The Diadema Die-Out

This topic submitted by Sarah Ehrman ( ehrmanse@muohio.edu) at 3:49 AM on 6/7/08.

A view of our boat at Gaulin Reef in the Bahamas

Tropical Field Courses -Western Program-Miami University

Sea urchins have tremendously varying impacts around the world. In the Northern Pacific, for example, they are primarily responsible for highly destructive grazing disturbances in the kelp forests when their populations are not kept under control. However, in the Caribbean their grazing impacts seem to be crucial for maintaining the trophic dynamics in the coral reefs where they live. In the 1980Õs the pivotal role of the black sea urchin (Diadema antillarum) as a keystone herbivore was brought to light when a current-born pathogen nearly wiped out the entire population of sea urchins in the Western Atlantic.
Toward the beginning of the Ô80s, Diadema populations were booming on the Caribbean reefs. Suddenly in 1983, Diadema began to die off rapidly throughout the Western Atlantic. A study conducted just south of the Virgin Islands showed that the first mass mortality in that location occurred in February of 1984. There, and in many other locations across the Caribbean, sea urchin populations were reduced by 95-99% over all reef zones, as well as other habitats (Carpenter, 1990). The study also found that a second mass mortality took place in October of 1985 after a very slight recovery, though the mortality rate was much lower, about 50%. Interestingly, the urchins were not size selected in either mortality event, though as time progressed, average urchin diameter increased. The growth in average size suggests that little to no recruitment took place, probably because there were no juveniles.
The slight recovery of the sea urchin population between the two mortalities was discovered to be due to the movement of urchins from deeper to shallower waters, possibly explained by the fact that better food is available shallow waters. The decrease in urchin density decreased competition for these resources. Additionally, the second mortality may have been less effective than the first due to increased immunity to the pathogen. Despite these two factors, the populations showed very little recovery. The low recovery might be explained by the timing of the first mortality, which occurred just before the peak spawning season, thus virtually eliminating the next generation (Carpenter, 1990). Even as late as 2004, there were still no real signs of recovery (Lessios, 2004).
The cause of the ÒDiadema die-offÓ was later determined to be a pathogen spread by surface currents (Davidson, 1998). Essentially, the pathogen caused the sea urchins to die from the outside in, first losing their skin over their protective spines, then the spines themselves. Without spines, urchins are defenseless against their natural predators. The urchins that werenÕt consumed eventually suffered neural damage, and then physically came apart. This process took anywhere from a few days to a few weeks, and was detrimental to the Caribbean sea urchins.
The sudden loss of the huge Diadema population produced noticeable effects on the coral reefs almost immediately. According to one paper by Robert Carpenter, within 5 days of the mortality occurring at St. Croix, U.S. Virgin Islands, the algal biomass on the reef increased by 20% (1988). Despite the increase in photosynthetic biomass, the paper noted that primary productivity decreased by 37% per unit area and by 61% per unit of algal biomass. Additionally, herbivory decreased by 51%. The decrease in herbivory can be explained by the loss of the Diadema populations, but why would the productivity decrease in the event of a gain in photosynthetic biomass?
Two years later, Carpenter answered this question when he published another paper with more detailed findings on the algae. He found that within the first two months of the mortality, not only did the changes mentioned above occur, but also there were rapid changes in algal species composition. Before the mass mortality, nearly 100% of the algal species in the reef were algal turfs and crustose algae. After the mortality, there was a 47% shift to macroalgae species, which were previously rare in the region (ÒLong term effectsÓ, 1990). Normally, regardless of the species shift, an increase in photosynthetic biomass would increase net primary productivity (NPP). However, unlike the detritus produced by turf and crustose algae, the detritus produced by macroalgae is swept out of the reef system by ocean currents, resulting in a loss of dead biomass and a decrease in NPP.
In the same year, Carpenter also published a companion paper further examining the effects of the mass mortality on herbivory (focused on the Tague Bay in St. Croix). Back in 1988, he noted an immediate decrease in overall herbivory directly following the Diadema die-out. However, in 1990 he gathered data on herbivorous fish populations, specifically the families of Acanthuridae (surgeonfishes) and Scaridae (parrotfishes). Carpenter found that in the Caribbean-wide absence of the sea urchin and abundance of algal growth, these herbivorous fish populations dramatically increased in numbers, though they were still unable to keep up with the booming algal biomass (ÒPopulation densitiesÓ, 1990).
The rapid algal growth has been blamed by many for the overall decline in Caribbean coral reefs. In 2003, a paper published in Science provided a comprehensive look at coral populations. Researchers used a software system to compile data from 263 sites and 65 separate studies, and the results were astounding: between 1975 and 2000, overall coral cover in the Caribbean Sea was reduced from 50% to 10%, an 80% loss (Gardner et. al., 2003). In some regions of the Caribbean, reefs have actually shown signs of recovery, but the decline in coral cover rapidly outpaces the gain.
The paper covered a whole gamut of possible explanations for the reef loss, ranging from local to global. The fact that coral reefs showed signs of recovery, according to the authors, is cause for guarded optimism, specifically because the recovering reefs showed a species shift from framework builders to sponges. On a global scale the authors speculated that with climate warming and weather pattern changes, which may bring more frequent storms, the sponges that dominate these new reefs will be more susceptible to damage and destruction. Also, other global stressors such as increased atmospheric carbon dioxide, which then dissolves into the ocean, and warmer temperatures may lead to coral bleaching (Gardner et. al., 2003).
