Habitats are places where plants and animals live, feed, find shelter, and reproduce. For human members sharing natural habitats, the knowledge of interdependence carries with it a responsibility. Human activity can threaten habitat in numerous ways--through direct loss, fragmentation, encroachment, disturbance, diminished water quality, altered drainage patterns, and barriers. To sustain the health of Casco Bay, we must pursue actions that benefit not only our own species, but also all the "neighboring" plants and animals that share the watershed's ecological community.

The marine habitats of Casco Bay cover 229 square miles. Due to topography and wide tidal variations characteristic of the Gulf of Maine, intertidal areas in Maine are the most extensive along the Atlantic Coast of the United States.

Salt marshes filter stormwater from upland developments and help moderate nutrient flow to adjoining waters. Marshes also act as giant sponges during storms and therefore reduce damage from flooding.

Approximately 150 species of waterbirds inhabit Casco Bay and the 500 acres of rocky shore in Casco Bay provide habitat for a wide range of species: from seaweeds, periwinkles, mussels, barnacles, and crabs to starfish and seals.

Plants, which support the food chain, are an important part of subtidal habitats. One particularly sensitive plant, eelgrass, is considered an indicator of ecosystem health. Casco Bay has the largest and most dense concentrations of eelgrass mapped along the coast of Maine, with over 7,000 acres of beds.

Casco Bay contains 758 islands, islets, and exposed ledges at mean high tide, a few of which are important habitat for colonial nesting seabirds.

Throughout the Casco Bay watershed, there are more than 1,356 miles of rivers and streams that offer habitat to muskrat, beaver, river otter, belted kingfisher, black duck, spotted sandpiper, shad, trout, bass, perch, pickerel, and salmon. Streams provide important habitats for juvenile fish and for anadromous fish like alewife and smelt, which use them for spawning. The riparian (streambank) zone and the 578 miles of edge habitat next to the shoreline of Casco Bay are important links between the terrestrial (land) ecosystem and the wetlands or water.

Invasive species are non-native species whose introduction causes or is likely to cause economic or environmental harm or harm to human health.