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Community ecology

I. StructureA. DiversityB. Trophic structureC. Species with big impacts

II. Dynamics and interactionsA. SuccessionB. Food web dynamicsC. Complex interactions

Community ecology

What’s a community?

Assemblage of populations co-occurring in space and time

• the diversity of species in a habitat

• interactions among these species

At the largest scale, communities are often characterized by the dominant vegetation.

Example:Pinyon-juniper community of parts of the Mojave

What is biological diversity?

1) Species Richness = number of species in a given area

Community structureA. Diversity (who’s there in a community)

What is biological diversity?

1)The number of species in a given area, also called species richness

2) Relative abundance of species in a given area

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2) The relative abundance of species in a given areaImagine that in each of two communities,

A and B, there were 10 species, and a total of 100 individuals

Do they have the same species richness?

What other information might you want to know about them?

How are the individuals distributed among species?

Imagine:

In Community A: 10 individuals of each species

In Community B: 91 individuals of one species and 1 individual of all of the other 9 species

In Community A:

In Community B:

Which do you think is more diverse?

In Community A:

In Community B:

Various measures, e.g. the Shannon-Wiener Diversity Index, measure the spread of individuals across species

Community A would score higher

I. Community structureB. Food webs (the trophic structure, or the “Who’s eating who” of a community)

At its simplest, a food chain:

Secondaryconsumer (predator)

Primary consumer (herbivore)

Producer (plant)

TrophicLevelsSee Table 55.1

I. Community structureB. Food websA more realistic example from the tropics

Lots of interactions between species. Many leafminers feed on more than one plant, many wasps attack more than one leafminer.Many species don’t feed on just one trophic level

Parasiticwasps

Leaf-miners

Plants

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I. Community structureC. Species with large impacts1. Dominant species – most abundant or

have the highest total biomass

Exert controls over the distribution of other species

e.g. Sugar maples in eastern forests –impact soil and shades other plants

2. Example: a sea star maintains intertidalanimal diversity

Treatments: Predatory sea stars removed from some tidal pools for several years, not from others

pool with sea stars

(eco beaker)2. The predatory sea star maintains animal diversity in the intertidal zone

‘63 ‘65 ‘67 ‘69 ‘71 ‘73Year

Control

Sea stars removed

20

15

105

0No.

of s

peci

es

Mussels(dominantcompetitor)take over

2. Example: a sea star maintains intertidalanimal diversity

The experimenter called the sea star a keystone species

The keystone keeps the arch from falling apart

In communities, a keystone species is one thatIs especially important in maintaining speciesdiversity

I. Community structureC. Species with large impacts

3. Ecosystem engineers – organisms that maintain diversity by create, modify, and maintain habitat

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3. Ecosystem engineers – organisms that create, modify, and maintain habitat

e.g. Beavers

3. Ecosystem engineers Beaver ecosystem engineering:

makes new habitats and increases plant diversity by 33%!!

I. StructureA. DiversityB. Trophic structureC. Species with big impacts

II. Dynamics and interactionsA. SuccessionB. Food web dynamicsC. Complex interactions

Community ecology

II. Community interactions

Communities are dynamic in time

A. Ecological succession:Gradual, sequential changes in species

composition

2 kinds depending on the starting point (both involve disturbance)

A. Ecological Succession

Primary successionEstablishment of the community where there had been no organisms

Examples?Following glaciersFollowing volcanoes

A. Ecological Succession

Secondary successionReestablishment of organisms on disturbed sites

Examples?Regeneration of forestsHurricanes

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A. Ecological Succession

As succession happens, plants change their environment

This allows new species of plants to grow there

N

Fig 55.14A. Ecological Succession

Not just a change in the plant community

i.e. fungi on fallen leaves (Fig 55.15)

TIME

II. Community interactionsB. Community interactions can influence community structure (e.g. species diversity, trophic structure)

II. Community interactionsB. Food webs

Predator

Herbivore

Plant

Omnivore

In community ecology omnivores are defined as animals that feed on more than one trophic level

Omnivore

Predator

Herbivore

Plant

• By feeding on competitors as well as herbivores, omnivores should win competitions with other predators

• but generally be less efficient at feeding on and suppressing the herbivore population

1. Predictions about omnivores

A test of the hypothesis from a cotton field...

