Beyond keystone predation Predation is a pairwise interaction Interference competition is a pairwise interaction Effects on the two species involved There

Beyond keystone predation

• Predation is a pairwise interaction• Interference competition is a pairwise interaction• Effects on the two species involved• There can be effects beyond the pair of species• Indirect effect: An effect of one species on

another that occurs via an effect on a third species

A surprisingIndirect effectPrey

Predator #2Predator #1

+ +--

RESOURCE COMPETITIONnegative effects caused via a shared victim

Indirect effect

Predator

Herbivore

Plant

+

+

-

-

Increase predator Decrease Herbivore Increase Plant

TROPHIC CASCADEeffects produced 2 or more trophiclevels down from top predator

Indirect effectPredator

Prey #2Prey #1

+ +

- -

Decrease prey #1 Decrease Predator Increase Prey #2

APPARENT COMPETITIONnegative effects caused via a shared enemy

Apparent competition• Can play a role in effects of

invasions• Novel pathogens can have

devastating effects on natives– American Chestnut– Pollen data for eastern forests

• White oak 25-65% of stems• Hickory 5-15%• Am. Chestnut 5-15%

• Parallel story for American Elm

Apparent competition Settle & Wilson 1990

• Invasion effects via native enemies– Variegated leaf hopper VLF

(Erythroneura elegantula)– Grape leaf hopper GLF

(Erythroneura variabilis)• Feed on grape• in California GLF native; VLF invasive• 1980s: as VLF spread in San Joaquin

Valley, GLF declined

Parasitoid• Anagrus epos• Egg parasitoid• Attacks both, prefers GLH• as proportion of VLH

increases, proportion of unparasitized eggs that are VLF increases

• and therefore proportion parasitism of GLH increases

Reductions of GLF

• Interspecific competition detectable, but not particularly strong or asymmetrical

• Apparent competition seems to be the main driver of replacement of GLF by VLF

Indirect effect

INTRAGUILD PREDATIONPreying on your competitor

Resource

Intraguild Predator

Intraguild prey

+

+

--

+-

Intraguild predation (IGP)

• Intraguild predator and intraguild prey are competitors

• For IGP to be stable, intraguild prey must be better competitors for the shared resource than intraguild predators– otherwise intraguild prey must have access to

resources unavailable to intraguild predators• high productivity favors intraguild predators• low productivity favors intraguild prey


Productivity (Carrying capacity for resource)

ResourceResource +Intraguild prey

Resource +Intraguild prey +Intraguild predator

Resource +Intraguild predator


• Diehl & Feissel 2001• Tested this with:

– Bacteria (=resource)– Tetrahymena (=intraguild prey)– Blepharisma (intraguild predator)

Indirect effect

Decrease predator #1 Increase Prey #1 Decrease Prey #2 Decrease Predator #2

INDIRECT PREDATOR MUTUALISMpositive effects of one predatoron another via competing prey

Predator #1

Prey #2Prey #1

+ +

- -

Predator #2

-

-

Indirect effects

• Possibilities are complex• Become more complex with more species• Two problems:

– 1. How do you detect indirect effects?– 2. How important are indirect effects in

determining community composition?

Kinds of indirect effects

• Up to this point – density mediated effects

• direct interactions produce effects that in turn have effects on other species

• other possibilities exist

Kinds of indirect effects

• Chains of interactions– effects of one species’ population propagate

through chains (or networks) of other direct interactions like competition and predation

– also called “density mediated interactions”• Interaction modification

– the presence of one species alters in some way the direct interaction of two other species

– also called “trait mediated interactions”

Density vs. Trait mediated interactions

A

B

C

increase C, increases B, which indirectly decreases A

A

B

C

A C

the presence of B changes something about how A and C affect one another

Examples of trait mediated interactions

• Apocephalus sp.– phorid fly– parasite of ants

• Pheidole diversipilosa– host

• Other ant species competing for food– presence of competitors improves

Apocephalus ability to find and to parasitize P. diversipilosa

• Presence of Apocephalus at food– reduces competitive ability of P.

