“My heart breaks,” one biodiversity advocate said Monday as the U.S. Fish and Wildlife Service announced that 21 species would be removed from the endangered species list due to their extinction.

The agency said it had conducted “rigorous reviews of the best available science” and determined that the animal species are no longer in existence, having been protected under the Endangered Species Act (ESA) starting in the 1970s and ’80s, when they were already in very low numbers—or potentially already extinct in some cases.

“These plants and animals can never be brought back,” said Noah Greenwald, endangered species director at the Center for Biological Diversity (CBD). “We absolutely must do everything we can to avert the loss of even more threads in our web of life.”

CBD noted that human exploitation of wildlife and the resulting spread of invasive species was directly linked to at least one of the species losses.

Eight types of the Hawaiian honeycreeper bird species […]