The Decline of Caribbean Coral Reefs... and an unlikely Hero
Why should we care about coral reefs when the economy is failing? Dr Anjani Ganase coral reef ecologist has argued that coral reef health is indicative of the health of the ocean and the earth. She provides an update.
For those of us aware of coral reef declines in the Caribbean, it was a series of unfortunate events over the last 50 years. For those of us who are unaware, here is a quick recap.
Today the Caribbean reefs are a shadow of what they used to be during the 1960s and 70s. I learned to scuba dive 15 years ago; and back then the reefs were already severely compromised. Unique in every way, Caribbean coral reefs have been explored by famous scientists. William Beebe dived in Haiti; Jacques Cousteau in Belize and Guadeloupe and some say even Buccoo Reef in the 1950 – 60s. However, by the time Trinidad and Tobago began exploring and recording our own reefs in the 1980s, they were already devastated by diseases and natural disasters. During the late 1970s to mid 1980s, a strange new disease, now referred to as the White Band disease (a pathogenic bacteria) plagued two of the most important branching coral species (Elkhorn and Staghorn corals) throughout the Caribbean. In the years following, the skeletal remains of the branching species eroded to rubble. Now, it might be hard to believe that many of the shallow reefs of the Caribbean were once covered by thickets of branching corals.
After the loss of the branching corals came the mass die off of common sea urchins (Diadema antillarum) in the early 1980s. These sea urchins were thought to be the main grazers of algae on Caribbean coral reefs after the long history of overfishing had left reefs devoid of fish herbivores. Without the appropriate players to regulate reef health – that is to control the abundance of algae on the reef or encourage coral survival - many Caribbean reefs are on the edge. Any disturbance that results in significant coral death would tip the reef ecology from a coral dominant to an algae dominant reef. Traditionally, herbivorous fish and invertebrates such as the sea urchin graze away the seaweed clearing spaces for corals to attach and grow. Reefs throughout the Caribbean have also been shifting to algae dominance following hurricane disasters, coral bleaching events or other local disturbance. In addition, as seaweed covered reefs become more permanent, the community that once was dependent on the corals also begin to shift. The seaweed encourages the recruitment of marine sponges that feed off the nutrients emitted by the seaweeds, leaving fewer opportunities for corals to settle. Seaweeds emit chemicals that deter fish larvae whose natural habitat is coral reefs. To date, very few recoveries have taken place without human intervention and coral reef management.
However, there are some silver linings in what seems to be a very grim future for Caribbean coral reefs. A group of scientists in Florida have shown that there may be a substitute to the now rare sea urchins to assist in the mass grazing of seaweeds on Floridian coral reefs and it is the Caribbean king crab (Maguimithrax spinosissimus). Experiments along tracts of the Florida Keys inner reefs have shown that elevated populations of the crab at test sites were reducing the algae cover by 50 to 80 percent. Alongside the reduction of seaweed, there was a four-fold increase in juvenile corals present after one year of crab grazing. Especially heartwarming is the fact that the coral species settling on the reef consisted of important reef builders that grow into large, long-lived corals that provide habitat for an array of reef life. What this means is that these reefs were given a major jumpstart to long-term reef recovery. The recovery was not limited to corals, fish species on the experimentally grazed sites showed a three-fold increase in diversity and up to a five-fold increase in reef fish abundance when compared to reefs that were seaweed dominant sites not grazed by the king crab. All hail the king crab, M. spinosissimus!
While studies like this highlight what little we know about the secret lives of the organisms of our Caribbean coral reefs, much more needs to be done before we can consider the king crab a suitable tool for restoration. The king crab naturally occurs on most Caribbean reefs, but they are rare because they are gobbled up on when they are juveniles. As they grow into adults, they have a much better survival rate. The next steps in the research must confirm the universality of this response as both studies were conducted in Florida, while the reefs of the Caribbean are geographically quite variable.
How would this experiment perform say in the nutrient rich waters of Tobago? The other consideration is how we increase their number on a broad scale, and will there be unintended consequences to their increased numbers? Again, further experiments carried out in other locations and for longer periods may provide answers. Once further testing is carried out, it can join the portfolio of coral restoration strategies through the use of conserving and advancing herbivory of coral reefs, which already includes the protection of parrotfish and the farming of sea urchins in captivity for release into the wild to increase grazing on the reefs. Individually the improvements on herbivory on reefs is limited without legislative and enforced protection, and problematic because parrotfish are more picky eaters than we thought, and sea urchins are difficult to cultivate. However, the combined methods may provide more robust restoration techniques on the back of coral reef management. Whatever strategies we implement, we need to avoid the woes of the past, especially considering the diseases that plague Caribbean corals across the region, and the climate threats of the future. Needless to say, this is good news for Florida’s reefs, which have been diminished in many places.
Scientists are already thinking outside the box when it comes to coral recovery and restoration against climate change, by breeding corals to preserve genetic diversity and breeding climate resistant strains. There are also cases of successful gardening of the branching corals in order to recover reef habitat structure. We do this, despite knowing that a single bleaching event or major storm event can wipe out all the efforts in an instant. More than 90 percent of the global coral reefs will die by 2050 if we do not drastically curb carbon emissions.
In Tobago, we’ve lost about 50 percent of our corals in the last ten years. Nevertheless, saving Caribbean reefs is becoming more and more urgent in light of regional and global inertia to protect what we cannot see. While coral reef scientists are willing to keep the coral lifeline going, by the time governments get their act together, there may be nothing left to save – of coral reefs or economies.
The Caribbean king crab, Maguimithrax spinosissimus (Lamarck, 1818) observed in United States of America by craigjhowe (licensed under http://creativecommons.org/licenses/by-nc-nd/4.0/)
References:
Spadaro, Angelo Jason, and Mark J. Butler IV. "Herbivorous Crabs Reverse the Seaweed Dilemma on Coral Reefs." Current Biology (2020).
Mumby, Peter J. "Ecology: Returning Caribbean Coral Reefs to Their Former Glory." Current Biology 31.4 (2021): R188-R190
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