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Chapter 53 campbell chapter 47 starr taggart l.jpg

Chapter 53 (Campbell)Chapter 47 (Starr/Taggart)

Community Ecology


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  • Overview: What Is a Community?

  • A biological community

    • Is an assemblage of populations of various species living close enough for potential interaction


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Figure 53.1

  • The various animals and plants surrounding this watering hole

    • Are all members of a savanna community in southern Africa


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  • Concept 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and disease

  • Populations are linked by interspecific interactions

    • That affect the survival and reproduction of the species engaged in the interaction


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Table 53.1

  • Interspecific interactions

    • Can have differing effects on the populations involved


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Competition

  • Interspecific competition

    • Occurs when species compete for a particular resource that is in short supply

  • Strong competition can lead to competitive exclusion

    • The local elimination of one of the two competing species


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The Competitive Exclusion Principle

  • The competitive exclusion principle

    • States that two species competing for the same limiting resources cannot coexist in the same place


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Ecological Niches

  • The ecological niche

    • Is the total of an organism’s use of the biotic and abiotic resources in its environment

    • If an organism’s “profession” and the habitat it its “address”.

    • An organism’s niche is its ecological role– how it “fits into” an ecosystem


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  • The niche concept allows restatement of the competitive exclusion principle

    • Two species cannot coexist in a community if their niches are identical


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EXPERIMENT

RESULTS

Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland.

When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area.

High tide

High tide

Chthamalus

Chthamalusrealized niche

Balanus

Chthamalusfundamental niche

Balanusrealized niche

Ocean

Ocean

Low tide

Low tide

In nature, Balanus fails to survive high on the rocks because it isunable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata.

CONCLUSION

The spread of Chthamalus when Balanus wasremoved indicates that competitive exclusion makes the realizedniche of Chthamalus much smaller than its fundamental niche.

Figure 53.2

  • However, ecologically similar species can coexist in a community

    • If there are one or more significant difference in their niches


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  • As a result of competition

    • A species’ fundamental niche may be different from its realized niche

    • Fundamental Niche: is the niche potentially occupied by that species

    • Realized Niche: the niche it actually occipies in a particular environment


Resource partitioning l.jpg

A. insolitususually percheson shady branches.

A. ricordii

A. insolitus

A. distichus perches on fence posts and other sunny surfaces.

A. alinigar

A. christophei

A. distichus

A. cybotes

A. etheridgei

Figure 53.3

Resource Partitioning

  • Resource partitioning is the differentiation of niches

    • That enables similar species to coexist in a community

Seven species of Anolis lizard live in close proximity, and feed on insects and other small arthropods. However, competition for food is reduced because each has a different perch, thus occupying a distinct niche


Character displacement l.jpg

G. fortis

G. fuliginosa

Beak depth

Santa María, San Cristóbal

40

Sympatric populations

20

0

Los Hermanos

Percentages of individuals in each size class

G. fuliginosa, allopatric

40

20

Daphne

0

40

G. fortis, allopatric

20

8

10

12

14

16

0

Beak depth (mm)

Figure 53.4

Character Displacement

  • In character displacement

    • There is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species

An example is the variation in beak size between different population of the Galapagos finches.


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Predation

  • Predation refers to an interaction

    • Where one species, the predator, kills and eats the other, the prey


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  • Feeding adaptations of predators include

    • Claws, teeth, fangs, stingers, and poison

  • Animals also display

    • A great variety of defensive adaptations


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Figure 53.5

  • Cryptic coloration, or camouflage

    • Makes prey difficult to spot


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Figure 53.6

  • Aposematic coloration

    • Warns predators to stay away from prey


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  • In some cases, one prey species

    • May gain significant protection by mimicking the appearance of another


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(b) Green parrot snake

(a) Hawkmoth larva

Figure 53.7a, b

  • In Batesian mimicry

    • A palatable or harmless species mimics an unpalatable or harmful model

Larva weaves head back and forth and hisses like a snake.


