A trophic cascade occurs when a higher trophic level has an indirect effect on a lower trophic level, mediated by a trophic level in-between (Carpenter, Kitchell & Hodgson 1985; Estes et al. 1998; Gilman et al. 2010)Since apex predators limit herbivores as well as mesopredators, they can initiate trophic cascades along two routes in an ecosystem. In one route they limit the herbivores that they prey on, and thereby indirectly affect the abundance of plants on whichthe herbivores feed. In the other route they limit mesopredators and thereby indirectly affect the abundance of smaller herbivores, i.e. the prey of mesopredators, and the plants that the small herbivores feed on (Elmhagen et al. 2010). Therefore, extinctions and reduction of apex predators can have extensive cascading effects on the ecosystem, and there are many examples of this from terrestrial as well as aquatic ecosystems all over the world (Estes et al.2011; Ripple et al. 2014). Loss of apex predators can cause a mesopredator release, i.e. an increase of mesopredator abundance as mesopredators are no longer limited top-down (Soulé et al. 1988). This can result in higher rates of predation in smaller prey species (Berger, Gese & Berger 2008), decline of prey populations (Palomares et al. 1995) and even extinction of prey species (Souléet al. 1988). Apex predators, even if introduced, can also limit introduced mesopredators which otherwise can have negative effect on native wildlife (Johnson, Isaac & Fisher 2007; Rayner et al. 2007). When herbivores are released from predation they can reach higher densities as well as utilize habitats that earlier were avoided due to predation risk (Creel et al.32005). This can have negative effects on vegetation and on other species that are related to the vegetation (Ripple & Beschta 2004).Because of their impact on ecosystems, apex predators have sometimes been suggested to be keystone species (Sergio et al. 2008), i.e. species that are important in structuring ecological communities (Paine 1969). However, the effect of apex predators is context dependent and can be affected by factors such as carrying capacity and food web structure, and they do therefore not always function as keystone species (Sergio et al. 2008). For example, trophic cascades can be relaxed in ecosystems with many predators (Finke & Denno 2004; Bruno & OConnor 2005). diva-portal.org/smash/get/diva2:695755/FULLTEXT01.pdf
Posted on: Tue, 22 Jul 2014 09:14:17 +0000