The Great Marsh is home to a number of habitats and species of concern spread out across the barrier beach and dune systems, maritime scrub forest, salt marsh, and estuarine shallow waters. Over 300 species of birds have been recorded in the area, of which 60 are nesting species. The area is an important migratory bird resting and feeding ground, and is beloved by birders and naturalists who come to look out at large rafts of winter seabirds or for an elusive bird of prey.

Mammals large and small cross the landscape and sea, including seals and whales in open water and deer, fox, coyote, raccoon, skunk, and many rodents over land. Intertidal waters provide an important nursery ground for small fish and invertebrates, though larger fish such as stripped bass are common in these coastal waters. Local clams are are a renowned local delicacy!

Coastal wetlands and barrier beaches provide protection to inland communities from storm surge. The marsh also provides important storage of carbon and other nutrients, and filter some pollutants from entering the sea.