Endangered species alone will not cause a collapse in the ecosystem yet the extinction will lead to collapse of specific ecosystems. Ecosystems are robust and complex systems though highly vulnerable to internal and external pressures. An internal pressure would be the near-extinction (endangerment) or extinction of a species. The sustainability of an ecosystem is based on many factors, including but not limited to number and type of species, internal and external pressure and food chain stability. The extinction of species is linked to ecosystem pressure and food chain stability and will produce a detrimental effect on ecosystems, possibly causing collapse within the ecosystem. Additionally, there are some species whose role within an ecosystem is visibly pronounced. For example, bees are critical to the process of pollination. Therefore, if bees were to go extinct, the effects would be drastic. Flowers would not be able to effectively pollinate and thus plant life within the ecosystem would be drastically reduced and the food chain destabilized leading to further endangerment and possible extinction. Since all animals depend on plant life as the basis of their food chain, a reduction in plant life may threaten the ecosystem as a whole.