Threats to Aquatic Environments - Multiple Threats Combine

Most aquatic lifeforms are not imperiled by a single threat to their survival, but by multiple threats that combine to produce daunting challenges to recovery. The lethal combination of historical overfishing and habitat degradation is blamed for the problems of many endangered and threatened aquatic species. Habitat degradation can be caused by many factors, including dams, water pollution, and invasive species.

Overcrowding of stressed fish populations into smaller and smaller areas has contributed to hybridization—uncharacteristic mating between closely related species resulting in hybrid offspring. According to the U.S. Fish and Wildlife Service environmental degradation appears to inhibit natural reproductive instincts that historically prevented fish from mating outside their species. In addition, shortage of suitable space for spawning has resulted in more mating between species. Cross-mating can be extremely detrimental to imperiled species, because the offspring can be sterile.

Coral Reefs

Coral reefs are the largest living structures on Earth. Biologically, the richness of coral reef ecosystems is comparable to that of tropical rainforests. The reefs themselves are formed from calcium carbonate skeletons secreted by corals. Corals maintain a close relationship with certain species of photosynthetic algae, providing shelter to them and receiving nutrients in exchange. Most people are familiar with the colorful coral reefs found in coastal, tropical waters. (See Figure 4.13.) These reefs are located in relatively shallow waters, making them more susceptible to human activities. Only 1% to 2% of warm-water corals are found in U.S. waters. Most warm-water corals are located in the waters of the South Pacific and around Indonesia. In addition there are numerous cold-water reefs around the world that scientists are just beginning to learn about. These reefs are found in cold deep waters from depths of 100 feet to more than three miles.

According to An Ocean Blueprint for the 21st Century, one-third of coral reefs around the world are severely damaged and all U.S. warm-water reefs have been damaged to some degree. Coral reefs are imperiled by diseases and coastal development that spurs the growth of unfriendly algae. Coastal development also increases the danger of the reefs being trampled by divers and boat anchors. Other serious threats to reef ecosystems include marine pollution, blast fishing, and cyanide fishing. Collection of tropical reef specimens for the aquarium trade has also damaged a number of species. Perhaps the greatest immediate threat to coral reefs is rising water temperature due to global climate change. This has caused extensive coral bleaching in recent years. (See Figure 4.14.)

FIGURE 4.13 Locations of tropical coral reefs "Figure 21.1. Tropical Waters Are Home to the Majority of Known Reefs," in An Ocean Blueprint for the 21st Century, Final Report, U.S. Commission on Ocean Policy, 2004, http://www.oceancommission.gov/documents/full_color_rpt/000_ocean_full_report.pdf (accessed February 1, 2006)

FIGURE 4.14 Coral reefs are among the most diverse ecosystems in the world. They are also immediately threatened by global warming, which has caused unprecedented episodes of coral bleaching in recent years. (AP/Wide World Photos.)