Inecology, acascade effect is a series of secondaryextinctions that are triggered by the primary extinction of a keyspecies in anecosystem. Secondary extinctions are likely to occur when the threatened species are: dependent on a few specific food sources,mutualistic (dependent on the key species in some way), or forced to coexist with aninvasive species that is introduced to the ecosystem. Species introductions to a foreign ecosystem can often devastate entire communities, and even entire ecosystems. These exotic species monopolize the ecosystem's resources, and since they have no natural predators to decrease their growth, they are able to increase indefinitely. Olsen et al.[1] showed that exotic species have caused lake andestuary ecosystems to go through cascade effects due to loss ofalgae,crayfish,mollusks, fish,amphibians, and birds. However, the principal cause of cascade effects is the loss of toppredators as the key species. As a result of this loss, a dramatic increase (ecological release) ofprey species occurs. The prey is then able tooverexploit its own food resources, until the population numbers decrease in abundance, which can lead to extinction. When the prey's food resources disappear, they starve and may go extinct as well. If the prey species isherbivorous, then their initial release and exploitation of the plants may result in a loss of plantbiodiversity in the area.[2] If otherorganisms in the ecosystem also depend upon these plants as food resources, then these species may go extinct as well. An example of the cascade effect caused by the loss of a top predator is apparent intropical forests. When hunters cause local extinctions of top predators, the predators' prey's population numbers increase, causing anoverexploitation of a food resource and a cascade effect of species loss.[3] Recent studies have been performed on approaches to mitigate extinction cascades in food-web networks.[4]
One example of the cascade effect caused by the loss of a top predator has to do withsea otters (Enhydra lutris). Starting before the 17th century and not phased out until 1911 when an international treaty was signed to prevent their further exploitation, sea otters were hunted aggressively for their pelts, which caused a cascade effect through thekelp forest ecosystems along the Pacific Coast of North America.[5] One of the sea otters' primary food sources is thesea urchin (Class:Echinoidea). When hunters caused sea otter populations to decline, an ecological release of sea urchin populations occurred. The sea urchins thenoverexploited their main food source,kelp, creating urchin barrens where no life exists. No longer having food to eat, the sea urchins populations became locally extinct as well. Also, since kelp forest ecosystems are homes to many other species, the loss of the kelp ultimately caused their extinction as well.[6] In conclusion, the loss of sea otters in local areas along the Pacific coast seems to have caused a cascade effect of secondary extinctions, continuing into the present day.
^Olsen, T.M. D.M. Lodge, G.M. Capelli, and R.J. Houlihan. 1991. ". Impact of the introduced crayfish,Orconectes rusticus in northern Wisconsin lakes".Canadian Journal of Fisheries and Aquatic Sciences 48:1853-1861
^Leigh, E.G., S.J. Wright, E.A. Herre, and F.E. Putz. 1993. The decline of tree diversity on newly isolated tropical islands: A test of a null hypothesis and the implications.Evol. Ecol. 7:76-102.
