Islands are stable in the long-term but very fragile to perturbation/disturbance. They have high levels of endemism, but only moderate species richness (population numbers). Extinction is not random; it is selective. Islands are often resource-poor e.g. little habitat or low nutrients. Therefore, a loss of some resource would be major. Also, as island size decreases, resource abundance and replenishment rate decreases (extinction rates are lower in groups that require few resources). Humans also have driven widespread and ongoing island extinctions.