Ecology 8310Population (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

N1 N2

Keystone predation

Exploitation competition

Trophiccascade

Apparent competition

Indirect mutualism

P

N2

R

+

-

-

--

+

P1 P2

N1 N2

-

+

-

+

--

++

P

N1 N2-

P1 P2

N

-+ -

+

--

-

-+-+

Classic experimental study: Wootton (1994)

Intertidal food web

Birds

MusselsGoose Barn.

Acorn Barn.

Snail

Starfish+

++

+

++-

-

- --

-

-

-

-

-

-+

+ -

-

GW Robinson

Gerald and Buff CorsiG&B Corsi

Data: Response to Bird Exclusion

Birds

MusselsGoose Barn.

Acorn Barn.

Snail

Starfish (no data)

What hypotheses might explain these patterns?

Hypothesis 1

Birds

MusselsGoose Barn.

Acorn Barn.

Snail

Starfish+

++

+

++

--

- --

-

-

-

-

-

-+

+ -

-

Another option?

How can we explain the data?

Hypothesis 2

Birds

MusselsGoose Barn.

Acorn Barn.

Snail

Starfish+

++

+

++

--

- --

-

-

-

-

-

-+

+ -

-

Hypothesis 3

Birds

MusselsGoose Barn.

Acorn Barn.

Snail

Starfish+

++

+

++

--

- --

-

-

-

-

-

-+

+ -

-

Hypothesis 3

Birds

Mussels

Goose Barn.

Acorn Barn.

Snail

Starfish

Hypothesis 1

Hypothesis 2

Birds Birds

Starfish

Acorn Barn.

Mussels

Mussels

Snail

SnailGoose Barn.

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 1

Hypothesis 2

Birds Birds

Starfish

Acorn Barn.

Mussels

Mussels

Snail

Snail

Goose Barn.

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 1

Hypothesis 2

Birds Birds

Starfish

Acorn Barn.

Mussels

Mussels

Snail

SnailGoose Barn.

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., aij is a

function of Nk): e.g.,

Prey (i)

Predator (j)

A third species (k)aij

aij is a function of Nk

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 email to Craig)