A New Inventory and Conservation Assessment of United States Islands
This study develops a 30‑m resolution spatial inventory of 19,023 nonriverine, nonlacustrine U.S. islands and evaluates their ecological and conservation significance. Results show that islands hold a disproportionate share of threatened species and global ecosystems, underscoring their critical role in national biodiversity conservation.
Subject Tags
- Marine protected areas
- Coastal
- Biodiversity
Abstract
To support conservation-focused research and management we developed a new 30-m resolution polygon data layer of the nonlacustrine and nonriverine islands of the United States, with associated attributes describing key physical and conservation geography characteristics. Islands were grouped into a three-tiered hierarchy of island regions (twelve), island provinces (twenty-eight), and individual islands (19,023). Islands were classified as either continental or oceanic based on their physiographic position relative to the North America continental shelf, and estuarine versus nonestuarine depending on their location within or external to estuaries. For each island we assessed the diversity of terrestrial and coastal ecosystems, the number of threatened and endangered (T&E) species listed under the Endangered Species Act, the number of T&E species critical habitats, the number of migratory bird species listed under the Migratory Bird Treaty Act, the number of Key Biodiversity Areas, and the number of and management responsibility for protected areas. We conclude that the conservation importance of islands is disproportionate to their total area as, for example, islands contain 52 percent of the T&E species yet their total area is only 2 percent of the area of the continental mainland. Similarly, of the global total of 431 World Terrestrial Ecosystems, 201 (47 percent) occur on U.S. islands compared with 286 (66 percent) that occur on the U.S. continental mainland.
Citation
Sayre, R., Martin, M., Naji, N., Sides, K. B., Cress, J., Butler, K., ... & Holmes, N. (2026). A New Inventory and Conservation Assessment of United States Islands. Annals of the American Association of Geographers, 1-24.
TNC Authors
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Alex Wegmann
Lead Scientist, Island Resilience. California
The Nature Conservancy
Email: alex.wegmann@tnc.org -
Charlotte Stanley
Spatial Data Analyst. California
The Nature Conservancy
Email: c.k.stanley@tnc.org -
Nick Holmes
Associate Director, Oceans Program. California
The Nature Conservancy
Email: nick.holmes@tnc.org