Forests Ecosystems are dynamic systems that are constantly under the influence of outside forces. Natural disturbances, such as storms, may negatively impact how an ecosystem functions. Human influences, such as pollution and exploitation, can also cause changes and place stress on a habitat. Ecosystem resilience refers to how easily an ecosystem can resist significant change, or bounce back, after a disturbance or new pressure is applied to the system. Within an ecosystem, populations of organisms are limited or regulated in different ways. Sometimes a lack of nutrients limits plant growth, therefore the ecosystem can only support a limited number of herbivores and the carnivores that prey on them. In other circumstances, plantlife is able to flourish and support a wide range of herbivores, only because predators regulate their populations. Predators keep the population of herbivores low, so that they do not devastate the available producers. For example, in Alaskan kelp forests, sea otters control populations of herbivorous sea urchins through predation. When sea otters are removed from the ecosystem (for example, by human exploitation), urchin populations are released from predatory control and grow dramatically. This leads to increased herbivore pressure on local kelp stands. Over consumption of the kelp itself results in the loss of physical ecosystem structure and subsequently, the loss of many other species that live in and use this habitat. In Alaskan kelp forest ecosystems, sea otters are the keystone species that serves to regulate a stable ecosystem. The otter is considered a keystone, because it exerts an inordinate amount of influence (compared to other species) in maintaining the ecosystem. When urchin populations are allowed to grow uncontrolled, they can completely destroy all of the kelp. This process leads to a new stable ecosystem, called urchin barrens, in which urchins dominate the area. Although this type of ecosystem is stable, it is much less biodiverse than the original kelp forest, because only a few species can exist there. Recovery can only occur if urchin populations are controlled, allowing the kelp to grow back. Generally, biodiverse ecosystems include more complex food webs that can withstand disturbances. Biodiversity encompases not only many different types of species that overlap in their roles, but the genetic diversity of a population. When a population decreases to low levels, individuals may have trouble finding suitable mates and producing sufficient numbers of genetically diverse offspring. A lack of genetic diversity in a population can increase genetic abnormalities and disease - making an already small population more vulnerable to extinction.