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Chapter 54 – Community Ecology. Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved. Ecologists call relationships between species in a community interspecific interactions Can you name some examples?

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slide2
Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved
  • Ecologists call relationships between species in a community interspecific interactions
  • Can you name some examples?
  • How about these: competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and facilitation
  • Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0)
slide3

Competition (–/– interaction) :

  • When one or more individuals depend on the same resource, competition arises.
  • Competition can be intraspecific (between members of the same species) or interspecific (between different species who rely on the same resource).

http://darwinsdarlings.blogspot.com/2013/02/darwins-finches-as-evidence-of.html

Intra-specific competition for food led to adaptive radiation of beak shapes in the evolution of the Galapagos finches.

slide4

Competitionfor mates is another example of intra-specific competition and another driving force behind evolution.

http://www.youtube.com/watch?v=C7HCIGFdBt8

In this video, you can see two male giraffes fighting with their heads. It is thought that the long neck of the giraffe evolved not only as a consequence of reaching higher food (also intra-specific competition with other giraffes), but as an advantage in fights for mates and territory. Longer necks and bigger heads allow for HARDER HITS!!

slide5

Competition:

  • In the rainforest, different types of trees compete for light by growing taller, wider, or producing more foliage. This is at the expense of other smaller species.
  • Smaller species driven by evolution to be more efficient at lower light intensities.

http://rainforests.mongabay.com/0201.htm

slide6

Competition:

  • Competition for food, water, nesting sites, and hunting grounds are also common types of interspecific competition.

http://www.youtube.com/watch?v=3IwDHvNxjJo

In this video, a cheetah mother protects her young from a lion attack. The cheetah and lion compete with each other for food, though the lion will try to kill the young of other big cats if possible.

slide7

Competitive Exclusion

  • Strong competition can lead to competitive exclusion, local elimination of a competing species
  • The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place
slide8

Competitive exclusion principle:

  • “No two species can occupy the same niche for extended periods of time”
  • If two species share a niche, this leads to inter-specific competition for resources.
  • Inevitably, one species will have an advantage over the other.
  • The less well adapted species will struggle to survive and reproduce.
  • It will inevitable be eliminated by: compete or total extinction, or population may migrate
slide9

Competitive exclusion principle example:

The Eurasian red squirrel (S. vulgaris) has suffered competitive exclusion due to the introduction of the Eastern grey squirrel (S. carolinensis) to the UK from the USA.

S. Carolinensis is larger, stronger, and can store more fat in winter, making it better able to survive and reproduce. It is more tolerant of habitat change than S. vulgaris

The red squirrel is now protected in many areas.

slide10

Ecological Niches and Natural Selection

  • The total of a species’ use of biotic and abiotic resources is called the species’ ecological niche
  • An ecological niche can also be thought of as an organism’s ecological role
  • Ecologically similar species can coexist in a community if there are one or more significant differences in their niches
slide11

The Niche Concept:

  • A niche is the specialized habitat of an organism including:
    • Space and territory
    • Nutrition and feeding habits
    • Interactions and relationships with other organisms
    • Reproductive habits
    • It role and impacts in the habitat or ecosystem.
  • Only one species or population can occupy the same niche for extended periods of time.
  • Example:Amphiprionocellaris
  • Habitat: In specific anemone types only, tropical reefs.
  • Nutrition: Copepods, plankton & undigested food from anemone (omnivores)
  • Predators: Larger fish species
  • Interactions: Mutualism with anemones:
          • Anemone provides protection and some food
          • Clownfish provides food (feces), cleans and circulates water around the anemone.
  • Reproduction: All born male. Dominant male becomes female and lays eggs on flat areas near the anemone.
slide12

GF Gause’s experiments into Niches

Paramecium is a genus of protists. Gause cultured two species:

P. aurelia & P. caudatum

When grown alone under ideal conditions (fundamental niche), each population grew to a higher maximum.

When grown together under the same conditions, one population would typically dominate and the other would die off.

http://ggause.com/gfg05.htm

These results are a great example of realized niches vs. fundamental niches and led to the competitive exclusion principal.

slide13

Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community

  • Niche partitioning can be spatial (dividing up space), morphological (different shapes or structures), or temporal (different times of the day or year).
figure 54 2

A. distichus perches on

fence posts and other

sunny surfaces.

A. insolitus usually

perches on shady

branches.