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(a) Cuckoo bee

(b) Yellow jacket

Figure 53.8a, b

  • In Müllerian mimicry

    • Two or more unpalatable species resemble each other


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Herbivory

  • Herbivory, the process in which an herbivore eats parts of a plant

    • Has led to the evolution of plant mechanical and chemical defenses and consequent adaptations by herbivores


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Parasitism

  • In parasitism, one organism, the parasite

    • Derives its nourishment from another organism, its host, which is harmed in the process


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  • Parasitism exerts substantial influence on populations

    • And the structure of communities


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Three types of parasitism

  • Endoparasites: parasites that live within the body of their host, such as tapeworms and malarial parasites

  • Ectoparasites: parasites that feed on the external surface of host, such as ticks and lice

  • Parasitoidism: Insects – usually small wasp-lay eggs on or in living host. The larvae then feed on the body of the host, eventually killing it.


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Disease

  • The effects of disease on populations and communities

    • Is similar to that of parasites


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  • Pathogens, disease-causing agents

    • Are typically bacteria, viruses, or protists


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Figure 53.9

Mutualism

  • Mutualistic symbiosis, or mutualism

    • Is an interspecific interaction that benefits both species


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Figure 53.10

Commensalism

  • In commensalism

    • One species benefits and the other is not affected


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  • Commensal interactions have been difficult to document in nature

    • Because any close association between species likely affects both species


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Interspecific Interactions and Adaptation

  • Evidence for coevolution

    • Which involves reciprocal genetic change by interacting populations, is scarce


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  • However, generalized adaptation of organisms to other organisms in their environment

    • Is a fundamental feature of life


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  • Concept 53.2: Dominant and keystone species exert strong controls on community structure

  • In general, a small number of species in a community

    • Exert strong control on that community’s structure, particularly on the composition, relative abundance, and diversity of its species.


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Species Diversity

  • The species diversity of a community

    • Is the variety of different kinds of organisms that make up the community

    • Has two components


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  • Species richness

    • Is the total number of different species in the community

  • Relative abundance

    • Is the proportion each species represents of the total individuals in the community


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A

B

C

D

Community 1

A: 25%B: 25%C: 25%D: 25%

Community 2

Figure 53.11

A: 80%B: 5%C: 5%D: 10%

  • Two different communities

    • Can have the same species richness, but a different relative abundance

Ecologist would say that community 1 has greater species diversity, a measure that includes both species richness and relative abundance


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  • A community with an even species abundance

    • Is more diverse than one in which one or two species are abundant and the remainder rare


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Trophic Structure

  • Trophic structure

    • Is the feeding relationships between organisms in a community

    • The transfer of food energy up the trophic levels from its source in plants and other photosynthetic organism (primary producers) through herbivores (primary consumers) to carnivores (secondary and tertiary consumers)

    • Is a key factor in community dynamics


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Quaternary consumers

Carnivore

Carnivore

Tertiary consumers

Carnivore

Carnivore

Secondary consumers

Carnivore

Carnivore

Primary consumers

Zooplankton

Herbivore

Primary producers

Plant

Phytoplankton

Figure 53.12

A terrestrial food chain

A marine food chain

  • Food chains

  • Link the trophic levels from producers to top carnivores


Food webs l.jpg

Humans

Smaller toothed whales

Baleen whales

Sperm whales

Elephant

seals

Leopard

seals

Crab-eater seals

Squids

Fishes

Birds

Carnivorous

plankton

Copepods

Euphausids

(krill)

Phyto-plankton

Figure 53.13

Food Webs

  • A food web

  • Is a branching food chain with complex trophic interactions


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Juvenile striped bass

Sea nettle

Fish larvae

Fish eggs

Figure 53.14

Zooplankton

  • Food webs can be simplified

    • By isolating a portion of a community that interacts very little with the rest of the community


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Limits on Food Chain Length

  • Each food chain in a food web

    • Is usually only a few links long (about 5 trophic levels)

  • There are two hypotheses

    • That attempt to explain food chain length


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  • The energetic hypothesis suggests that the length of a food chain

    • Is limited by the inefficiency of energy transfer along the chain

    • Hypothesis predicts that food chains should be relatively longer in habitats of higher photosynthetic productivity

    • Generally known as the 10% Law: That states that only 10% of energy will be transferred from one trophic level to the next.


