Lecture 13: Trophic Cascades & Community Structure

Question 1

Keystone predators

Answer 1

Disproportionate effect on community diversity relative to biomass

Question 2

Trait-mediated vs density-mediated indirect effects of consumers on lower trophic levels

Answer 2

Actively hunting spider vs. sit-and-wait spider on grasshopper and herbs
* suppression of competitive dominant plant species by grasshoppers, mediated by avoidance of sit-and-wait predator, increases plant diversity

Question 2

P. rimator (active hunter) spider effect

Answer 2

• density reduction in grasshoppers alleviates herbivory on Solidago (indirect positive)
• Other herbs outcompeted by Solidago (indirect negative)

Question 3

P. mira (sit-and-wait) spider effect

Answer 3

• grasshopper switches from foraging on grass –> Solidago –> reduced competition w/ Solidago v. other herbs

Question 4

Follow-up study to active v sit –> “non-lethal” spider

Answer 4

• more grass present when spider in community –> place cages around to set environment (some w/ glued spiders)
• –> addition of wither lethal or nonlethal spiders decreased herbivory
• –> reduction in herbivory occurs because spider change grasshopper behavior
• grasshoppers that consume less grass add less N –> slow decomposition of grass (limited amount of nutrients in soil)

Question 5

Phytochemical landscape concept

Answer 5

• linking nutrient availability to the herbivore + consumer distributions
• trophic interactions <–> plant/autotroph phytochemistry <–> nutrient dynamics
• reflection of the history of past biological and physical processes & constrain those than can occur in the future

Question 6

Bottom-up control

Answer 6

when the abundances of organisms at each trophic lvl are determined by the amount of energy available from consumers

Question 7

Top-down control

Answer 7

when the abundances of organisms are determined by the existence of predators at the top of the food web

Question 8

“The world is green” hypothesis (HSS-Fretwell)

Answer 8

Constraints on population growth depend on trophic level (ex. predators keep herbivores in check –> allow most ecosystems to have excess PP biomass)

Question 8

HSS-Fretwell model for predicting PP biomass

Answer 8

odd levels = green; even = barren

Question 9

Consumer-resource interactions working alongside competition & abiotic competition to structure species distributions

Answer 9

Sea stars on rocky shores affecting lower distribution of mussels
* how to test? –> exclude sea stars