Corals have always been under attack by Mother Nature, but they have also always been able to withstand these threats. Now, however, corals may be becoming more susceptible to natural threats as human activity creates additional stressors that weaken the health of the coral animals.
> Natural Threats to Coral Reefs <
Storms are a regular event in the world's oceans, and the waves they create can easily smash fragile coral formations. In addition, heavy coastal rains can wash excessive amounts of sediment, along with freshwater, into the clear saltwater the corals need for optimum health.
Just as earthquakes can cause buildings to collapse on land, they can cause corals to shatter or buckle and crack. Corals already weakened by human activity may be less able to recover from these shocks.
Predators exist in all ecosystems, but some are capable of destroying entire populations of prey if left unchecked. The crown-of-thorns starfish is one such predator, which can quickly destroy all the living coral on a reef. Invasions of this voracious creature have become regular occurrences on some reefs.
Most coral diseases were unknown until a few decades ago, but now new diseases seem to be cropping up with frightening regularity. Scientists know very little about the cause of most coral diseases, but bacteria, fungi and some algae may be to blame for much of the disease.
> Human Threats to Coral Reefs <
Overfishing disrupts the natural ecosystem and the balance between the residents of a reef. There are fewer fish, and those that remain are smaller juveniles. Overfishing may be the reason the crown-of-thorns is able cause such destruction. Humans have removed most of its natural predators.
In addition, when fish become scarce, fishermen turn to more destructive techniques to try to increase their catch. Some fishermen, especially in Southeast Asia, use explosives to kill fish within a certain area. Unfortunately, the explosion also damages nearby coral, which may not recover for years. With repeated blasts, it may become impossible for the coral to ever recover.
Agricultural runoff and untreated sewage making its way into the water increases the nutrient content of the water. Increased nitrogen concentrations, especially, cause sudden increases in algal growth, called an algal bloom. Depending on the type of algae involved, this overgrowth may block out sunlight, necessary for photosynthesis, or it may simply cover the algae, choking it.
Oil spills can suffocate existing corals as well as cause long-lasting pollution that impedes growth and reproduction. Early stages of the coral life-cycle are especially sensitive to oil, so oil spills during the coral's reproductive season are potentially more damaging than at other times.
* Global Warming
Corals are very sensitive to increased water temperatures. When the temperature stays even one degree above their preferred range for more than a day or two, the corals expel their symbiotic algae, resulting in the phenomenon known as coral bleaching. Corals will slowly starve to death if temperatures remain warm and the algae are not able to return.
In addition to the immediate effects of warmer temperatures, the increased atmospheric carbon dioxide is slowly acidifying the oceans. Acidic waters reduce the availability of carbonates, which corals use to build their limestone skeletons. The acidity may also be dissolving some of the existing skeletons, weakening them and making them more vulnerable to other assaults, such as storms or explosion fishing.
Corals grow best in clear waters, but coastal development causes erosion and increased sediment flowing into the ocean. These sediments can block vital sunlight as well as directly smother the corals if too much settles onto the reef.
Mangrove trees, which often grow in coastal waters, protect coral reefs by filtering sediment from the water before it reaches the reef. However, mangrove stands are often removed or greatly thinned during coastal development. This, too, allows excess sediment to reach the reefs.
While there is nothing we can do about Mother Nature, we can modify our own behavior to minimize our impact on coral reef ecosystems. By keeping the reefs healthy we can help to ensure their continued survival.