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  • The dynamic stability hypothesis

    • Proposes that long food chains are less stable than short ones

    • Population fluctuations at lower trophic levels are magnified at higher levels potentially causing the local extinction of top predators.

    • In variable environment, top predators must be able to recover from environmental shocks (such as extreme winters) that can reduce the food supply all the way up the food chain.


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6

6

No. of species

5

5

No. of trophic links

4

4

Number of species

Number of trophic links

3

3

2

2

1

1

0

0

Low

Medium

High (control)

Productivity

Figure 53.15

  • Most of the available data

    • Support the energetic hypothesis


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Species with a Large Impact

  • Certain species have an especially large impact on the structure of entire communities

    • Either because they are highly abundant or because they play a pivotal role in community dynamics


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Dominant Species

  • Dominant species

    • Are those species in a community that are most abundant or have the highest biomass (the total mass of all individuals in a population)

    • Exert powerful control over the occurrence and distribution of other species

    • Example: Sugar maples in North American forest communities has major impact on abiotic factors such as shading and soil, which in turn affect which other species live there


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  • There is no single explanation for why a species becomes dominant in a community.

  • One hypothesis suggests that dominant species

    • Are most competitive in exploiting limited resources

  • Another hypothesis for dominant species success

    • Is that they are most successful at avoiding predators


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Keystone Species

  • Keystone species

    • Are not necessarily abundant in a community

    • Exert strong control on a community by their ecological roles, or niches


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With Pisaster (control)

20

15

Number of species present

10

Without Pisaster (experimental)

5

0

1963

´70

´71

´73

´64

´65

´69

´66

´72

´67

´68

(b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity.

(a) The sea star Pisasterochraceousfeeds preferentially on mussels but will consume other invertebrates.

  • Field studies of sea stars

    • Exhibit their role as a keystone species in intertidal communities

A good way to identify the keystone species is to removal experiment (like the experiment here)

Figure 53.16a,b


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100

80

60

Otter number (% max. count)

40

20

0

(a) Sea otter abundance

400

300

Grams per 0.25 m2

200

100

0

(b) Sea urchin biomass

10

8

6

Number per 0.25 m2

4

2

0

1972

1985

1989

1993

1997

Year

Food chain beforekiller whale involvement in chain

(c) Total kelp density

Food chain after killerwhales started preyingon otters

Figure 53.17

  • Observation of sea otter populations and their predation

  • Shows the effect the otters haveon ocean communities


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Ecosystem “Engineers” (Foundation Species)

  • Some organisms exert their influence

    • By causing physical changes in the environment that affect community structure


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Figure 53.18

  • Beaver dams

    • Can transform landscapes on a very large scale


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8

6

Number of plant species

4

2

0

With Juncus

Without Juncus

Salt marsh with Juncus (foreground)

Conditions

  • Some foundation species act as facilitators

    • That have positive effects on the survival and reproduction of some of the other species in the community

Juncus helps prevents the salt marsh soils form becoming anoxic (decrease in oxygen level) as it transports oxygen to its below ground tissue

Figure 53.19


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Bottom-Up and Top-Down Controls

  • The bottom-up model of community organization

    • Proposes a unidirectional influence from lower to higher trophic levels

  • In this case, the presence or absence of abiotic nutrients

    • Determines community structure, including the abundance of primary producers


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  • The top-down model of community organization

    • Proposes that control comes from the trophic level above

  • In this case, predators control herbivores

    • Which in turn control primary producers


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100

75

50

Percentage of herbaceous plant cover

25

0

0

100

200

300

400

Rainfall (mm)

Figure 53.20

  • Long-term experiment studies have shown

    • That communities can shift periodically from bottom-up to top-down

Moisture limitation on a plant growth during dry non-El-Nino years (red points) creates strong bottom up control on this community, whereas the abundant moisture during El Nino years (blue points) stimulates increased plant and animal abundance, inducing strong top-down control


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Restored State

Polluted State

Fish

Rare

Abundant

Abundant

Zooplankton

Rare

Abundant

Algae

Rare

  • Pollution

    • Can affect community dynamics

  • But through biomanipulation

    • Polluted communities can be restored


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  • Concept 53.3: Disturbance influences species diversity and composition

  • Decades ago, most ecologists favored the traditional view

    • That communities are in a state of equilibrium


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  • However, a recent emphasis on change has led to a nonequilibrium model

    • Which describes communities as constantly changing after being buffeted by disturbances


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What Is Disturbance?

