Charles Birkeland

Life and death of coral reefs

Introduction. Reefs and reef limestones in earth history. Reefs as dynamic systems. Bioerosion and coral-reef growth: a dynamic balance. Interactions between corals and their symbiotic algae. Diseases of coral-reef organisims. Organic production and decomposition. Reproduction and recruitment in corals: critical links in the persistence of reefs. Invertebrate predators and grazers. Effects of reef fishes on coral and algae. Indirect interactions on the coral reefs. Geographic differences in ecological processes on coral reefs. Ecosystem Interactions in the tropical coastal seascape. Diversity and distribution of reef organisms. Disturbances to reefs in recent times. Traditional coral-reefs fisheries management. Resource use: conflicts and management solutions. Implications for resource management. References. Index.

Feeding Behavior of Asteroids and Escape Responses of their Prey in the Puget Sound Region

Observations were made with scuba on the diet and behavior of 18 species of undisturbed sea stars in their natural habitats along the shores of Washington state through all seasons. Some sea stars are specialists. Hippasteria spinosa feeds almost exclusively on a sea pen; Solaster stimpsoni eats holothurians; full—sized Orthasterias koehleri consume the venerid clam Humilaria; Solaster dawsoni eats its congener S. stimpsoni. Others show remarkably variable diets. In different habitats, Dermasterias imbricata specializes on either anemones, holothurians or sea pens, but within these habitats its diet is consistent throughout the year. The diet of Mediaster aequalis varies with both habitat and season. Pycnopodia helianthoides feeds on sea urchins on rocky substrata but digs clams from sand and cobble. Many other sea stars, including Luidia foliolata, Pteraster tesselatus, Pisater ochraceus, Evasterias troschelii and Leptasterias hexactis, are quite generalized in their diet and, though often demonstrating preferences in laboratory studies, will feed on a variety of prey determined largely by relative abundance of prey species in the particular habitat. The areal and seasonal variation in the diet of sea stars coupled with the reluctance of some to eat any prey in the laboratory makes extension of laboratory observations to the field diet suspect. Laboratory observations can be used to provide a more detailed understanding of field observations. Several previously undescribed behavioral mechanisms of food capture in asterioids and escape or defense responses or prey are described: Orthasterias pulls chips of shell off Humilaria until an opening is made, thus allowing exploitation of a prey species unavailable to other clam—eating asteroids. A number of sea stars, such as Luidia, Hippasteria, Mediaster, Pisaster brevispinus, Orthasterias and Pycnopodia dig into mud, sand or cobble substrata to obtain infaunal prey. The locomotory patterns of S. dawsoni and Crossaster papposus tend to allow encounter with their prey in such a way as to reduce the effectiveness of the escape response. A pushing response by Solaster stimpsoni and autotomy in Pycnopodia and Evasterias are thought to be effective responses to S. dawsoni predation. The similar swimming responses to two anemones, a nudibranch, and a holothurian are discussed in relation to asteroid predation. The avoidance responses that many invertebrates show to asteroids are correlated with predator—prey relationships. Two mechanisms may obscure this correlation. The very success of the response may effectively remove the responding organism from the predators diet. Thus scallops, although abundant, form a very inconspicuous portion of starfish diet. Also biochemical similarities between the predator and other organisms may make a prey species unable to distinguish only its predator. Thus Pycnopodia will move rapidly away from Crossaster and Solaster stimpsoni as well as from S. dawsoni, although the latter is its only asteroid predator. The similarities may be based either on a close taxonomic relationship with the predator, or on the diet of the other organism including species closely related to the responding prey species.

Geographic Differences in Ecological Processes on Coral Reefs

Although basic ecological processes are generally consistent on a local scale, there are geographic differences in geological processes, the physical environment, and dimensions of the areas involved that change the nature of coral-reef systems. The structure of reefs is affected by plate tectonics, e.g., there is a greater prevalence of atolls in the central Pacific and Indian Oceans than in the Atlantic or eastern Pacific. The vast dimensions of the Pacific Ocean produces a sharp gradient of decrease from west to east in species, generic, class, and habitat diversity across the Pacific that is not so pronounced in the smaller western Atlantic and Indian Oceans. Diseases are generally contained within distantly separated archipelagoes in the Pacific but quickly spread across the relatively small and interconnected greater Caribbean. The differences in sediment input into the oceans (2 % west coasts of continents, 82 % east coasts) and the degree to which river output affects biogeographic patterns (eastern vs western coasts of continents), show the overwhelming global influences of trade winds and the Coriolis effect on coral reefs. The relative prevalence of corals and other sessile photosymbiotic invertebrates compared to sessile heterotrophic invertebrates can be affected on a geographic scale as well as locally by the strength and dependability of nutrient input. Adult coral colonies can persevere in areas with substantial nutrient input such as upwelling, but recruitment in these areas is extraordinarily difficult in competition with algae and heterotrophic invertebrates in areas with abundant nutrients. The effects of disturbances and effects of altering the system, for example by reducing the stock of grazers, can be substantially greater in areas of high nutrient input.