Figure 54.2

A. ricordii

A. insolitus

A. aliniger

A. christophei

A. distichus

A. cybotes

A. etheridgei

slide15

A species’ fundamental niche is the niche potentially occupied by that species. The full range of environmental and social conditions under which it could potentially survive and reproduce.

  • A species’ realized niche is the niche actually occupied by that species with the limitations of other species being present.
  • As a result of competition, a species’ fundamental niche may differ from its realized niche
    • For example, the presence of one barnacle species limits the realized niche of another species
figure 54 3

EXPERIMENT

Figure 54.3

High tide

Chthamalus

Chthamalus

realized niche

Balanus

Balanus

realized niche

Ocean

Low tide

RESULTS

High tide

Chthamalus

fundamental niche

Ocean

Low tide

slide17

The common spiny mouse and the golden spiny mouse show temporal partitioning of their niches

  • Both species are normally nocturnal (active during the night)
  • Where they coexist, the golden spiny mouse becomes diurnal (active during the day)

The golden spiny mouse

(Acomysrussatus)

slide18

Character Displacement

  • Character displacement 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 variation in beak size between populations of two species of Galápagos finches
figure 54 4

G. fuliginosa

G. fortis

Figure 54.4

Beak

depth

Los Hermanos

60

40

G. fuliginosa,

allopatric

20

0

60

Daphne

40

G. fortis,

allopatric

Percentages of individuals in each size class

20

0

Sympatric

populations

60

Santa María, San Cristóbal

40

20

0

8

10

12

14

16

Beak depth (mm)

slide20

Predation

  • Predation (+/– interaction) refers to an interaction in which one species, the predator, kills and eats the other, the prey
  • Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison
slide21

Predation:

  • A consumer kills and eats another consumer.
  • Predators have evolved adaptations to best equip it for finding, chasing, catching, killing and eating or digesting its prey.

Crocodiles hunt wildebeest

http://www.youtube.com/watch?v=PiG3yokCOug

http://www.arkive.org/collared-anteater/tamandua-tetradactyla/image-G81304.html

The giant anteater has an extended nose and a VERY long tongue for foraging for ants and termites.

slide22

Prey display various defensive adaptations

  • Behavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm calls
  • Animals also have morphological and physiological defense adaptations
  • Cryptic coloration, or camouflage, makes prey difficult to spot

Sea Horses

figure 54 5
Figure 54.5

(a) Cryptic coloration

(b) Aposematic

coloration

Canyon tree frog

Poison dart frog

(c) Batesian mimicry: A harmless species mimics a harmful one.

(d) Müllerian mimicry: Two unpalatable species

mimic each other.

Hawkmoth

larva

Cuckoo bee

Yellow jacket

Green parrot snake

slide24

Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration

  • Predators are particularly cautious in dealing with prey that display such coloration

Poison dart frog

(b) Aposematic

coloration

slide25

In some cases, a prey species may gain significant protection by mimicking the appearance of another species

  • In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model

(c) Batesian mimicry: A harmless species mimics a harmful one.

Hawkmoth

larva

Green parrot snake

slide26

Herbivory

  • Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga
  • It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores
slide27

Symbiosis

  • Symbiosis is a relationship where two or more species live in direct and intimate contact with one another.
  • Can you name some of the types?
slide28

Parasitism

  • In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process
  • Parasites that live within the body of their host are called endoparasites
  • Parasites that live on the external surface of a host are ectoparasites
slide29

Parasitism:

  • A symbiotic (co-living) relationship.
  • Parasites gain an advantage at the cost of the host which may be killed.
  • We usually consider only eukaryotes as parasites. Bacteria and viruses we count as infections diseases.

Jewel wasps turn cockroaches into ‘zombies’ and lay their eggs inside them

http://www.youtube.com/watch?v=9fO0zHiAIG8

slide30

Parasitism:

  • Some common human examples are tapeworms, roundworms, and malaria (caused by Plasmodium falciparum)

Loa loafilariasis (african eye worm)

Hookworms – Intestinal mucosa

The hookworm is much smaller than a tapeworm. These parasites are rarely more than a centimeter long, and burrow into your small intestine to feast on your blood. Since hookworms latch onto your small intestine and divert nutrients away from the bloodstream, they're actually more problematic than tapeworms. Hookworm infection can lead to anemia, slower cognitive growth, and malnutrition.

slide31

Mutualism

  • Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species
  • A mutualism can be
    • Obligate, where one species cannot survive without the other
    • Facultative, where both species can survive alone