Omnivory in a cotton field

Cotton aphid- the herbivore

Lacewing- specialist

predator

Assassin bug- omnivore

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Omnivory in a cotton fieldA team of researchers caged various combinations of aphids and predators on plants.

1st prediction: Omnivore will win competition for aphids

1st prediction: Omnivore (assassin bug) will dominate specialist (lacewing) in competition for aphids

Do the results support the first prediction?

Lace- L + L+ Lacewings +wings pred.1 pred.2 assassin bugsPr

op. l

acew

ings

sur

vivi

ng 0.6

0.3

2nd prediction: Omnivore will be less efficient at reducing the prey population

+

vs.

+

2nd prediction: Omnivore will be less efficientat reducing the prey population.

Do the results support the second prediction?

Aphids +lacewings

Aph

idpo

pula

tion

grow

th

Aphids Aphids +assassin bug

I. Community interactionsB. Food webs

2. Trophic cascades

Models say that plant biomass depends on the number of trophic levels –

if even number = plant biomass low

if odd number = plant biomass high

2. Trophic cascades

When only a herbivore is present, the plant biomass is kept low (2 trophic levels)

Plant

Herbivore (The thickness of the arrow indicates degree of impact on lower level)

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2. Trophic cascades

When only a herbivore is present, the plant biomass is kept low (2 trophic levels)

Plant

Herbivore

Example - Galapagos tortoises have no natural predators, keep plant biomass low

2. Trophic cascades

(3 trophic levels)

Predators should keep herbivore population in check plant biomass relatively high Plant

Herbivore

PredatorFish fry

Midgelarvae

Algae

Experimenter predicted a N. California river food web without large fish would look like this

2. Trophic cascades(4 trophic levels)

Secondarypredator should keep predator population in check herbivores flourish plant biomass relatively low

Large fish

Fish fry

Midgelarvae

AlgaePlant

Herbivore

Predator

Secondary predator

2. Trophic cascades Large fish

Fish fry

Midgelarvae

Algae

Experimenter tested the trophic cascade hypothesis in a stream with large fish excluded and included in cages

Prediction?

With large fish more midges, less algae

Without large fish fewer midges, more algae

2. Trophic cascades

With large fish more midges, less algae

Without large fish fewer midges, more algae

w/ large fish w/o large fish

1) Midge density

40

20

0

2. Trophic cascades

With large fish more midges, less algae

Without large fish fewer midges, more algae

Algal sp. A Algal sp. B

w/o large fishw/ large fish

2) Weight of algae (mg/cm2)

1200

600

0

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2. Trophic cascades

The predictions of trophic cascade theory are not always supported

What might reduce the impacts of one trophic level on the level below?

C. Community interactions in an oak forest – some background on Gypsy Moths

The population peaks every 6-10 years

83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Gyp

sy M

oth

Den

sity

Primarily feed on eat oak leavesVoracious feeders

Community interactions in an oak forest –some background

When the populations peaks, serious defoliation occurs

Small rodents feed on acorns and gypsy moth pupae

Oaks produce acorns every few years (called mast)(remember moth pop fluctuates in time too)

Community interactions in an oak forest –some background

Community Interactions in an oak forest

Researchers asked:What is controlling the dynamics of gypsy moth

populations? Of rodent populations?

Conducted several large scale manipulations….

Community Interactions in an oak forest

ManipulationsRemove all mice from forest (simulate

predation)Add acorns to forest (simulate mast)

Monitored:Moth & mice populationsAcorn production, lead defoliationPredation on moth pupae

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Community Interactions in an oak forest

Mice pops increase in response to mast

Dense mice pops prey on moth pupae and prevent emergence

Keeps moth pop low

+

+

+

Fig. 55.2

Community Interactions in an oak forest

Predators reduce mice population

Mice eat less pupaeMoth pop increaseMoths defoliate treesAcorn production drops

mice

Community Interactions in an oak forestIf gypsy moth pops recovered following

defoliationThis would really hammer the oaks –

preventing their recovery

Instead – the mice reduce moths population, allowing trees to recover

Studying more than one pair of species lets us see the INDIRECT interactions and feedback controls in a community