  • A disturbance

    • Is an event that changes a community

    • Removes organisms from a community

    • Alters resource availability


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Figure 53.21a–c

(a)Before a controlled burn.A prairie that has not burned forseveral years has a high propor-tion of detritus (dead grass).

(b)During the burn. The detritus serves as fuel for fires.

(c)After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

  • Fire

    • Is a significant disturbance in most terrestrial ecosystems

    • Is often a necessity in some communities

Sometimes enhances a community like the following example


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  • The intermediate disturbance hypothesis

    • Suggests that moderate levels of disturbance can foster higher species diversity than low levels of disturbance


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(a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.

(b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

  • The large-scale fire in Yellowstone National Park in 1988

    • Demonstrated that communities can often respond very rapidly to a massive disturbance

Figure 53.22a, b


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Human Disturbance

  • Humans

    • Are the most widespread agents of disturbance

    • Examples: Agricultural development, Logging and clearing for an urban area


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  • Human disturbance to communities

    • Usually reduces species diversity

  • Humans also prevent some naturally occurring disturbances

    • Which can be important to community structure


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Ecological Succession

  • Ecological succession

    • Is the sequence of community and ecosystem changes after a disturbance


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  • Primary succession

    • Occurs where no soil exists when succession begins

    • No Soil has ever existed!!!

    • Example: Volcanic Island that has just formed.


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  • Secondary succession

    • Begins in an area where soil remains after a disturbance

    • Soil Exist!!

    • Example: Logging, Abandoned farm


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  • Early-arriving species

    • May facilitate the appearance of later species by making the environment more favorable

    • May inhibit establishment of later species

    • May tolerate later species but have no impact on their establishment


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Canada

GrandPacific Gl.

Alaska

Riggs Gl.

0

5

10

McBride Gl.

Muir Gl.

1940

1912

1948

Miles

1941

1899

Plateau Gl.

Casement Gl.

1931

1911

1907

1879

1948

1900

1879

1879

1892

1913

1949

1935

1860

Reid Gl.

1879

Johns HopkinsGl.

GlacierBay

1830

1780

1760

Pleasant Is.

McBride glacier retreating

  • Retreating glaciers

    • Provide a valuable field-research opportunity on succession

Figure 53.23


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(a) Pioneer stage, with fireweed dominant

(b) Dryas stage

60

50

40

30

Soil nitrogen (g/m2)

20

10

0

Dryas

Pioneer

Alder

Spruce

Successional stage

(c) Spruce stage

Figure 53.24a–d

(d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content.

  • Succession on the moraines in Glacier Bay, Alaska

    • Follows a predictable pattern of change in vegetation and soil characteristics


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  • Concept 53.4: Biogeographic factors affect community diversity

  • Two key factors correlated with a community’s species diversity

    • Are its geographic location and its size


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Equatorial-Polar Gradients

  • The two key factors in equatorial-polar gradients of species richness

    • Are probably evolutionary history and climate


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  • Species richness generally declines along an equatorial-polar gradient

    • And is especially great in the tropics

  • The greater age of tropical environments

    • May account for the greater species richness


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Climate—Weather History

  • Climate

    • Is likely the primary cause of the latitudinal gradient in biodiversity

    • These factors can be considered together by measuring community rate of evaportranspiration, the evaporation of water from soil plus the transpiration


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180

200

160

140

100

120

Tree species richness

100

Vertebrate species richness(log scale)

50

80

60

40

20

10

1

0

1,500

2,000

1,000

500

900

500

700

1,100

300

100

Potential evapotranspiration (mm/yr)

Actual evapotranspiration (mm/yr)

(a) Trees

(b) Vertebrates

Figure 53.25a, b

  • The two main climatic factors correlated with biodiversity

    • Are solar energy input and water availability


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Area Effects

  • The species-area curve quantifies the idea that

    • All other factors being equal, the larger the geographic area of a community, the greater the number of species


