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Lecture 25: Trophic Cascades Top-down Control: • Abundance/biomass/diversity of lower trophic levels depends on effect of consumers from higher trophic levels • Trophic levels alternate between consumer & resource limitation • In terrestrial systems: top-down control is limited to low diversity systems The world is GREEN (Hairston et al. 1960) • e.g. Predators keep herbivores in check, primary producers compete, carnivores compete • Removal of herbivores = no effect on plants Bottom-up Control • Structure of community depends on factors from lower trophic levels (nutrient concentrations & prey availability) • Organisms limited by trophic level below them. • Plants have primacy of control but… 1 The world is prickly & tastes bad (Murdoch 1966) • Plant defenses make herbivory hard. • Herbivores may compete, predators compete • Removal of predators or herbivores does not affect plants The world is white, white yellow & green (Fretwell 1977, Oksanen et al. 1981) Predator effects vary along a gradient of 1° productivity Predictions: • Unproductive : herbivores rare (not enough food) • High productivity: few herbivores (pred. limited), green • Intermediate: plants are pred. limited (prey can’t support predators) • High & Low productivity: plant competition • Intermediate: herbivore competition • High productivity: predator competition Evidence for WYG • Biomass: productivity relationship is confirmed • Herbivore removals = increase plants • Plants only compete in low & high prod. 2 Top-down vs. Bottom-up Fath (2004) • Control of community structure & change is more complicated than a dichotomy • Control is distributed among ecosystem elements (all have input/output effects) • Thus, must examine all elements to determine control: • After initial E input what affects energy flow & storage? Network Analysis • Maximum donor control: sequential food chain • Maximum recipient control: one node is final recipient for all energy flow • Unrealistic & Idealized, but can use as a comparison for relative strength of donor/recipient control in real systems Trophic Cascades Paine (1980): • Top-down effects of predators influenced abundance of species at lower trophic levels. • e.g. Lake productivity : 50% of the variability can be attributed to top-down effects of top predators But, trophic effects can cascade up & down 3 Types of Trophic Cascades Species-level: • Changes in predator abundance affect the success of a subset of plant species • Generally do not explain habitat-wide changes in biomass distribution. Community-level: • Substantially alter the distribution of plant biomass throughout an entire community Consensus (Polis et al. 2000) • Top-down influence on structure • Change in plant biomass must be significant or plant community composition (amount of change?) • Community-level cascades are relevant, species-level cascades are not Terrestrial vs. Aquatic Systems Strength of Cascades (Shurin et al. 2002): • Strongest in lentic & marine benthos • Weakest in marine plankton & terrestrial • Effects on herbivores stronger than on plants 4 More… • Stronger effects in aquatic systems but, among the aquatic systems, differences were as significant as between wet & dry systems • Stronger attenuation in terrestrial environments • Weak representation of terrestrial experiments Aquatic Systems a) Discrete & homogeneous habitats b) Prey population dynamics fast rel. to pred. c) Common prey “uniformly edible” (unlikely) d) Simple & trophically stratified communities, strong interactions (but, omnivory common & diverse –scale issue) Terrestrial: Antiherbivore defenses (weakens effect of carnivore removal) a) Heterogeneous & poorly separated habitats b) Variable prey dynamics rel. to predator (effect appears weak) c) Prey rarely uniformly edible d) Reticulate & complex systems, weak & diffuse interactions 5 Other differences a) Body size ratios b/w prey & predator larger in aquatic systems b) Aquatic primary producers are more nutritious c) Aquatic: higher mass-specific biomass production d) Herbivores stronger effect on primary producers in aquatic systems But… • Differences among systems account for 35% or less of variation in herbivore/plant response Within community differences are more important: • e.g. diversity, behaviour, productivity, structural complexity… Empirical Evidence: Aquatic • Lots of evidence • Primarily from lentic systems (marine & streams as well) • Community-level results rather than species-level • But, much more highly studied… 6 Empirical Evidence: Terrestrial North American forests (& European tundra): • Wolves structure populations of moose & deer, cascades to plant abundance Canadian Boreal Forest: • Strong predator-herbivore & herbivore-plant relationships • Weak predator-plant effects Chilean Scrubland: • Predator removal: herbivore increase, plant response Empirical Results: Schmitz et al. (2000): • 41 studies of carnivore removals • Quantified direct & indirect effects • Species-level cascades not community-level cascades Schmitz et al. Results • Species-level cascades are common (45/60) • Similar magnitude of indirect & direct effects as in aquatic systems • Indirect effects are obvious quickly (< 6 mos.) • Trophic cascades are weaker in food webs with high intermediate species diversity 7 Plant Biomass vs. Plant Damage • When biomass was used, carnivore effects tended to be attenuated • Biomass considered better measure of community effects, damage tends to be a short-term response Mechanism • Important to understand pathways, especially because presence/absence studies do not always demonstrate control e.g. Spiller & Schoener (1998): • Lizard removal: no effect on plants, but direct effect on herbivore abundance, therefore attenuation • But, lizard removal: increase in spiders, intraguild replacement Behavioural Changes • Predator removal/addition may not change herbivore density but may affect herbivore behaviour • e.g. Herbivores may adjust prey preference, habitat preference in response to predation risk • May affect plant species composition (must examine species!) 8 Problems • Studies that show NO trophic cascade generally not published (bias) • Inadequate information about species composition, modes of interaction, degree of interconnectedness • More emphasis on mechanism (behav., physiol., morpho.) 9