A few key species of coral have been essential to the survival of Caribbean coral reefs, according to scientists at the Natural History Museum.
Biodiversity is an important factor in the health of ecosystems. However, it is a small number of coral species that have allowed the reefs in the Caribbean to grow and become what they are today.
By analysing fossils, Museum palaeontologist (fossil expert) Ken Johnson and his team were able to look back at the 28-million-year history of the Caribbean coral reefs. During this time, there were many environmental changes on global and regional scales that had important impacts on coral reefs and associated biodiversity.
The team discovered past episodes of widespread extinction of corals and other reef animals in response to changing environmental conditions. The fossil record has revealed that different species lived at different times.
'More interestingly, our research revealed that during any particular time, the growth and development of ancient reefs did not depend on the diversity of the coral species living in the region,' says Johnson.
The survival of coral reefs in the Caribbean during a previous environmental crisis that occurred approximately 1.5 million years ago, was due to the response of a few species of staghorn and elkhorn corals that were adapted to the new environmental conditions.
Staghorn and elkhorn corals of the genus Acropora are among the dominant species living on modern Caribbean reefs. They are particularly fast growing and able to cope both with damage caused by hurricanes and by rapid changes in sea level.
A bicolour damselfish from the Caribbean seas
Although these corals have existed in the region for tens of millions of years, their dominance increased remarkably during the last few million years.
This ecological change coincided with a regional extinction event that included the demise of more than 50% of the coral species in the Caribbean.
The Acropora species were able to thrive in the new environmental conditions that had proved so deadly to the other species.
'Significantly, the importance of the staghorn and elkhorn corals on modern-day reefs could not have been predicted millions of years ago,' says Johnson, 'and modern Caribbean coral reefs might not exist if these kinds of corals had not been alive just before the crisis.'
'The relative importance of coral species varies as environments change through time,' adds Johnson. 'Biodiversity is therefore crucial to the long-term stability of coral reefs because future changes cannot be predicted. Systems with a diverse team of players are more likely to survive changing conditions.'
Coral reefs are some of the most diverse and productive ecosystems on the planet and are widespread throughout the tropical seas. The animals and plants living on coral reefs secrete and bind together limestone to form very large natural structures or reefs. These reefs are resistant to storm waves yet provide habitats for large numbers of marine life.
Corals are animals, related to sea anemones and jelly fish. They can thrive in tropical ocean habitats not suitable to other types of marine life. This is because they have large numbers of single-celled algae living inside their bodies that provide the coral animal with food, in a relationship that is known as symbiosis. The waste produced by the coral animal serves as fertiliser for the tiny plants.
Coral reefs are vulnerable to human-caused impacts such as over-fishing, which disrupts the balance of species in the ecosystem. Pollution is also a major threat to many coral reefs, particularly from increased run-off caused by agriculture and other development on land that can change ocean conditions and can cause an overgrowth of algae. Human waste, herbicides, and pesticides inadvertently released in the coastal zone also impact the health of coral reefs in many parts of the world.
Larger-scale climate changes such as global warming can also damage coral reefs. For example, as the temperature of sea water increases, the internal-plant-and-animal symbiosis, which provides much of the food required by corals to grow, breaks down. This leaves the coral animals looking bleached.
Besides the direct impacts of human development and changes in the global climate, ocean acidification is a newly recognised threat to coral reefs throughout the tropics.
Rapid injection of carbon dioxide into the atmosphere during the past century of industrial growth could overwhelm the ocean's natural ability to deal with carbon dioxide dissolved in surface water. The resulting acidification makes it difficult for corals and other reef organisms to secrete limestone skeletons.
This reduced growth would have serious consequences when combined with increasing sea levels, by compromising the ability of reefs to keep up with rapid sea-level change during the next few centuries.
Many experts call for more protection of coral reefs, such as by creating protected areas to conserve the diversity of the habitat, reducing pollution, over-fishing and tackling climate change.
'This insight from the fossil record shows how conserving biodiversity on living reefs can increase their resilience during the coming decades of rapid global-environmental-change,' concludes Johnson.
This research was reported in the journal Science in March.