diversipilosa

Detecting indirect effects

• You must know something about the pairwise direct interactions within the community

• You often must do experiments, typically species removals and additions

• If you don’t know which pairwise interactions are present, indirect effects may be interpreted incorrectly even in an experiment

Misinterpreting an indirect effect in an

experiment

Predator #1

CompetitorPrey

+

+

-

-

Predator #2

-

-

• Remove predator #2• Predator #1 increases• Prey decreases• Competitor increases• If you don’t know the

interactions, it looks like Predator #2 might prey on Competitor

The importance of indirect effects

• Commonly assumed that – direct effects are strong– indirect effects are weak

• Relative to any single direct effect, indirect effects may be stronger, more important determinants of species composition and diversity

• Data? (Wootton 1994)

Intertidal invertebrates (again)

Predatory snailNucella

Goose N. Barn.Pollicipes

Acorn BarnacleSemibalanus

+

+Birds(crows, gulls)

--

MusselMytilus

-

--

-

-

+

-

+

-+

-

-

Sea starLeptasterias -

+

+ -

-

+

Interactions in intertidal• Observation: Exclude bird predation (cages)

– Nucella: decreases relative to control (2 - 4 X)– Pollicipes: increases relative to control (~5 X)– Semibalanus: decreases relative to control (3 - 7 X)– Mytilus: decreases relative to control (to 70%)

• Excluding predator:– 2 prey species decrease – 1 non-prey species decreases– 1 prey species increases

Understanding this effect

• A hypothesis to explain this result• Which direct interactions are strong?

– affect numbers of individuals• Which direct interactions are weak?

– do not affect numbers of individuals

Hypothesis #1: strong & weak interactions




+


--

MusselMytilus

-

--

-

-

+

-

+

-+

-

-


+

+ -

-

+





+


--

MusselMytilus

-

--

-

-

+

-

+

-+

-

-


+

+ -

-

+





+


--

MusselMytilus

-

--

-

-

+

-

+

-+

-

-


+

+ -

-

+

Hypotheses new predictions

• Remove Pollicipes with birds excluded–H #1: Mytilus, Semibalanus, Nucella all

increase–H #2: Mytilus, Semibalanus increase–H #3: Mytilus only increases

• vs. birds excluded only

Hypotheses new predictions

• Exclude birds after removing Pollicipes –H #1: no effects–H #2: Nucella decreases, Leptasterias

increases–H #3: Semibalanus, Nucella decrease,

Leptasterias increase • vs. removing Pollicipes only

Experiment 1Manipulate Pollicipes without birds




+

-

-

MusselMytilus

-

--

-

+

-

-

BirdsEXCLUDED

SeastarLeptasterias

+

-

+-

Experiment 2.Manipulate birds without Pollicipes

REMOVE Pollicipes



+

+

-

Birds(crows, gulls)

MusselMytilus--

-

-

+

-

SeastarLeptasterias

+

-

+-

Results of experiment 1

• Remove Pollicipes in cages that exclude birds– Mytilus increases (2 X)– Semibalanus increases (7 X)– Nucella increases (3.6 x)

• compared to cages with Pollicipes• As predicted by hypothesis #1• Inconsistent with hypotheses #2 & #3

Results of experiment 2

• Exclude birds (cages) after removing Pollicipes – Mytilus unaffected– Semibalanus unaffected– Nucella unaffected

• compared to no exclusion of birds after removing Pollicipes

• As predicted by hypothesis #1• Inconsistent with hypotheses #2 & #3

More...• Experiment 3. Removal of Nucella

– no effects on Pollicipes, Semibalanus, Mytilus

– As predicted by hypothesis #1– Inconsistent with hypotheses #2 & #3

• Experiment 4. Removal of Semibalanus– Nucella decreases– As predicted by hypothesis #1– Inconsistent with hypotheses #2 & #3

Path analysis• Statistical technique for estimating

direct and indirect effects among observational variables

• Analysis predicts important direct paths are:– birds Pollicipes– Pollicipes Mytilus, Semibalanus, Nucella– Semibalanus Nucella– Mytilus Semibalanus

