OUR Ecological Footprint - 61. Recycle; pay tax for it.2. Live near work; Ride bike; minimize car use.3. Buy energy-efficient furnace.4. Programmable thermostat: winter/summer5. Turn off lights when leave room; unplug appliance6.

Next week’s Lab:

Data Analysis in Computer Lab Data Entry to Excel: Each individual MUST enter all data before lab

SDP2 Manuscript 1 draft ONLY Introduction Methods Literature Cited

Ch 23: Community Development

Ch 21: Community Structure

Objectives• How are communities measured/compared?• Is the community a natural unit?• Are communities in equilibrium? stable?• How strong are connections among species?• How are food febs measrued/compared?• Do complex food webs aid stability?• What controls abundance within trophic levels?• Do keystone species aid stability?

• Next lecture: How do communities change following disturbance?

What is a community? assemblage of species that co-occur in the same place.

What is a community here? How many are there?

How many are here? How abrupt are boundaries of these communities?

How can community structure be quantified and compared?

*** Which variables can be used to describe the species diversity of a community?

Which community is more diverse?

• Species richness

• Species relative

abundance

Species richness (# of species) = S• S varies with sample size so…• Compare S by rarefaction:

equal-sized subsamples randomly drawn.

• Species richness: # of species BUT species differ in abundance and thus in role

• Species diversity: weight species by their relative abundance

• Simpson’s index: D = 1 / pi2

p = proportion of each species in total sample

• Shannon-Wiener index: H' = - pi ln pi

Measures of community structure

Calculate Species Diversity:

Species No. Ind. pi pi2

ln pi pi ln pi

1 5 .25 .0625 -1.386 -.3465 2 4 .20 .0400 -1.609 -.3218 3 3 .15 .0225 -1.897 -.2846 4 4 .20 .0400 -1.609 -.3218 5 4 .20 .0400 -1.609 -.3218Total (N) 20 1.00 ∑=.205 ∑= -1.5965

• D = 1/ ∑pi2 = 1/.205 = 4.878

• H' = -∑ pi ln pi = 1.5965

Comparisons of diversity indices among communities.

•***What factors increase species diversity?• more species.• less difference in relative abundance among species.

C1C2C3C4C5

‘Community’ has many meanings:

• Spatially defined• Functionally defined --> focus on

interactions• Community = an association of

interacting populations

• First: focus on spatially defined…

Is the community a natural unit of ecological organization?Closed vs. open community structure…

Holistic

Individualistic

Soil conditions reflecting underlying geology may cause prominent community boundaries = ecotone.

Concentrations of soil minerals change sharply across ecotone.

Replacement of species across an ecotone.

Historical arguments…What is the ‘nature’ of the community?

• Clements: • holistic concept• superorganism/interdependent• whole greater than sum of parts• closed with discrete boundaries• emphasis descriptive and on classification

Gleason (UI!):

• individualistic concept• haphazard assemblages of species• not interdependent• open with no natural boundaries• emphasis on community dynamics

and functional organization

• Which concept has most support?

Whittaker (UI!): gradient analysisHow are species distributed along a physical gradient?

• Open or closed?• Ecotones?• Continuum?

• Most communities are open…

• species distribute independently of other species.

Continuum concept: Within broadly defined habitats, species replace one another continuously along gradients of physical conditions.

***Do these data support the ‘continuum concept”?

***Do these data support a stable (equilbrium) or transient (non-equilbrium) view of the community?

Question: Do identical communities develop in identical environments?• ‘Clemensian’ hypothesis: yes

• ‘Gleasonian’ hypothesis: no

• Experimental set-up?

• ‘Clemensian’ prediction: Identical plankton

communities will develop in all ponds.

•‘Gleasonian’ prediction: Different plankton

communnities will develop in different

ponds.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

2. Examine water samples;3. ID each speciesPresent in each pond.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

What is conclusion?

• What is the ‘nature’ of the community?• How tightly connected are species?• Are communities stable or transient?

Later models Redundancy Rivet

Non-equilibrium Equilibrium

OPEN CLOSED

How can community structure be quantified and compared?

*** Which variables can be used to describe the species diversity of a community?

Which community is more diverse?

