Ecology 8310 Population (and Community) Ecology "Indirect Effects" A terminological milieu Classic indirect effects (1 example) Higher order interactions (2 examples)

Indirect effect Trait-mediated indirect effect Trait-mediated indirect interaction Trait-mediated interaction Interaction modification Non-lethal effects Non-consumptive effects Trait-modified indirect effect Density-mediated indirect effect Behavioral indirect effect Higher order interaction

Indirect effect Higher order interaction

Indirect Effect An effect of one species on another, mediated by a change in the density of an “intermediate” species (a series of >2 direct effects)

P N1N1 N2N2 Keystone predation Exploitation competition Trophic cascade Apparent competition Indirect mutualism P N2N2 R P1P1 P2P2 N1N1 N2N P N1N1 N2N2 - P1P1 P2P2 N

Classic experimental study: Wootton (1994)

Intertidal food web Birds Mussels Goose Barn. Acorn Barn. Snail Starfish

Data: Response to Bird Exclusion Birds Mussels Goose Barn. Acorn Barn. Snail Starfish (no data) What hypotheses might explain these patterns?

Hypothesis 1 Birds Mussels Goose Barn. Acorn Barn. Snail Starfish Another option? How can we explain the data?

Hypothesis 2 Birds Mussels Goose Barn. Acorn Barn. Snail Starfish

Hypothesis 3 Birds Mussels Goose Barn. Acorn Barn. Snail Starfish

Hypothesis 3 Birds Mussels Goose Barn. Acorn Barn. Snail Starfish Hypothesis 1Hypothesis 2 Birds Starfish Acorn Barn. Mussels Snail Goose Barn. Acorn Barn. + + Can we come up with a test of these hypotheses?

Two approaches 1.Manipulate intermediate species directly (and look at effects) 2.Remove intermediate species: does indirect effect of birds disappear?

It the absence of snails: what will happen to the other species, if we then manipulate birds?

Hypothesis 3 Birds Mussels Goose Barn. Acorn Barn. Snail Starfish Hypothesis 1Hypothesis 2 Birds Starfish Acorn Barn. Mussels Snail Goose Barn. Acorn Barn.

Response thru snails (Nucella): Weak effects of snail manipulation Bird effects do not depend on presence of snails Supports Hypothesis 1 Other focal taxa…

Hypothesis 3 Birds Mussels Goose Barn. Acorn Barn. Snail Starfish Hypothesis 1Hypothesis 2 Birds Starfish Acorn Barn. Mussels Snail Goose Barn. Acorn Barn.

Effects thru gooseneck barnacles (Pollicipes): Large effects of Pollicipes (reduced acorn barn, mussels, snails) Bird effects depend on presence of Pollicipes (key result) Supports Hypothesis 1 (not 2 & 3)

Creative experimental approach based on clear definition of "indirect effect"

Higher order interaction the direct effect of one species on another depends on the density of a third (i.e., a ij is a function of N k ): e.g., Prey (i) Predator (j) A third species (k) a ij a ij is a function of N k

Higher order interaction == a direct effect (but influenced by another species) Indirect effect == a result of a series of direct effects

Higher Order Interactions: Non-linearities (e.g., a type II functional response with >2 prey) "Trait-moderated" indirect effects: Habitat shifts (e.g., predator-induced; aka “behavioral indirect effects”) Morphology (tadpole tails, barnacle lips, Daphnia helmets) Physiology (activity, stress) Life history (e.g., early reproduction)

The key players: Predator Prey Resource

Three treatments (3 years): Predictions? --plant abundance --plant diversity

Richness: No. species Evenness: a measure of equitability (1=equal abundances; 0=one dominant) Symbols are different years

Solidago rugosa

Non-lethal effects of predator on prey can influence prey's resource assemblage

1)3 mortality levels (removed 9, 4, 25% per 2.5 d) X 4 non-lethal levels (0, 1, 2, 4 caged Anax) 2)Effect of 2 lethal Anax

Use these data to partition lethal and non-lethal effects

Subsequent reviews have shown non-consumptive effects can be very large relative to consumptive effects. Preisser et al. 2005

HW6: Consider a 3-species system: 1 predator with 2 prey. The predator eats both prey types, and exhibits a type 2 functional response. The prey do NOT compete. Consider an experiment in which you reduce (and maintain) prey species A to half of its ambient density. You monitor the demographics and density of prey species B. In the absence of any trait-moderated effects, explain the subsequent dynamics of the predator and the other prey species (relative to a control system that remains at its equilibrium). Make sure you include the short-term effects of the manipulation as well as the long-term outcomes. Due by Friday, 5pm (via to Craig)