Predicting local extinction rates is complex due to differences in biological diversity, species distribution, climate, vegetation, habitat threats, invasive species, consumption patterns, and enacted conservation measures. One constant, however, is human population pressure. A study of 114 nations found that human population density predicted with 88-percent accuracy the number of endangered birds and mammals as identified by the International Union for the Conservation of Nature . Current population growth trends indicate that the number of threatened species will increase by 7 percent over the next 20 years and 14 percent by 2050. And that's without the addition of global warming impacts.