The U.S. government has declared them extinct.

Although it’s rare for wildlife officials to abandon hope on an animal or plant, government scientists claim they have exhausted all efforts to locate these 23. They also warn that climate change could increase the likelihood of such disappearances , as a warmer planet increases the dangers for endangered wildlife and plants.

Perhaps the most well-known species in the U.S. was the ivory-billed woodpecker. The extinct species will be announced by Fish and Wildlife Service Wednesday. It was persistently announced and celebrated with great fanfare. Recent appearances made in unconfirmed areas in Florida, Louisiana, Mississippi, and Arkansas in recent decades sparked a frenzy of futile searches in the swamps of Florida, Mississippi, and Louisiana.

Other species, such as the flat-pigtoe freshwater mussel from the southeast U.S.A., were only identified in the wild a few times, and were never seen again. By the time they got a name, they were already fading away.

Anthony Ford, a U.S. citizen, said, “When I see one those really rare ones it’s always in my mind that it might be the last time I see this animal again.” Fish and Wildlife Service biologist in Tennessee, who specializes on freshwater mussels.

There are many factors that can lead to the disappearance of wildlife, including too much development, water pollution and logging. In all cases, humans were the main cause.

They also share another thing: All 23 species were considered to have a low chance of survival after being added to the endangered species register in the 1960s. In the nearly 50 years since the Endangered Species Act became law, only 11 species have been extinct. The Wednesday announcement opens a three month comment period before final species status changes.

Globally, 902 species are considered extinct. It is believed that the actual number may be higher, as some species are not formally identified. Many scientists believe the earth is facing an “extinction crisis”, with fauna and flora disappearing at a rate of 1,000 times historical rates.

Scientists believe it’s possible that one or more of 23 species announced Wednesday could reappearance.

After millions of dollars spent on habitat preservation and searches, a leading figure in the hunt to find the ivory-billed woodpecker claimed it was premature for them to end the effort.

John Fitzpatrick (Cornell University bird biologist) said that an extinction declaration can lead to “little is gained and great loss.” He was the lead author of a 2005 study in which determined that the woodpecker had been rediscovered within eastern Arkansas.

He said, “A bird like this, and such a representative of the major old growth forests in the southeast, keeping them on the endangered species list keeps attention on it, keeps the states thinking about how to manage habitat to the off chance that it still exists.”

Craig Hilton-Taylor, of the International Union for Conservation of Nature (based in Switzerland) said that the International Union for Conservation of Nature isn’t putting the ivory-billed Woodpecker in its extinction list because it’s possible they still exist in Cuba.

Hilton-Taylor stated that premature extinction can have unintended, but potentially damaging consequences. He stated, “Suddenly, the (conservation money) is not there and then suddenly it does drive it to extinction as you stop investing in them.”

Federal officials claimed that the declaration of extinctions was motivated by the desire to eliminate a backlog in recommended status changes for species that hadn’t been taken action on for many years. They claimed it would allow for more resources to be used for conservation of species with a chance of recovery.

When these efforts fail, creatures that are uniquely adapted to their environment are what is lost. Freshwater mussels, such as the ones that the government claims have become extinct, reproduce by drawing fish with a lure-like attachment. Then they send out a cloud larvae to attach to the gills of the fish until they are large enough to live independently.

Ford of the wildlife services says that there is a 1 in a million chance of a mussel living to adulthood. However, those who do live can often last for over 100 years.

The most species are found in Hawaii — eight woodland birds, one plant. This is partly because many of the species on the list are from islands, which have many animals and plants that have very small ranges. Many can also blink quickly.

The smallest po’ouli was the most recent bird to die, an endangered species of honeycreeper that was discovered in 1973.

In the late 1990s, only three of them remained: a male and two women. After failing to mate in the wild, one male was captured and bred in 2004. They were never seen again.

Stuart Pimm, Duke University’s extinction expert, was inspired by the fate of Hawaii’s birds to pursue his career. Pimm stated that despite the dire nature of the government’s proposal to add more species to the extinct column and the lack of an Endangered Species Act, the toll would have been higher.

He said, “It’s a pity we didn’t reach those species in the time we had, but when they do, we are often able to save species.”

After recovering from their injuries, 54 species have been removed from the endangered list since 1975. These include the bald Eagle, Brown Pelican, and most Humpback Whales.

Climate change is making it harder for species to recover, with droughts, floods, wildfires, and temperature swings that increase the dangers species face.

It is also changing how they are protected. The focus is no longer on specific species, let alone individual birds. Officials now say that the larger goal is to conserve their habitats, which will boost all species that live there.

Michelle Bogardus, a biologist with Hawaii’s wildlife service, said that she hopes they can meet the challenge. We don’t have the resources or ability to stop extinctions unilaterally. Given all the threats, we must think proactively about ecosystem health and how to maintain it.