Plant life in the North Cascades is extremely varied, reflecting differences in rock and soil types, exposure, slope, elevation, and rainfall. Eight distinctive life zones support thousands of different plant species in the North Cascades greater ecosystem. No other US National Park surpasses North Cascades National Park in the number of plant species recorded. Over 1,627 vascular plant species have been identified, and estimates of non-vascular and fungal species could more than double this number for total plant species in the North Cascades. Some of these plants are threatened or endangered, and changes such as air pollution and global warming might affect their survival. Other threats include invasive non-native plants that are referred to as exotic species. Exotic species are capable of displacing native species and changing biotic communities. Resource managers at North Cascades National Park are taking action to reduce this threat by removing these invasive plants. This can be particularly difficult because these plants utilize trails, waterways, wind, and roads to colonize the area. Restoration of habitats changed by human activity has been a priority since the park was established in 1968. As leaders in developing methods of revegetation in the National Park Service, the plant propagation crew has grown thousands of native plants from seeds and cuttings. Taken from areas adjacent to damaged sites, these seeds and cuttings are later returned as young plants to restore campsites and trampled areas of the park.