There are 42 islands in Biscayne National Park, which anchor together the northern end of the coral rock Florida Keys and transition to the sand barrier islands in the north. They offer a glimpse into what all of the Keys looked like before development. Elliott Key is the park's largest island and is considered the first of the true Florida Keys. Elliott and the keys to the south are the remains of coral reefs, which formed when ocean waters were much higher than they are now. Today, you can see remains of the coral around Elliott Key. The islands to the north of Elliott Key, from Sands Key to Soldier Key, are considered "transitional" islands. They share features of the hard rock coral keys to the south and some with the sand barrier islands to the north. All of the park islands provide a protective barrier for Biscayne Bay and the mainland.
Although Biscayne National Park has an extensive human history spanning over 10,000 years, the park was established for its natural features. The park's four primary ecosystems are comprised of a variety of smaller communities like seagrass meadows, hardbottom areas and hardwood hammocks. The geology of the area has been influenced by changing sea levels, currents, hurricanes, and reef-building organisms like corals. South Florida's subtropical climate produces forest types that are more typical of the Caribbean than of mainland North America.