Environmental versus genetic influences on growth rates of the corals Pocillopora eydouxi and Porites lobata (Anthozoa: Scleractinia)

ABSTRACT Reciprocal transplant experiments of the corals Pocillopora eydouxi Milne Edwards & Haime and Porites lobata Dana were carried out for an 18-month period from September 2004 to March 2006 between two back reef pools on Ofu Island, American Samoa, to test environmental versus genetic effects on skeletal growth rates. Skeletal growth of P. eydouxi showed environmental but not genetic effects, resulting in doubling of growth in Pool 300 compared with Pool 400. There were no environmental or genetic effects on skeletal growth of P. lobata. Pool 300 had more frequent and longer durations of elevated seawater temperatures than Pool 400, characteristics likely to decrease rather than increase skeletal growth. Pool 300 also had higher nutrient levels and flow velocities than Pool 400, characteristics that may increase skeletal growth. However, higher nutrient levels would be expected to increase skeletal growth in both species, but there was no difference between the pools in P. lobata growth. P. eydouxi is much more common in high-energy environments than P. lobata; thus the higher flow velocities in Pool 300 than in Pool 400 may have positively affected skeletal growth of P. eydouxi while not having a detectable effect on P. lobata. The greater skeletal growth of P. eydouxi in Pool 300 occurred despite the presence of clade D zooxanthellae in several source colonies in Pool 300, a genotype known to result in greater heat resistance but slower skeletal growth. Increased skeletal growth rates in higher water motion may provide P. eydouxi a competitive advantage in shallow, high-energy environments where competition for space is intense.

Dredging in the Spratly Islands: Gaining Land but Losing Reefs

Coral reefs on remote islands and atolls are less exposed to direct human stressors but are becoming increasingly vulnerable because of their development for geopolitical and military purposes. Here we document dredging and filling activities by countries in the South China Sea, where building new islands and channels on atolls is leading to considerable losses of, and perhaps irreversible damages to, unique coral reef ecosystems. Preventing similar damage across other reefs in the region necessitates the urgent development of cooperative management of disputed territories in the South China Sea. We suggest using the Antarctic Treaty as a positive precedent for such international cooperation.

Safety in Numbers? Abundance May Not Safeguard Corals from Increasing Carbon Dioxide

Marine conservation efforts are often focused on increasing stocks of species with low population abundances by reducing mortality or enhancing recruitment. However, global changes in climate and ocean chemistry are density-independent factors that can strongly affect corals whether they are scarce or abundant—sometimes, the abundant corals are most affected. Because reproductive corals are sessile, density-independent effects of global changes such as physiological stress and resultant mortality can decouple stock abundance from recruitment and may accelerate the downward spiral of their reproductive rates.

Coral Reefs in the Anthropocene

Although coral reefs cover only 0.00063 of the surface of Earth, they have had important effects on the atmosphere, ocean chemistry, the shape of the surface of Earth, the diversity of life, the biogeographic distribution of life, and they provide hundreds of billions of dollars in value per year in goods and services to tens of millions of humans. All the continents, islands and freshwater habitats of Earth together occupy more than 460 times the total surface area of coral reefs, yet host only 19 phyla while coral reefs host at least 30 phyla of animals. The per square meter value of coral reefs in goods and services has substantially increased since estimated in 1997, but the total value has decreased from loss of coral-reef habitat and stock of large fishes. Coral reef ecosystems in natural undisturbed states can be inverted trophic biomass pyramids with especially high primary production, but meagre yield or net production. Extractive commercial fishing is potentially sustainable if medium-sized individuals and not large individuals are taken. The net yield for human consumption can be increased by removing the upper trophic levels, but the system is more sustainable and beneficial for humans when managed as a service-based economy rather than an extraction-based economy. The present interglacial period (the Anthropocene) has been exceptionally favorable to coral reefs for thousands of years until the recent three or four decades, in which the living coral cover has abruptly declined about 53% in the western Atlantic, about 40% in the general Indo-Pacific, and about 50% on the Great Barrier Reef. Reefs are presently threatened by increasing CO2. Although there have been few, if any, extinctions, reefs are declining in topographic complexity and ecosystem services. This is most likely the trajectory for future decades and reflects the norm for much of the geologic history of coral reefs.

Biology Trumps Management: Feedbacks and Constraints of Life-History Traits

The geologic record suggests a diverse array of reef-building corals will survive increasing CO2, but the relative prevalence of different types will shift and the reefs will become degraded and eroded. Although many corals may not go extinct, if pH decreases effectively, reef ecosystem services will deteriorate because bioerosion will accelerate and, for some coral species, net skeletal construction will require more energy when the aragonite saturation state decreases. A similar pattern of many genera of reef-building scleractinian corals surviving, but with relatively little reef accretion, was seen through the roughly 140 million years from the Late Jurassic to the late Paleogene when the calcite seas (Mg/Ca mole ratio <2) and pH <7.8 were unfavorable for aragonitic reef accretion. The geologic record suggests that the corals most vulnerable to extinction were the fast-growing branching species because the traits that provide fast growth have tradeoffs with traits that provide tolerance of stressful environments. Iteroparous animals such as corals are adapted for survival under stressful conditions at the expense of fecundity. Surveys have recorded widespread decreases in living coral cover, but the less visible decrease in fecundity from stress may be more insidious to population recovery. Reduced fecundity and less dense population distribution can act synergistically to produce Allee effects in sessile animals such as corals. Natural coral-reef ecosystems give the appearance of inverted trophic pyramids, but when fished down by about 80 %, recovery has usually not happened, possibly because the larger individuals in the populations were a major source of fecundity. Although biomass of eukaryotes appear to be in inverted trophic pyramids, the turnover and energy is in the form of standard pyramids and although large individuals in the upper trophic levels are especially sensitive to exploitation, subsistence economies can be maintained by harvesting the medium-sized individuals. Large individuals matter more than population biomass because of the distinct roles of large individuals in ecological processes maintaining coral-reef ecosystems and the relatively large reproductive potential of big fishes. The functional traits of both the coral-reef ecosystem and its component animals provide a greater potential for exploitation by globalization in a service-based economy than with an extractive economy, as exemplified by Palau.