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Direct vs. Indirect Effects Predators and Food Webs Direct effect- a change in the phenotype or abundance of a species mediated by y an interaction with another species. p I. Direct vs. Indirect Effects II. Keystone predators Indirect effect- a change in the phenotype or abundance of a species caused by a species it does not interact with directly. III. What controls food webs? Keystone Predators What controls food webs? A keystone species is one whose impact on its community or ecosystem is disproportionately large relative to its abundance. Mitella Pisaster Mytilus Top down control- food webs are controlled by the negative impacts of predators on p p prey y ((demand driven). ) Bottom up control- food webs are controlled by the availability of prey for predators (supply driven). Nucella [Source: Paine 1966, Am. Nat. 100: 65-75] Why is the world green? Where have all the fish gone? [Source: Hairston et al. 1960, Am. Nat. 94: 421-425] 1 [Source: Estes et al. 1998, Science 282: 473-476] 2