Education Themes: Island Ecosystems

Boston Harbor is an "estuary" system where the salt water of Massachusetts Bay mixes with fresh water from three rivers: the Charles, the Mystic, and the Neponset. The harbor shores include six of Boston's neighborhoods (East Boston, Charlestown, North End, Fort Point, South Boston, and Dorchester) and seven other municipalities: Hull, Hingham, Weymouth, Quincy, Chelsea, Revere, and Winthrop. Although it has extensive development on its edge, the estuary provides valuable habitat for wildlife, a nursery for marine organisms, water filtration, and flood control.

The islands and surrounding estuary have been home to a rich diversity of plant and animal life for millennia. The Boston Harbor Islands support complex natural communities adapted to coastal and island life. For more about and how the hydrologic environment of the park affects park management, go to "Water Quality."

Wetland and Aquatic Marine Vegetation and Wildlife

  • The Boston Harbor Islands provide shelter and food-rich habitats for marine birds, mammals, fishes and invertebrates, as well as nurseries for their young.
  • Much of the Gulf of Maine fauna can be found in Boston Harbor, especially around the Brewsters.
  • The once-plentiful eelgrass is the only type of seagrass now present in Boston Harbor. It is now confined to only four isolated areas, the largest of which is near the south coast of Bumpkin Island. Seagrass beds are critical wetlands components of shallow coastal ecosystems where they hold sediment, providing food and cover for a great variety of animals.
  • Salt marshes, the most highly productive ecosystems in the world, are dominated by saltwater cordgrass and provide habitat for many marine organisms. More than 50 percent of the state's salt marshes have been filled. The largest remaining salt marshes on the islands are found on Thompson and Snake islands. Smaller brackish marshes have been identified on Calf, Grape, Lovells, and Peddocks. Mud flats, which generally occur on the periphery and at the expanding edges of salt marshes, are found on Raccoon, Snake, and Thompson islands.
  • Lobsters, crabs, and clams inhabit submerged portions of the islands. Mussels and barnacles cling to the intertidal zone. Jellyfish live in the surrounding waters. Several species of fish including striped bass, bluefish, and winter flounder, live in waters surrounding the islands. Little Brewster, Nixes Mate, Shag Rocks, and other islands characterized by bedrock outcroppings contain rocky intertidal communities of rockweed and barnacles.
  • Harbor seals haul out on some of the outer islands. Because their feeding grounds or migratory routes are nearby, humpback, fin, minke, and North Atlantic right whales and white-sided and striped dolphins are potential, though rare, visitors, as are harbor porpoises.

Terrestrial Wildlife

  • The diversity of upland and marine habitats provides good nesting and feeding opportunities for a number of bird species. Field surveys have identified more than 100 bird species including gulls, terns, herons, ducks, geese, hawks, plovers, sandpipers, doves, owls, woodpeckers, and perching birds.
  • During migration, large numbers of shorebirds use the mudflats and salt marshes around the harbor, while transient hawks and songbirds use the more remote islands. In late fall and winter great flocks of waterfowl gather in the harbor waters.
  • A few species of terrestrial mammals, including exotic species, occur throughout the islands. These include cottontail rabbits, raccoons, skunks, gray squirrels, mice, muskrats, voles, and Norway rats. Some species have been known to devastate populations of small vertebrates and nesting birds.
  • Eastern garter snakes, Northern brown snakes, and Eastern smooth green snakes are known to live on the islands.
  • Dozens of Great Black-Backed and Herring gulls nest on Gallops Island each spring.
  • Hundreds of Brant, a small goose, stop on Georges Island each spring and Snowy owls have been seen there during the winter.
  • Sea gulls may seem numerous today. But, in the 1800s gulls were hunted almost to extinction for their feathers. Apple and pear trees remain on Bumpkin from the island's agricultural past.
  • Lovells Island has a large population of European hares, introduced during the 1940s and 1950s.
  • Little Brewster and nearby Shag Rocks offer roosting sites for Cormorants that fish the waters nearby; Cormorants are sometimes called Shags in Britain.
  • Tidal flats at Worlds End attract thousands of migrating shore birds each autumn.
  • Marshes on Thompson Island serve as a nursery for fish and shellfish and a stop-over for migrating shore birds in the spring and fall.
  • For more about wildlife on the islands, go to "Animals."

Upland Vegetation

  • The flora of the islands reflects a long history of human alteration, including introduction of a large number of invasive exotic species.
  • It is thought that the islands' drumlins were covered with mature forests of hemlock, maple, oak, pine, and hickory, which were cleared to support agriculture and pasturage, and to supply firewood for fuel. In addition, the construction of the islands' massive fortifications severely disrupted much of the native flora.
  • Thorough documentation of the characteristics of the terrestrial environment is just beginning, but successional species including aspen, pine, birch, and white poplar are clearly evident on most of the islands.
  • Native Americans farmed the land as did the colonial settlers. The remnants of attempts at subsistence farming are evident in the appearance of apples, pears, grapes, chives, garlic, asparagus, and horseradish.
  • Today, patches of undisturbed native flora are rare on the islands. The vegetation on most of the islands is dominated by grasses and sumac.
  • The owners of Worlds End and Thompson Island have continued to manage expansive grasslands that are part of the cultural landscape. Worlds End and Thompson Island have communities of mixed oak forest; on Thompson they cover approximately one-tenth of the island.
  • For more about vegetation on the islands, go to "Plants."

Protected species

  • The park protects six rare species including four protected bird species (Least Tern, Northern Harrier, Common Tern and Barn Owl) and two protected plant species (Sea-beach Dock and American Sea Blite).
  • The U.S. Fish and Wildlife Service reports several federally listed endangered and threatened species of fish, turtles, birds, and mammals near or in coastal waters of Massachusetts, but not known to be found among the Boston Harbor Islands. There are no island species on the federal list.

Last updated: February 26, 2015

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