Top-Down vs. Bottom-Up Control Community Structure: Top-Down vs. Bottom-Up Control

Hairston, Smith and Slobodkin (1960) Community Dynamics Carnivores Detritivores control Resource limited Frees plants from herbivore control Herbivores 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 X 2) All communities have 3 trophic levels 3) Omnivory not important 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 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) Observed (bottom up) Urchin 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 Low flow Leonard et al, 1998

Interaction of Systems • increased seaweed growth • increased filter feeder growth • increased consumer pressure • increased larval settlement • increased sedimentation • low consumer efficiency • lower densities of organisms with planktonic larvae • higher densities of organisms with planktonic larvae • increased consumer mortality • more spatial competition • less spatial competition

Leonard et al, 1998 Hydrodynamics Flow rate Time

Leonard et al, 1998 Community structure High flow Low flow Tide height Percent cover Percent cover barnacles Fucus mussels Bare space

Leonard et al, 1998 High flow Recruitment rates Low flow Barnacles Mussels Snails Density (#/100 cm2)

Leonard et al, 1998 Crab predation High flow Low flow On Littorina, Nucella, Mytilus Predation Intensity (% mortality)

Leonard et al, 1998 crabs grazers mussels barnacles diatoms Nutrients Larvae Plankton

crabs whelks grazers mussels barnacles Leonard et al, 1998 crabs diatoms diatoms Nutrients Larvae Plankton Nutrients Larvae Plankton

(-) Predators (-) (-) Consumers Plants Interference competition, exploitative competition for resources other than food Depletion of more nutritious, palatable or accessible prey (-) Predators (-) (-) - (+) Induced morphological or chemical defenses, hiding, retreat to refuges Consumers + (-) Cover from (for) predators Stimulation of area-specific primary productivity Plants (+) Powers. ‘92. Ecology 73: 733