OUR FRAGILE ECOSYSTEM

Sea and shore.

The clear warm waters surrounding St. John support a diverse and intriguing complex of coral reefs. The health of these reefs is closely tied to its component plants and animals as well as adjacent non-coral marine environments such as sandy bottoms, seagrass beds, and mangrove forests. Also the effects of waves and currents and the impacts of human use and nearby development directly effect the well-being of the reef. Here decisions the National Park Service makes regarding management of these resources and their human visitors in an important determining factor in what will happen to these important ecosystems in the future.

The term coral reef refers to a specific type of marine community-a functioning assemblage of a large variety of interacting organisms. Coral itself is made up of tiny soft animals called polyps. Hard corals such as brain, star, and elkhorn secrete calcium carbonate creating a cup around their soft bodies. Once established, polyps reproduce asexually by budding (making clones of themselves.) As the coral colony grows outward by budding it also grows upward; the polyps remain on the surface and continue to secrete a calcium carbonate base. Other animals such as worms and some fish break down, rearrange, and aid in cementing this base.

At night, close observers may see the coral polyps extend tentacles from their protective cups as they feed on planktonic food. An extensive variety of animals make their homes in the coral reef. Fish living and feeding in the reef include groupers, parrotfish, angelfish, damselfish, and wrasses. Invertebrates include sponges, urchins, bristle worms, mollusks, lobsters, and anemones in addition to the diverse species of hard and soft corals.

The growth of an individual coral colony is slow with many species averaging less than 1" each year. Faster growing elkhorn and staghorn coral can increase by 3-4" and 6-8" respectively. A specific set of environmental conditions is required to sustain healthy reef growth. Adequate sunlight, proper temperature, and the right nutrients are important to promote reef growth. Dependable sources of oxygen and planktonic food, the transportation function of currents, and proper salinity levels are also critical. Favorable temperature range is 70- 80 deg. F; optimum salinity level is 30-36 parts per thousand. Corals are very fragile and suffer when exposed to chemical pollution. Siltation caused by erosion in water runoff can smother corals and cause light deprivation that will eventually kill the coral. Human actions also damage coral. These actions can include improperly anchoring boats, touching the coral with hands or flippers, harvesting coral for the aquarium trade and development along shorelines and in harbors. Coral diseases are also taking their toll in many parts of the world, including the Atlantic and the Pacific. Scientific researchers are analyzing the amount of such stress reef colonies can endure. They have proven that coral reefs worldwide are in serious jeopardy.

The health of coral reefs also depends on the viability of surrounding ecosystems. Seagrass beds help stabilize sandy bottoms reducing suspended sediment. Red mangrove trees can grow directly out of shallow salt water and their maze of prop roots trap sediment runoff helping to keep offshore water clear. The mangrove areas in turn provide protection and create nutrient rich water serving as nurseries for juvenile fish and other marine organisms. As sediment fills the water around their roots, red mangroves move outward and white, black, and buttonwood mangroves begin to grow in the anaerobic mudflats left behind-stabilizing the shorelines in the process.

Terrestrial vegetation.

Over two hundred years of intensive agriculture and the introduction of many non-native species of plants has drastically altered the vegetation on St. John. Now that much of the island is protected within Virgin Islands National Park, various plant communities will have the opportunity to pass through successional stages toward more mature and balanced ecosystems. Extremes of rainfall at different locations on this small island as well as the effects of salt, wind, and varying soil types, have created several different terrestrial ecosystems.

Moist forest areas receiving up to 65" of rain annually are located along the island's north-central shoreline and at higher interior elevations. Primarily evergreen, canopy trees may exceed 75 feet in height and include bay rum, West Indian locust, sandbox, kapok, and hogplum. Shade-tolerant trees, shrubs, and vines like wild coffee, sweet lime, guavaberry, and hoop vine comprise the understory. Ferns, mosses, and bromeliads are also common.

Dry forests and arid scrublands grow on the eastern and southern portion of St. John and in some low-lying coastal areas. Dry forests include turpentine tree (gumbo limbo), black and water mampoo, acacia, and white cedar (pink poui). The slow-growing lignum vitae tree, once a dominant tree in the dry forest is almost non-existent now due to over-harvesting of its valuable wood. Shoreline stabilizers like seagrape, maho and seaside purslane grow in coastal areas. The poisonous manchineel tree can be found along rocky and sandy shorelines. Do not touch or eat its toxic leaves and fruits. Organ pipe and turks cap cacti and century plants grow in the very driest places such as Ram Head.