Currently between 50 and 100 species are added to the list every year. Roderick Nash, a environmental historian, believes a conservative estimate of the current extinction rate is 10,000 known species a year and rising, mostly because of the destruction of forests and other key habitats. The problem for the enforcement and regulation of the ESA has been the lack of money. With more animals being added to the ESA list, the allotment of the money is getting smaller for each species. Thus, humans have responded by allotting money to protect and save species deemed worthy of preserving.
Some species provide a more direct value to humans.
For example, a cancer fighting drug had been derived from the bark of a yew that was native to the Pacific Northwest forests, which were being cut for timber. Although, this Pacific Yew is scarce and a threatened species, it was deemed unworthy of protection under the Endangered Species Act, because scientists have synthesized taxol, which was the substance obtained from the Pacific Yew . In the United States, it is believed at least 500 known species and subspecies of plants and animals have become extinct since the 1500s. Natural causes appear to have claimed just one of the animals, a marine snail that used to live off New England's shores. Keep in mind that we had hardly gotten to know the others and many species have perished without ever being discovered.