• Species richness

• Species relative

abundance

Species richness (# of species) = S• S varies with sample size so…• Compare S by rarefaction:

equal-sized subsamples randomly drawn.

• Species richness: # of species BUT species differ in abundance and thus in role

• Species diversity: weight species by their relative abundance

• Simpson’s index: D = 1 / pi2

p = proportion of each species in total sample

• Shannon-Wiener index: H' = - pi ln pi

Measures of community structure

Calculate Species Diversity:

Species No. Ind. pi pi2

ln pi pi ln pi

1 5 .25 .0625 -1.386 -.3465 2 4 .20 .0400 -1.609 -.3218 3 3 .15 .0225 -1.897 -.2846 4 4 .20 .0400 -1.609 -.3218 5 4 .20 .0400 -1.609 -.3218Total (N) 20 1.00 ∑=.205 ∑= -1.5965

• D = 1/ ∑pi2 = 1/.205 = 4.878

• H' = -∑ pi ln pi = 1.5965

Comparisons of diversity indices among communities.

•***What factors increase species diversity?• more species.• less difference in relative abundance among species.

C1C2C3C4C5

Community: functionally defined

How strong are the connections among species in a community?

• Rivet model• tight linkage in ‘web of life’• obligate associations of species• or obligate exclusion of species• Redundancy model• loose ‘web of life’• most species have no impact on other species• species and ecological processes are redundant

***What are conservation implications of the two contrasting models?

• Would focus be on community dynamics or single-species dynamics?

• In which model are keystone species important?

Feeding relationships organize communities in food webs. Communities of similar diversity can havevery different food webs.

***What changes with increased food web complexity? What doesn’t change?

# trophic levels 3 4 6

Variables that quantify food webs:

• # species• # guilds (groups of species with different feeding or

foraging ecology) • Total # feeding links• # feeding links per species

• Connectance = # interactions/total possible• = # interactions/[S(S-1)/2]

• Linkage density = # interactions/# species

Does greater complexity of food web increase community stability?

***Does food web complexity lead to increased community stability?

• Pro:• alternative resources--->less dependent on

fluctuations in any one resource• energy can take many routes --> disruption

of one pathway shunts more energy to another

• Con: • more links may create pervasive,

destabilizing time lags in population processes

Question: Which are more stable when disturbed: species-rich or less-diverse communities?

Hypothesis: Resistance to disturbance increases with increasing species richness.

Null hypothesis: No relationship exists between resistance and species richness.

Experimental Design?

Prediciton: Plots with more species before drought are more resistant to change.

Null prediction: All plots have similar resistance regardless of species richness.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

What is the conclusion?

Different approaches to depict food webs--->show different ways in which populations influence one another.

Connectedness

Energy flow

Functional

What influences abundance with each trophic levels?H1: predation (top-down control) H2: production by plants (bottom-up control).

Trophiccascadelinks alltrophiclevels.

Bottom-up Top-down

Trophic cascade: indirect effects of consumer-resource interactions extended through additional trophic levels of the community.

• top-down effect• consumers depress size of trophic

level immediately below them, which indirectly increases populations two tropic levels below.

Keystone species: non-redundant, key species in maintaining community stability and diversity

• can be: • plants• herbivore • top predator

If a beetle is the keystone species,then…removal of the beetle will ---> lower species diversity.

Objectives• How are communities measured/compared?• Is the community a natural unit?• Are communities in equilibrium? stable?• How strong are connections among species?• How are food webs measrued/compared?• Do complex food webs aid stability?• What controls abundance within trophic levels?• Do keystone species aid stability?

• Next lecture: How do communities change following disturbance?

VocabularyChapter 21 Community Structure biological community rivet model redundancy model holistic concept organismic concept* individualist concept* species richness guilds closed community open community ecotones continuum concept gradient analysis food web connectednesswebs energy flow webs functional webs connectance linkage density keystone species trophic cascade top-down control bottom-up control dominants species-area relationship* diversity indices relative abundance Simpson’s index Shannon-Wiener index rarefaction equilibrium force* non-equilibrium force* species diversity* species evenness* species richness*