Despite all these complicated global mechanisms, however, the authors concluded that the most significant reasons for the loss of coral reefs in the Caribbean were local: hurricanes, fisheries, and white band disease of Acropora corals being the three most important. Interestingly, the authors concluded that the changes in the algal community due to the mass mortality of sea urchins did not play as significant a role in coral reduction as those factors above, shown largely because the coral cover was already decreasing before the Diadema die-out occurred (Gardner et. al., 2003).
Still, there can be no doubt that the loss of the sea urchins has had a negative impact on the coral reefs, even if it plays just one small part in the grand scheme. If nothing else, CarpenterÕs data reflects this truth. From his conclusions, it can be inferred that because of the tremendous impact of the mass mortality on herbivory, algae, and by extension, corals, Diadema antillarum is probably a keystone herbivore. Many researchers, including Carpenter himself, agree on this point (ÒLong term effectsÓ, 1990), but they disagree on the role humans have played in the big picture.
In 2005, another group of researchers looked at how the same species of black sea urchin had different effects on the Eastern and Western Atlantic, and how human overfishing of herbivorous fish species in both cases have caused Diadema populations to have disproportionate effects on the environment. Diadema antillarum is endemic to both the ends of the Atlantic, though the populations have been genetically isolated for about 1.5 million years. Even at the same latitude, the Eastern and Western Atlantic are completely different environments, partly due to the temperature differences caused by the ocean currents; there are no coral reefs off the west coast of Africa. Instead, the Eastern Atlantic is dominated by macroalgae. These two different aquatic environments are examples of alternate stable states that are heavily influenced by the presence of sea urchins, and the authors compared the ecological functions of the urchins in each case (Tuya et. al., 2005).
In the Western Atlantic, the authors found that over the last thirty years, fisheries increased dramatically, thus decreasing the number of herbivorous fish. Because of this, echinoids like the sea urchins took a more dominant role as herbivores. When the mass mortality of urchins occurred, the lack of grazing pressure allowed macroalgae to increase in abundance. The corals then decreased because the macroalgae out-competed them for open space and inhibited their outward growth (Tuya et. al., 2005).
Another researcher argued that the human influence in the Western Atlantic goes back even further than overfishing in the last thirty years. He claimed that the elimination of green sea turtles (Chelonia mydas) from the Caribbean back in the days of Christopher Columbus caused herbivorous fishes to play an overly dominant role in algae control. Then, when overfishing reduced the populations of herbivorous fishes, sea urchins were forced into an overly dominant role. Finally when the sea urchins were eliminated, there was nothing for the reef systems to fall back on to restrict algal growth (Davidson, 1998).
In the Eastern Atlantic the scenario is completely different, though the human impacts are eerily similar. Due to the removal of too many top predators, a result of over-fishing, the sea urchins have become extremely abundant. In a classic example of a trophic cascade, the sea urchins then eliminate the dominant erect vegetation, macroalgae, and create barrens as an alternate stable state. The loss of physical structure and primary resources that the macroalgae provide limits the available food for a great number of species, which then go locally extinct. The region then becomes dominated by crustose algae, and lacks biodiversity (Tuya et. al., 2005).
Thus, the two sides of the Atlantic represent two opposite scenarios: the east has too many sea urchins, the west has too few. Before the mass mortality occurred, both had too many urchins due to either a reduction in the abundance of top predators, or a reduction in the amount of competition. Both the advocates of the sea turtle theory and these authors claim that human interference in these systems, resulting in the distortion of the food web, has reduced ecosystem resilience (Tuya et. al. 2005). The restoration of the original states on each side of the Atlantic may be possible, but that will involve a change in human influence on the systems.
A very recent paper published in 2007 suggests a way that humans might be able to at least help Diadema populations in the Caribbean recover via a density-dependent positive feedback system. Basically, as soon as urchins overcome the barrier set up by predation, recovery can truly start because a positive feedback system would bolster the process. Predation plays a key role in the number of juvenile sea urchins in a system, and therefore it indirectly determines the number of adults. Additionally, many sea urchin predators selectively feed on juveniles, who seek protection by the adults. The dramatic reduction in urchin populations has made it difficult for juveniles to find the protection of adults, simply because the density is so low. Once a population begins to recover, as shown experimentally, higher densities of adults lead to higher juvenile survival rates, which lead to higher densities of adults, and so on (Miller et. al., 2007).
By taking advantage of this knowledge, these researchers suggest that humans assist juveniles by placing them in patches with high densities of adults. While this may not solve every problem the coral reefs in the Caribbean are facing, it is one step in the right direction. It has taken the better part of thirty years to come up with this one seemingly simple solution, but the more we understand about how reef systems work, the better we will be able to recover what we have lost.

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