• Most similar to Hypothesis #1

Overall...• Experiment, alterntive hypotheses, new

predictions, new experiments• Sophisticated experiments to test

indirect effects• Statistical technique combined with

experiments• Hypothesis #1 clearly supported• Indirect effects of primary importance in

this system

Trophic cascades

• Hairston, Smith, Slobodkin, 1960. Am. Nat.– Green earth argument– predators limit herbivorous prey and so enhance

production & populations of plants• Examples: Morin pp. 214-221

Predator

Herbivore

Plant

+

+

-

-

Trophic cascades• May involve more than trophic interactions• May cross ecosystem bondaries• Ecosystem engineers: species affect

others, but the interaction has no effect on their own fitness or population growth– Large herbivores– Burrowing species– Fire-prone species

• Trophic cascades can work through ecosystem engineers

Foxes on Aelutian IslandsCroll et al. 2005

• Beginning 1900– Foxes

introduced– Absent on

some

• Effects– Reduced bird

density– Vegetation

change– Change in

nutrient import

Resource subsidy from marine system

hunting

defecating

N, P

Effects of foxes as predators• Without Foxes• Large nesting bird

populations• Lots of guano input

– N, P– high soil P

• More grass, less shrub• Greater grass biomass

• With Foxes• Bird populations reduced

(100x)• Reduced guano input

– low soil P (60x)• Less grass (3x), more shrub

(10x)• Less grass biomass (3x)

significance• Importance of subsidies from one

ecosystem to another• Importance of predation, even predation

several trophic levels removed – trophic cascade

• Trophic cascades can include nontrophic interaction.– Birds impact via ecosystem engineering,

not feeding– This type of effect rarely demonstrated

Trophic cascades across system boundaries(Knight et al. 2005)

• Species with complex life cycles– Aquatic larvae – terrestrial adults– Amphibians, Odonates, Mosquitoes, many

insects– How do predators in one environment

(aquatic) affect trophic systems in the other (terrestrial)?

Fish predation

• Dominant factor in freshwater systems• Influences abundances of many invertebrates

Knight et al.• Eight ponds

– 4 with fish (Sunfish)– 4 without fish– Not experimental

• Dragonflies– Abundances

significantly lower in and around fish ponds vs. no fish ponds.

– Particularly for medium and large dragonflies

Plants and pollinators• St. John’s Wort• More pollinators near fish

ponds– More Diptera, Lepidoptera,

& especially Hymenoptera

Knight et al.

• Fish – Reduce dragonflies– Increases pollinators

• Does this matter to the plants?• Does reduced pollinator density near

fishless ponds reduce plant reproductive success?

Knight et al.• Pollen supplementation

– St. John’s wort

• Supplemental pollen increases seed set near both fish and fishless ponds– Magnitude of increase ~3X greater near fishless

ponds (where pollinators are reduced)– Similar for Sagittaria as well

Effects on pollinators

• Data suggest that effects of dragonflies on pollinators is both density mediated and trait mediated

• Pollinators avoid behaviorally areas with lots of dragonflies

Effects of fish• Solid –

direct• Dashed

- indirect

Significance

• Interactions cross community boundaries• Complex life cycles

– Dragonflies– Other insects– Link terrestrial and aquatic communities

Disturbance and stress

• Disturbance and stress can be accommodated with isoclines

• Assessment of the conditions necessary for coexistence of e.g., competitors

• Chase & Leibold Fig. 2.11

Stress-Resource isoclines

R

Ssp. 1

sp. 2

species 1 only

species 2 only

Nonequilibrium coexistence

• Chase & Leibold– Ch. 6– especially pp. 99-101

• Tradeoffs create equilibrium conditions• Analysis has primarily concerned

conditions where dN / dt = 0• conditions with dN / dt ≠ 0 …

Variation

• intrinsic– variation in e.g., species abundance

produced by deterministic dynamics of population(s)

