Wildlife protection officials added 15 imperiled species to the federal endangered list Tuesday for Hawaii, where native plants and animals are among the most vulnerable to extinction in the nation.

The species are all native to Hawaii island. They include plants vital to traditional Hawaiian cultural practices and "one of the most primitive shrimp species in the world," according to one prominent conservation group.

The new status gives them special protections designed to stave off extinction from development, pollution and invasive plants and animals, as well as other threats to their survival.

Hawaii already is the state with the most entries by far on the Endangered Species Act list. With the additions Tuesday, the island state now has 526 species listed, according to Oahu-based U.S. Fish and Wildlife Service Information and Education Specialist Ken Foote. California comes closest with about 315 listings, according to the service’s Environmental Conservation Online System. Most states have double-digit listings.

Some conservationists and wildlife officials call Hawaii the "endangered species capital of the world," Foote said.

"The geographical isolation is the main reason" for Hawaii’s large number of endangered species compared with the rest of the country, Foote said. "Anything that offsets that natural balance, that out-competes them, they’re pretty much goners," he added.

The additions stem from a 2011 agreement with conservation groups for the wildlife service to catch up on a backlog of candidates for the Endangered Species Act list, and to consider 757 total species for federal protections, according to the Arizona-based Center for Biological Diversity.

"It’s very exciting," Tierra Curry, a center biologist, said Tuesday. So far, 99 percent of all species given protections under the act have survived, she said. "The service has known for many years that so many Hawaiian species are endangered, and they’ve just been on the waiting list" due to lack of federal funds and resources to take action, Curry said.

The 15 new species represent the fourth wave of endangered listings for Hawaii under the 2011 agreement, Foote said. Federal wildlife protection officials previously added 48 species on Kauai, 23 on Oahu, then 38 on Maui, Molokai, Lanai and Kahoolawe combined, he said. The agency likely will list more vulnerable Hawaiian species — mainly those that are found across multiple islands, he said.

For these most recent endangered species listings, Hawaii-based Fish and Wildlife officials focused on shared threats so that they could focus on conserving entire ecosystems instead of treating species as isolated units, Foote said.

It’s a somewhat new approach for the service spearheaded by its personnel in Hawaii and the southeastern U.S., Curry said. Even so, the 15 new species are found in 10 different ecosystems across Hawaii island, according federal officials.

The listings include the anchialine pool shrimp, an eyeless, 2-inch-long creature with 2-inch antennae that lives in landlocked pools with subterranean connections to the sea. The pools are threatened by pollution, development and other factors, Foote said. The nonprofit Center for Biological Diversity maintains that only five members of the shrimp species have ever been seen.

One of the 13 plant species added Tuesday, Bidens micrantha ctenophylla(commonly known in Hawaiian as kookoolau) was traditionally used to treat infections and ailments, according to the wildlife service. It was further used to purify the body, and herbal teas were made from its leaves to treat throat and stomach pains, according to the agency.