential to reduce or eliminate those historical differences. The latter possibility is of particular concern with respect to coral reefs, which have undergone dramatic changes in the last 25–30 years. This paper reports a case in which two reef systems with different biotic histories were recently perturbed to a single, novel state. We compared millennial-scale records of species dominance from reefs in Bahia Almirante, a coastal lagoon in northwestern Panama, to previously published records from reefs in the shelf lagoon of Belize. Reef cores extracted from Bahia Almirante at 5–10 m water depth revealed that the Panamanian reefs were persistently dissimilar from the Belizean reefs for at least 2000–3000 years prior to the last several decades. The Panamanian reefs were dominated continuously by branching finger corals, Porites spp. (primarily P. furcata). Shifts from the Porites-dominated state to dominance by other coral species were rare, were restricted to small areas, and lasted for decades to centuries. The Belizean reefs were dominated continuously by the staghorn coral Acropora cervicornis in the same depth range during the same period. Excursions from the Acropora-dominated state were again rare and spatially localized. Populations of Ac. cervicornis in the Belizean lagoon were nearly extirpated by an outbreak of white-band disease in the late 1980s, and changes in water quality were apparently detrimental to branching Porites in Bahia Almirante in recent decades. These large-scale perturbations caused the two reef systems to converge on a third, historically unprecedented state: dominance by the lettuce coral Agaricia tenuifolia. Ag. tenuifolia possesses life-history attributes and environmental tolerances that enabled it to become dominant in both disturbed ecosystems. Although the two phase shifts to Ag. tenuifolia differed in both their general mechanisms and specific causes, they had the effect of eliminating the salient difference in benthic composition between the Panamanian and Belizean reefs. The changes in species composition thus obliterated the influence of several thousand years of reef history.