Four plants that are among the "rarest of the rare" in the world are now being considered for protection under the Endangered Species Act, along with three Hawaii damselflies and 16 other plants that can be found on Oahu.

An agreement announced Monday between the U.S. Fish and Wildlife Service and the Center for Biological Diversity, an Arizona-based, nonprofit environmental organization, would add to the 437 species currently listed as threatened and endangered by the Pacific Islands Fish and Wildlife Service Office in Hawaii, home to some of the rarest and most endangered species on earth.

It a federal offense to harm any plants, or kill or harass any animal, on the list.

The agreement is part of a settlement to fast-track 757 species across America to get them on the federal endangered species list by 2018.

The crimson Hawaiian damselfly, blackline Hawaiian damselfly and oceanic Hawaiian damselfly are all threatened by non-native insects, development and changes to streams, the Center for Biological Diversity said.

The 20 plants include an annual herb, shrubs, trees and a fern. They’re all threatened by the disappearance of their native habitat — and by foraging and trampling from invasive goats, pigs and rodents, as well as by invasive insects that eat the plants’ pollinators.

In what the Center for Biological Diversity called a landmark legal settlement, the agreement also protects 43,491 acres across seven different types of ecosystems in Oahu’s Koolau and Wai­anae mountain ranges. The habitats are considered essential for the conservation of the 23 plants and damselflies.

The Fish and Wildlife Service will designate critical habitat for the 23 plant and damselfly species. It also will designate critical habitat for two additional plant species already listed as endangered — and revise critical habitat for 99 plant species currently listed as endangered or threatened, the agency said.

The designation of critical habitat does not affect landownership or establish a refuge, wilderness, reserve, preserve or other special conservation area — nor does it allow government or public access to private lands, the Fish and Wildlife Service said.

But the agreement will help protect a fragile Hawaii ecosystem under stress after "tens of millions of years of unique relationships," said Christy Martin, coordinator for the Coordinating Group on Alien Pest Species, which was not a party to the settlement. "When you have an ecosystem like Hawaii that’s so isolated, there are interrelationships that we really don’t understand."

The agreement was reached last month after the Center for Biological Diversity filed lawsuits against the U.S. Fish and Wildlife Service to put 16 Oahu plants and three damselflies on the list of candidates for the endangered species list.

The agency added four additional Oahu plants that are listed as among the "rarest of the rare" by the Plant Extinction Prevention Program, a multi-agency program in Hawaii, said Tierra Curry, a conservation biologist with the Center for Biological Diversity.

The four plants — haha (Cyanea purpurellifolia), haiwale (Cyrtandra gracilis), haiwale (Cyrtandra waiolani) and ohe (Tetra­plasandra lydgatei) — are among approximately 180 Hawaii plants that each have fewer than 50 surviving members, Curry said.

"All of the species (announced on Monday) have been in trouble for a long time," Curry said. "The Fish and Wildlife Service has had a backlog of species that have long been known to need protection but hasn’t had the resources to get around to (protecting) them."

Loyal Mehrhoff, field supervisor for the Pacific Islands Fish and Wildlife Office, said in a statement, "Hawaii is a unique and special place in the natural world, and for that reason requires an innovative, holistic approach to conservation. We are on the forefront of endangered species conservation and are the first Fish and Wildlife Office in the nation to utilize the ecosystem-based approach for listing species and designating critical habitat, which will allow us to address the backlog of candidate species and, ultimately, the health of entire ecosystems."

The public has until Oct. 3 to comment on whether to place the plants and damselflies on the endangered species list. The Fish and Wildlife Service has 10 months to publish the species on the federal register of endangered species, Curry said.

The Center for Biological Diversity has been campaigning for a decade "to safeguard 1,000 of America’s most imperiled, least protected species," which include 70 in Hawaii, the organization said.

"The Southeast, West Coast, Hawaii and Southwest are America’s extinction hot spots," Kierán Suckling, executive director of the Center for Biological Diversity, said in a statement. "Most of the species lost in the past century lived there, and most of those threatened with extinction in the next decade live there as well."

Some 499 species included in the agreement are not scheduled for inclusion on the endangered species list, Suckling said.

They include:

» The scarlet Hawaiian honeycreeper, or iiwi, a bright-red bird that the Center for Biological Diversity describes as hovering "like a hummingbird and has long been featured in the folklore and songs of native Hawaiians." The Hawaiian honeycreeper has been eliminated from low elevations on all islands from diseases such as avian pox and malaria that were carried by mosquitoes, which are now moving into the honeycreeper’s higher-elevation refuges, according to the center.

Under the agreement, the Fish and Wildlife Service will consider the Hawaiian honeycreeper for protection in 2016, the center said.

» The black-footed albatross, a large, dark-plumed seabird that lives in the Northwest Hawaiian Islands, is threatened by longline swordfish fisheries, which kill it as bycatch, the center said.

The center and others petitioned to have the black-footed albatross listed as endangered in 2004. According to the agreement, the Fish and Wildlife Service will propose the black-footed albatross for protection later this year, the center said.