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Biomedical Engineering
BMEN 515
William Huddleston

HOW DO SPECIES INTERACTIONS CAUSE TROPHIC CASCADES • Interactions of a single predator species in a community can cause a progression of indirect effects across successively lower trophic levels-trophic cascade • can be illustrated by wolf pop’n in Lamar Valley of Yellowstone National Park • wolves feed on pronghorn, mule, deer, elk, bison • share prey with coyotes, lions and bears • unrestricted hunting continued after park establishment • by 1926, wolves were extirpated from park • elk pop’n rapidly increased, when wolves were absent, elk pop’n browsed aspen trees so intensely that no young trees were recruited after 1920 • 1995, wolves were reintroduced to Yellowstone and pop’n grew rapidly • Wolves preyed on elk-elk pop’n decreased • The presence or absence of a single predator influences pop’n of prey and structure of vegetation and pop’n of other species • Organisms that create structure-ecosystem engineers • Species that exerts influence out of proportion to its abundance-keystone species • May influence species richness of communities and the flow of energy and materials thorugh ecosystem HOW DO DISTURBANCES AFFECT ECOLOGICAL COMMUNTIES? • Disturbances: event that changes the survival rate of one or more species in an ecological community-remove some species but may open up space and resources for others • Small disturbances are much more common than large disturbances • Few large events may cause most changes (hurricanes) • Succession: change in composition of an ecological community following a disturbance • Two major types: primary succession: begins on sites that lack living organisms; secondary succession: begins on sites where some organisms have survived most recent disturbances • Succession i
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