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Predation and Community Structure • Predator influence on community structure/diversity – Predation decreasing diversity • Predators select for competitive inferiors, allowing competitive superiors to dominate. • Eg. lizards preferred rarer spiders as prey. Predation and Community Structure • When productivity is high – Frequency dependent predation increases diversity. – Generalist predators – less or no change in diversity (they will not specialize in the competitively dominant prey). – Competitive dominants put more resources into production, less into avoiding predation. Predation and Community Structure • As with disturbance, intermediate levels of predation maximize diversity. Expected and unexpected effects • The fact that we observe unexpected effects hints at additional complexity in communities. • Intermediate predation pressure more likely to increases diversity – Too high – preferred prey driven to extinction – Too low – superior competitors dominate • Predation = biotic factor. Role or predators most pronounced in deterministic communities. 1 Direct/Indirect Effects Overview Food Webs Trophic Cascade • Increase in predator abundance → decreased consumer abundance → increased producer abundance 2 Trophic Cascades • Best documented in aquatic systems • Criticized as being – “all wet” – Species cascades and not trophic cascades – Assume top-down control – Less applicable to >3 level communities • Community level cascades most common when: – Habitat is homogeneous – Prey are uniformly edible – Producer and Prey populations both capable of rapid increase – Tropic levels are discrete, omnivores or multi-level feeders rare Top down vs. Bottom up control • Hairston, Smith and Slobodkin (1960) – The world is green, top-down forces more important Top-Down vs. Bottom-Up • Top-down control - consumers control abundance of their prey. Predator Prey Producer • Bottom-Up (donor) control – consumer abundance controlled by resource availability. Predator • Trophic cascade – alternating topdown and bottom-up control. • Are communities dominated by topdown or bottom-up forces? Prey Producer Power (1990) • Manipulated fish presence in in-stream mesocosms that allowed insect movement. chironomids • Oksanen (1981) – Much of the world is white – Productivity shifts the balance from top-down to bottom-up control Pred. insects Stickleback fry 3 Power (1990) Food Webs • Second experiment excluded fish and included only one (or none) of the second level consumers. • Strong vs. weak interactors • Historically, most important species (keystone) viewed as top predators, controlling top-down forces • Modern view - does not have to be predator (eg. figs). • Pollinating insects = Keystone mutualists Change in Ecosystem Function if Removed Keystone Species Keystone species – rare, but vital to ecosystem Abundant species And vital role in ecosystem Moderate abundance moderate role in ecosystem Rare species not a vital role in ecosystem Species Biomass 4