Human impacts on coral includes:

Coral reefs are threatened by human activities throughout the world. In southeast Asia, the hot spot of coral diversity, more than 80% of the reefs are at risk from coastal development and harmful fishing practices.

Pollution

Population growth and development threaten the survival of coral reefs.

Macroalgae. Photo courtesy South Florida Water Management District
Macroalgae. Photo courtesy South Florida Water Management District

Population growth and urban development currently rank among the greatest threats to coral reefs. Development activities cause erosion, resulting in the run-off of sediments which eventually reaching the reefs. Also stormwater runoff carries fertilizers and sewage into the ocean, damaging coral reefs. Increase of nutrient concentrations within the reef environment is followed by increase of algae, which may smother corals. Whether it is from direct sedimentation or an increase in turbidity from nutrient-loading, the amount of sunlight reaching the corals is reduced.

Overfishing

Overfishing has altered ecological dynamics of marine communities.

Shrimp boats. Photo courtesy NOAA
Shrimp boats. Photo courtesy NOAA

Along with human population growth, the harvest of resources from the sea is ever-increasing. Overfishing has changed the ecological dynamics of marine communities, allowing some organisms to dominate reefs that once were kept in check by large reef fish populations. Fishing practices have also become more destructive as fish populations decrease. In some regions, indiscriminate fish traps are used while in other areas the use of dynamite and cyanide have become common practice.

Tourism

Increase in tourism is responsible for increase in reef damage.

Recreational SCUBA diver. Photo courtesy NOAA
Recreational SCUBA diver. Photo courtesy NOAA

Coral reefs attract ecotourists, bringing much needed currency to tropical regions. Ironically, the increase in ecotourism is usually accompanied by an increase in reef damage. Divers and snorkelers may harm the corals by touching the polyps, while the boats transporting ecotourists to the reef may damage the reef by dragging anchors. Although ecotourism is responsible for reef damage, if it is carefully managed and monitored, ecotourism may actually help preserve these valuable habitats.

Aquarium and Souvenir Trade

Fish and invertebrates are harvested from the world’s coral reefs.

Marine collectibles. Photo © Jan Bester
Marine collectibles. Photo © Jan Bester

Many species of fish and invertebrates are harvested from the world’s coral reefs for the aquarium industry. Unfortunately, poisons such as cyanide are often used in the Indo-Pacific region, causing the deaths of many fish and surrounding corals. After the live fish are collected, up to half die as a result of poisoning or stress. Those that do survive have high mortality rates in home aquaria due to dietary and physical requirements. Corals and shells are also collected throughout the world and sold as souvenirs to tourists, made into jewelry, or exported as curios.

Global Climate Change

Rising sea temperatures cause coral bleaching.

Climate change research - data collection. Photo courtesy NOAA
Climate change research – data collection. Photo courtesy NOAA

Global climate change may cause increases in sea temperature and sea level as well as ocean current patterns, all of which can cause damage to reefs. Corals are very sensitive to water temperature changes, with rises in temperature resulting in coral bleaching episodes.

Natural impacts on corals include:

Hurricane cloud formation. Photo courtesy SeaWIFS/NASA
Hurricane cloud formation. Photo courtesy SeaWIFS/NASA

Storms

Storms and hurricanes may cause extensive damage to reef structures and communities.

Although much of coral reef destruction is blamed on human activity, natural disturbances are also capable of causing extensive damage to coral reefs. Strong waves caused by storms and hurricanes may smash into the reef, breaking up large corals and creating rubble fields. Storms also harm reefs indirectly by disrupting nearshore habitats, resulting in the release of large amounts of sediments and freshwater to come into contact with the reef. These disturbances open spaces for new organisms to colonize, preserving overall biodiversity.

Thermal Stress

Thermal stress can induce the loss of symbiotic algae from corals, often referred to as “coral bleaching.”

Coral bleaching. Photo courtesy U.S. Geological Survey
Coral bleaching. Photo courtesy U.S. Geological Survey

Heat-related stress often occurs during late summer months when the sun is intense. Corals can survive water temperatures as high as 86-95°F (30-35°C), depending upon the species. Increases in water temperature often causes loss of symbiotic algae from coral tissues, referred to as coral bleaching. Corals are also susceptible to cold stress at temperatures at or below 57°F (14°C). Cold water masses originate in Florida Bay, causing coral death in patch reefs long the water current’s path. However, most offshore corals experience little damage due to the mixing of the cold water current with the warm offshore waters, moderating temperatures and lessening the impact on surrounding reefs.

Diseases

Coral reef organisms, such as the long-spined sea urchin, are susceptible to many pathogens.

Sea Urchin (Diadema antillarum). Photo © James L. Van Tassell
Sea Urchin (Diadema antillarum). Photo © James L. Van Tassell

Along with coral bleaching, black band disease, and white band disease, coral reef organisms are subject to many pathogens. During the 1980s, a water-borne pathogen caused the massive die off of the long-spined sea urchin, Diadema antillarum. Long-spined sea urchin populations were reduced as much as 95% in some locations. This urchin is an important herbivore, keeping algae growth in control in reef habitats. The decline in sea urchin populations resulted in algal overgrowth throughout the reefs of the Caribbean.

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