– cycles, chaos– e.g., Lotka-Volterra predation, logistic

population growth with discrete generations

Variation

• extrinsic– variation imposed on populations or communities

by changing environmental conditions– typically postulated as temporal variation– historical argument: Temporal variation disrupts

equilibrium determined by species interactions– Thus facilitates nonequilibrium coexistence of

competitors

Environmental harshness

• ideas parallel those on extrinsic variation

• harsh environment (stress)– causes mortality– reduces impact of competition– facilitates coexistence

Harsh or fluctuating conditions• coexistence of competitors is actually not favored by

harsh conditions• harsh conditions may actually reduce the likelihood of

coexistence• fluctuating conditions sometimes can increase the

likelihood of coexistence– do so when extrinsic variation provides "niche opportunities"– species benefit differentially from fluctuations– different species favored at different points along

environmental variable that fluctuates

Some relevant references• Chesson, P 2000. Mechanisms of maintenance of

species. Annual Review of Ecology & Systematics 31:343-366

___________________________________________• Chesson, P & N Huntly 1997. American Naturalist

150:519-553• Pake, CE & DL Venable 1995. Ecology 76:246–61• Pake, CE & DL Venable 1996. Ecology 77:1427–35• Cáceres, CE 1997. Proceedings of the National

Academy of Sciences USA 94:9171-9175

Harsh conditions• increase mortality (m)• in resource competition that raises R*

– R* = K1/2m / [ pFmax - m ]

• affects all species the same way– does not alter outcome of competition – may slow down approach to equilibrium

• affects species differently– may reverse competitive outcome– isocline model for effect of stress

Stress-Resource isoclines

R

Ssp. 1

sp. 2

species 1 only

species 2 only

Mechanisms of coexistence

• Fluctuation independent– resource differences, trade offs, etc.– the previous lectures on competitive coexistence– can operate in either fluctuating or constant

environments

• Fluctuation dependent– mechanisms that require environmental fluctuation– deterministic (e.g., seasonal)– stochastic (random)

Fluctuation dependent mechanisms• Storage effect

– differential responses to environment– buffered population growth– covariance between effects of environment

and competition (+)• Relative nonlinearity of competition• Both involve "temporal niches"

– concentrates intraspecific effects in time– greatest intraspecific effect at those times

that most limit its population

Storage effect

• differential responses to the environment– different species have greatest population

growth at different values of environmental variable(s) that fluctuate

Storage effect

• covariance between environment and competition– intraspecific competition greatest when a

species is favored by the environment– interspecific competition greatest when the

species’ competitors are favored• sounds as though species would be

greatly harmed by competition when rare

Storage effect• buffered population growth

– resting, inert, or otherwise invulnerable stages– resting eggs– dormant stages– invulnerable, long-lived adults

• limits impact of competition when a species is not favored– species escapes those times when it does not

have an advantage

Storage effect• differential responses, covariance of

environment and competition, & buffered population growth

• combined they render the impact of intraspecific competition on population growth greater than that of interspecific competition

Examples of storage effect

• Cáceres 1997– Daphnia– dormant eggs

• Pake & Venable 1995– desert rodents

• Pake & Venable 1996– desert plants– seed banks

Nonlinearity of competition (fluctuating environment)

Nonlinearity• species a has advantage (greater resource dependent

growth) on average in the fluctuating environment or in a constant environment (arrow)

• greater fluctuation of environment favors growth of species b

• nonlinearity of species a causes fluctuation of competitive factor F when a is abundant and b is rare (benefits b)

• species b causes less fluctuation of competitive factor F when b is abundant and a is rare (benefits a)

Implications• "Niche differences" essential for

coexistence of competitors– differences in limiting factors

• Fluctuations are important as alternative aspects of the environment that limit a species

• In a sense variation becomes another resource axis– Chase & Leibold Fig. 6.3

variation as a resource

Mean Resource

Var

iabi

lity

of R

esou

rce

1 2 3

Documents

Beyond keystone predation Predation is a pairwise interaction Interference competition is a pairwise interaction Effects on the two species involved There