An absolute description of the threat hanging over the world's mammals, reptiles, amphibians(, , ) and other life forms has been published by the well-known scientific journal, Nature. A special analysis carried out by the journal indicates that an astonishing 41% of all amphibians on the planet now face extinction while 26% of mammal species and 13% of birds are similarly threatened. Many species are already critically endangered and lose to extinction, including the Sumatran elephant, Amur leopard and mountain gorilla. But also in danger of vanishing for the wild, it now appears, are animals that are currently rated as merely being endangered. In each case, the finger of blame points directly at human activites The continuing spread of agriculture is destroying million of hectares of wild habitats every year, leaving animals without homes, while the introduction of newly-come species, often helped by humans, is also damaging native populations. At the same time, pollution and overfishing are destroying ocean ecosystems. "Habitat destruction, pollution or overfishing either skill off wild creatures and plants or leaves them badly weakened," said Derek Tittensor, an ocean ecologist at the World Conservation Monitoring Centre in Cambridge. "The trouble is that in coming decades, the additional threat of worsening climate change will become more and more common and could then kill off these survivors." The problem, according to Nature, is worsened because of the huge gaps in scientists knowledge about the planet's biodiversity. Evaluations of the total number of species of animals and plants alive vary from 2 million to 50 million. In addition, evaluations of current rates of species' disappearances vary from 500 to 36, 000 a year. "That is the real problem we face," added Tittensor. "The scale of uncertainty is huge." In the end, however, the data indicate that the world is heading cruelty towards a mass extinction-which is defined as one involving a loss of 75% of species or more. This could arrive in less than a hundred years or could take a thousand, depending on extinction rates. What does "This" in the last sentence refer to   _  ?
mass extinction extinction rates extinction time 75% of species or more