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Community ecology
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
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
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
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
A. Ecological Succession
As succession happens, plants change their environment
This allows new species of plants to grow there
N
Fig 55.14
A. 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
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
thAphids 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)
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
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?