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1,000

100

Number of species (log scale)

10

1

1

10

100

103

104

105

106

107

108

109

1010

Area (acres)

Figure 53.26

  • A species-area curve of North American breeding birds

    • Supports this idea


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Island Equilibrium Model

  • Species richness on islands

    • Depends on island size, distance from the mainland, immigration, and extinction


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Extinction

Extinction

Immigration

(small island)

Immigration

Immigration

Extinction

(far island)

(near island)

(large island)

Extinction

Immigration

Rate of immigration or extinction

Rate of immigration or extinction

(far island)

(large island)

Rate of immigration or extinction

Extinction

Immigration

(near island)

(small island)

Small island

Equilibrium number

Large island

Far island

Near island

Number of species on island

Number of species on island

Number of species on island

(a) Immigration and extinction rates. The equilibrium number of species on anisland represents a balance between the immigration of new species and theextinction of species already there.

(b) Effect of island size. Large islands may ultimately have a larger equilibrium num-ber of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands.

(c) Effect of distance from mainland. Near islands tend to have largerequilibrium numbers of species thanfar islands because immigration ratesto near islands are higher and extinctionrates lower.

  • The equilibrium model of island biogeography maintains that

    • Species richness on an ecological island levels off at some dynamic equilibrium point

Figure 53.27a–c


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FIELD STUDY

Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galápagos Islands, which vary greatly in size, in relation to the area of each island.

RESULTS

400

200

100

Number of plant species (log scale)

50

25

10

5

0

0.1

10

100

1

1,000

Area of island (mi2) (log scale)

CONCLUSION

The results of the study showed that plant species richness increased with island size, supporting the species-area theory.

Figure 53.28

  • Studies of species richness on the Galápagos Islands

    • Support the prediction that species richness increases with island size


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  • Concept 53.5: Contrasting views of community structure are the subject of continuing debate

  • Two different views on community structure

    • Emerged among ecologists in the 1920s and 1930s, based primarily on observations of plant distribution.


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Integrated and Individualistic Hypotheses

  • The integrated hypothesis of community structure

    • Describes a community as an assemblage of closely linked species, locked into association by mandatory biotic interactions that cause the community to function as an integrated view includes the observation that certain species of plants are consistently found together as a group.


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  • The individualistic hypothesis of community structure

    • Proposes that communities are loosely organized associations of independently distributed species with the same abiotic requirements


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Population densities of individual species

Environmental gradient(such as temperature or moisture)

(a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together.

Figure 53.29a

  • The integrated hypothesis

    • Predicts that the presence or absence of particular species depends on the presence or absence of other species


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Population densities of individual species

Environmental gradient(such as temperature or moisture)

(b) Individualistic hypothesis. Species are independently distributed along gradients and a community is simply the assemblage of species that occupy the same area because of similar abiotic needs.

Figure 53.29b

  • The individualistic hypothesis

    • Predicts that each species is distributed according to its tolerance ranges for abiotic factors


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  • In most actual cases the composition of communities

    • Seems to change continuously, with each species more or less independently distributed

600

Number of plantsper hectare

400

200

0

Moisture gradient

Dry

Wet

(c) Trees in the Santa Catalina Mountains. Thedistribution of tree species at one elevation in the Santa Catalina Mountains of Arizona supports the individualistic hypothesis. Each tree species has an independent distribution along the gradient, apparently conforming to its tolerance for moisture, and the species that live together at any point along the gradient have similar physical requirements. Because the vegetation changes continuously along the gradient, it is impossible to delimit sharp boundaries for the communities.

Figure 53.29c


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Rivet and Redundancy Models

  • The rivet model of communities

    • Suggests that all species in a community are linked together in a tight web of interactions

    • Also states that the loss of even a single species has strong repercussions for the community


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  • The redundancy model of communities

    • Proposes that if a species is lost from a community, other species will fill the gap


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  • It is important to keep in mind that community hypotheses and models

    • Represent extremes, and that most communities probably lie somewhere in the middle


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