Habitat degeneration through loss of plant cover, soil erosion and nitrification opens areas to aggressive, fast-growing species. In natural ecosystems plants colonising newly opened areas help to stabilise the soil and in later years are overtaken by other species as succession progresses to the climax vegetation type. However, unnatural habitat degradation alters this balance and enables colonising species to persist and dominate. The problem is compounded by the arrival of foreign plants (aliens) brought in by human intervention. Some aliens are very aggressive, spreading to dominate large areas to the exclusion of native plants. Invasive aliens are a problem in all parts of the world (see the IUCN Invasive Species Specialist Group website), and one of the more difficult threats to wild populations to deal with. There are over 100 non-native plant species that are so well established in Nepal that they have become weeds. Currently alien species are not considered a priority conservation issue by the Government of Nepal.