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Community Structure: Top-Down vs. Bottom-Up Control Hairston, Smith and Slobodkin (1960) Community Dynamics Carnivores Detritivores control Resource limited Herbivores Frees plants from herbivore control Plants Critiques Too Simple 1) Species differences matter 2) Plant dominance could be explained by good defences Other inferences of Hairston et al, 1960 1) Exceptions not important ?? 2) All communities have 3 trophic levels 3) Omnivory not important X X 4) External abiotic factors - not controllers X Robles et al, 1995 Mean Density (seastars/m2) Recruitment index of Mytilus Menge and Sutherland, 1976 Effects of predation by whelks. Predation is weak High wave energy - effects of predation -weak Moderate wave energy - effects of predation - strong Menge Sutherland Bottom Up Control Fretwell, 1977, 1987 - availability of plant material governs structure of food chains - Low productivity - 1 link (plants) - Higher productivity - add links Ecological Relationships in Kelp Forests Kelp Orca Sea Otter Urchins Transplant mussels and barnacles (filter feeders) to urchin-dominated and kelp-dominated substrates Expected (top down) Urchin Kelp Transplant mussels and barnacles (filter feeders) to urchin-dominated and kelp-dominated substrates Expected (top down) Urchin Observed (bottom up) Urchin Kelp Kelp Clearly - can be a complex interaction Increased nutrient Increased algae Increased benthic filter feeders Increased consumers (predation) control Interaction of Systems High flow Leonard et al, 1998 Low flow Interaction of Systems • increased seaweed growth • increased filter feeder growth • increased larval settlement • low consumer efficiency • higher densities of organisms with planktonic •larvae more spatial competition • increased consumer pressure • increased sedimentation • lower densities of organisms with planktonic larvae • increased consumer mortality • less spatial competition Leonard et al, 1998 Hydrodynamics Flow rate Time Leonard et al, 1998 Community structure High flow Low flow T i d e h e i g h t Percent cover Percent cover barnacles Fucus mussels Bare space Leonard et al, 1998 High flow Recruitment rates Barnacles Density (#/100 cm2) Low flow Mussels Snails Leonard et al, 1998 Crab predation High flow Low flow Predation Intensity (% mortality) On Littorina, Nucella, Mytilus Leonard et al, 1998 crabs grazers diatoms Nutrients mussels Larvae barnacles Plankton Leonard et al, 1998 crabs crabs grazers diatoms Nutrients whelks grazers mussels Larvae barnacles Plankton diatoms Nutrients mussels barnacles Larvae Plankton Interference competition, exploitative competition for resources other than food (-) Predators (-) (-) Induced morphological or chemical defenses, hiding, retreat to refuges Depletion of more nutritious, palatable or accessible prey - (+) Consumers + (-) Cover from (for) predators Stimulation of areaspecific primary productivity Plants (+) Powers. ‘92. Ecology 73: 733