Community Ecology

Climate and the Distribution ofEcological Communities

Communities are assemblages of large numbers of speciesthat all interact with each other.

Areas with different climatic characteristics contain different ecological communities.

Climate types are classified using the Koeppen Classification System.

• Categorizes climate types based on annual temperatureand precipitation, as well as variations occurring in these two variables.

• Examples: tropical wet forests, subtropical deserts, temperate grasslands, temperate forests, boreal forests, and tundra.

Climate and the Distribution ofEcological Communities

Productivity is positively correlated with temperature and humidity.

Communities have a characteristic pattern or type of disturbance.

How Predictable Are Community Assemblages?

Two views of community dynamics exist.

• Clements believed that communities are stable, integrated, orderly, and predictable entities.

• Gleason believed communities are neither stable nor predictable, but a matter of history and chance.

• Historical and experimental data support Gleason’s view.

Belém, BrazilTemperature

(ºC)

40

30

20

40

30

20

50

10

0J J JF M MA A S O N D

Variation

Annual total:HIGH

Variation:HIGH

Precipitation(cm)

Very lowvariation

Average:HIGH

Variation:VERY LOW

Month

Climate characteristics

Appearance

Temperature(ºC)

40

30

20

20

10

0J J JF M MA A S O N D

Annual total:VERY LOW

Variation:LOW

Precipitation(cm)

Variation

Average:HIGH

Variation:MODERATE

Month

10

0

Yuma, Arizona

(Freezing)

Climate characteristics

Appearance

Temperature(ºC)

40

30

20

20

10

0

J J JF M MA A S O N D

Annual total:LOW

Variation:MODERATE

Precipitation(cm)

Average:MODERATE

Variation:MODERATE

Month

10

0

Denver, Colorado

(Freezing)

Climate characteristics

Appearance

Temperature(ºC)

40

30

20

20

10

0

J J JF M MA A S O N D

Annual total:MODERATE

Variation:LOW

Precipitation(cm)

Average:MODERATE

Variation:MODERATE

Month

10

0

Chicago, Illinois

(Freezing)

Climate characteristics

Appearance

Temperature(ºC)

40

30

20

20

10

0

J J JF M MA A S O N D

Annual total:LOW

Variation:LOW

Precipitation(cm)

Average:LOW

Variation:VERY HIGH

Month

10

0

Dawson, Yukon, Canada

Climate characteristics

(Freezing)

–10

–20

–30

Appearance

Temperature(ºC)

40

30

20

20

10

0

J J JF M MA A S O N D

Annual total:VERY LOW

Variation:LOW

Precipitation(cm)

Average:VERY LOW

Variation:HIGH

Month

10

0

Barrow, Alaska

(Freezing)

–10

–20

–30

Climate characteristics

Appearance

1 2 3 4 5 6 7 8 9

10 11 12

1

10

20

30

40

50

60

Ponds

Pla

nkt

on

sp

eci

es(n

um

ber

ed r

ath

er t

han

nam

ed,

for

sim

pli

city

)

EXPERIMENT TEST ON COMMUNITY STRUCTURE

1. Construct 12 identical ponds.Fill at the same time with sterilewater so that there are nopreexisting organisms.

2. After one year, examinewater samples from eachpond under the microscope.Count the number of planktonspecies in each sample.

3. Plot results.

Clement hypothesis:Biological communitieshave a predictablecomposition.

Gleason hypothesis:The composition of biologicalcommunities is largely amatter of chance.

REJECTED SUPPORTED

Totalspeciesin eachpond

35 31 38 35 39 31 35 30 31 37 33 34

1234

Ponds12341 2 3 456789

10 11 12

1

10

20

30

40

50

60

Pla

nkt

on

sp

eci

es(n

um

ber

ed r

ath

er t

han

n

amed

, fo

r si

mp

lic

ity)

Clement hypothesis:Biological communitieshave a predictablecomposition.

Gleason hypothesis:The composition of biologicalcommunities is largely amatter of chance.

REJECTED SUPPORTED

Totalspeciesin eachpond

35 31 38 35 39 31 35 30 31 37 33 34

1. Construct 12 identical ponds.Fill at the same time with sterilewater so that there are nopreexisting organisms.

2. After one year, examinewater samples from eachpond under the microscope.Count the number of planktonspecies in each sample.

