There are many causes of the current extinction crisis, but all of them stem from unsustainable management of the planet by humans. Despite efforts to conserve key habitats for animal and plant species, widespread destruction of important habitats continues unchecked. Perhaps even more serious, the quality of habitats continues to deteriorate as a result of harmful activities such as overgrazing, selective logging, removal of dead wood, and burning. A rapidly growing threat, and one which is very hard to control, is the spread of alien, invasive species. These species, both animals and plants, spread to areas and habitats where they do not naturally occur, displacing native species through predation, competition, disease and hybridisation.
People move thousands of alien species around the globe, either deliberately or accidentally.
Other species become established outside their natural range by taking advantage of new habitats modified by humans. Over-harvesting of species, both animals and plants, that are valuable, either for economic or cultural reasons is another universal threat. Many species now appear to be suffering serious declines as a result of climate change, yet the full effects of the problem are little understood. The list of threats goes on - pollution, freak weather events, and disease can all have devastating effects. When species decline so that only very small populations survive, additional, biological factors begin to operate against them.