Invasive species have likewise been introduced by human populations. Following the 2014 discovery in Madagascar of the Asian common toad, a relative of a toad species that has severely harmed wildlife in Australia since the 1930s, researchers warned the toad could "wreak havoc on the country's unique fauna." Habitat destruction and hunting have threatened many of Madagascar's endemic species or driven them to extinction. The island's elephant birds, a family of endemic giant ratites, became extinct in the 17th century or earlier, most probably because of human hunting of adult birds and poaching of their large eggs for food. Numerous giant lemur species vanished with the arrival of human settlers to the island, while others became extinct over the course of the centuries as a growing human population put greater pressures on lemur habitats and, among some populations, increased the rate of lemur hunting for food. A July 2012 assessment found that the exploitation of natural resources since 2009 has had dire consequences for the island's wildlife: 90 percent of lemur species were found to be threatened with extinction, the highest proportion of any mammalian group. Of these, 23 species were classified as critically endangered. By contrast, a previous study in 2008 had found only 38 percent of lemur species were at risk of extinction.