3. Plot results.

EXPERIMENT TEST ON COMMUNITY STRUCTURE

How Predictable Are Community Assemblages?

Disturbance and change in ecological communities.

• Disturbance is any event that removes some individuals or biomass from a community.

• The characteristic type of disturbance found in a community is known as its disturbance regime.

• Important management decisions hinge on understanding the disturbance regimes of any community.

Giant sequoias after a fire

Fire scars in the growth rings

Reconstructing history from fire scars

50

40

30

20

10

00 400 800 1200 1600 2000

Years A.D.

Nu

mb

er o

f fi

res

per

cen

tury

How Predictable Are Community Assemblages? Succession

• Succession is the recovery and development of communitiesafter a disturbance occurs.

• Primary succession removes all organisms and soil, while secondary succession leaves soil intact.

• A distinct sequence of communities develops as succession proceeds.

• Succession is greatly impacted by the particular traits of the species involved, how species interact, the short-term weather conditions, and the overall environmental conditions.

• Glacier Bay, Alaska provides an excellent case study in succession.

Old field

Pioneering species

Early successionalcommunity

Mid-successionalcommunity

Late-successionalcommunity

Climax community

Disturbance ends, site is invaded byshort-lived weedy species.

Weedy species replaced bylonger-lived herbaceous speciesand grasses.

Shrubs and short-lived trees beginto invade.

Short-lived tree species mature; long-lived trees begin to invade.

Long-lived tree species mature.

Climax community

Old field

Disturbance ends, site is invaded byshort-lived weedy species.

Pioneering species

Weedy species replaced bylonger-lived herbaceous speciesand grasses.

Early successionalcommunity

Shrubs and short-lived trees beginto invade.

Mid-successionalcommunity

Short-lived tree species mature; long-lived trees begin to invade.

Late-successionalcommunity

Long-lived tree species mature.

Hypothesis 1: Only one successional pathway occurs in Glacier Bay.

GlacierBay

Alaska

Proposed successionalpathway:

Soils exposed less than 20 years:willow and Dryas

Soils exposed 45-80 years:sitka alder, scatteredcottonwood

Soils exposed 100years: sitka alder,scattered spruce

Soils exposed150-200 years:dense sitkaspruce andwestern hemlock

Glacier Bay

Direction ofglacial retreat

20 km

N

Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.

Early-mid successional Late-mid successional Climax

Alder

Spruce

CottonwoodHemlock

PATHWAY 1

Early successional Mid-successional

PATHWAY 2

Late-successional Climax

No hemlock?

?

Early successional Mid-successional

PATHWAY 3

Late-successional Climax

No hemlock?

?

Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.

Early-mid successional Late-mid successional Climax

Alder

Spruce

CottonwoodHemlock

PATHWAY 1

Early successional Mid-successional

PATHWAY 2

Late-successional Climax

No hemlock?

?

Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.

Early successional Mid-successional

PATHWAY 3

Late-successional Climax

No hemlock?

?

Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.

Recent data indicate that all three pathways exist.

In pathway 1, spruce arrive early and in high densities. Alder never forms continuous groves and western hemlock arrive after spruce.

In pathway 2, alder arrives early and forms continuous thickets. Spruce arrives early as well but develops low-density stands under the alder thicket. Western hemlock is absent.

In pathway 3, alder arrives early and forms continuous thickets. Cottonwood and spruce form a low-density forest int eh alder thickets. Western hemlock is absent.

Species Diversity in Ecological Communities

Quantifying diversity can be simple or complex.

Research has focused on why some communities are more diverse than others and why diversity is important.

Tropical forest Boreal forest

Canopy

Subcanopy

Epiphytes

Vines

Understorytrees and shrubs

Canopy

Understoryshrubs

Species Diversity in Ecological Communities

On a global scale, a latitudinal gradient of species diversity exists for most taxa.

• Species diversity declines as latitude increases.

• Several hypotheses have been proposed to explain thisgradient, but no simple answer exists.

10000

8000

6000

4000

2000

00 10 20 30 40 50 60

Latitude (degrees North or South)

Nu

mb

er o

f va

scu

lar

pla

nt

spec

ies

per

10,

000

km2

Equator

60º

30º

0º

60º

30º

Species Diversity in Ecological Communities

Communities with high diversity are more productive, more resistant, and more resilient than those with low diversity.

Shading indicatesburned areas

LakeYellowstone

Park boundary