Clownfish and Sea anemone

Ant Army Defends Tree

figure 54 7
Figure 54.7

(a) Acacia tree and ants (genus Pseudomyrmex)

(b) Area cleared by ants at the base of an acacia tree

slide33

Commensalism

  • In commensalism (+/0 interaction), one species benefits and the other is neither harmed nor helped
  • Commensal interactions are hard to document in nature because any close association likely affects both species
slide35

Facilitation

  • Facilitation (/ or 0/) is an interaction in which one species has positive effects on another species without direct and intimate contact
    • For example, the black rush makes the soil more hospitable for other plant species

Number of plant species

(a) Salt marsh with Juncus

(foreground)

With Juncus

Without Juncus

slide36

Concept 54.2: Diversity and trophic structure characterize biological communities

  • In general, a few species in a community exert strong control on that community’s structure
  • Two fundamental features of community structure are species diversity and feeding relationships
slide37

Species Diversity

  • Species diversity of a community is the variety of organisms that make up the community
  • It has two components: species richness and relative abundance
    • Species richness is the number of different species in the community
    • Relative abundance is the proportion each species represents of all individuals in the community
figure 54 10
Figure 54.10

A

B

C

D

Community 1

Community 2

A: 25%

B: 25%

C: 25%

D: 25%

A: 80%

B: 5%

C: 5%

D: 10%

slide39

Diversity and Community Stability

  • Ecologists manipulate diversity in experimental communities to study the potential benefits of diversity
    • For example, plant diversity has been manipulated at Cedar Creek Natural History Area in Minnesota for two decades

Figure 54.12

slide40

Communities with higher diversity are

    • More productive and more stable in their productivity
    • Better able to withstand and recover from environmental stresses
    • More resistant to invasive species, organisms that become established outside their native range
slide41

Trophic Structure

  • Trophic structure is the feeding relationships between organisms in a community
  • It is a key factor in community dynamics
  • Food chains link trophic levels from producers to top carnivores

Shark Eating Seal

figure 54 13
Figure 54.13

Quaternary

consumers

Carnivore

Carnivore

Tertiary

consumers

Carnivore

Carnivore

Secondary

consumers

Carnivore

Carnivore

Primary

consumers

Zooplankton

Herbivore

Primary

producers

Phytoplankton

Plant

A terrestrial food chain

A marine food chain

figure 54 14

Food Webs

Figure 54.14

Humans

  • A food web is a branching food chain with complex trophic interactions

Smaller

toothed

whales

Sperm

whales

Baleen

whales

Elephant

seals

Crab-

eater

seals

Leopard

seals

Fishes

Squids

Birds

Carniv-

orous

plankton

Euphau-

sids

(krill)

Cope-

pods

Phyto-

plankton

slide44

Species may play a role at more than one trophic level

  • Food webs can be simplified by
    • Grouping species with similar trophic relationships into broad functional groups
    • Isolating a portion of a community that interacts very little with the rest of the community
figure 54 15
Figure 54.15

Juvenile striped bass

Sea nettle

Fish larvae

Fish eggs

Zooplankton

slide46

Species with a Large Impact

  • Certain species have a very large impact on community structure
  • Such species are highly abundant or play a pivotal role in community dynamics
slide47

Dominant Species

  • Dominant species are those that are most abundant or have the highest biomass
  • Dominant species exert powerful control over the occurrence and distribution of other species
    • For example, sugar maples have a major impact on shading and soil nutrient availability in eastern North America; this affects the distribution of other plant species
slide48

One hypothesis suggests that dominant species are most competitive in exploiting resources

  • Another hypothesis is that they are most successful at avoiding predators
  • Invasive species, typically introduced to a new environment by humans, often lack predators or disease
slide49

Keystone Species and Ecosystem Engineers

  • Keystone species exert strong control on a community by their ecological roles, or niches
  • In contrast to dominant species, they are not necessarily abundant in a community
  • Observation of sea otter populations and their predation shows how otters affect ocean communities
figure 54 18

100

Figure 54.18

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

Number per

0.25 m2

6

4

2

0

1972

1985

1989

1993

1997

Year

(c) Total kelp density

Food chain

slide51

Concept 54.3: Disturbance influences species diversity and composition

  • Decades ago, most ecologists favored the view that communities are in a state of equilibrium
  • It was suggested that species in a climax community function as a superorganism.
  • But the idea that communities were at equilibrium was challenged by other ecologists
slide52

Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances

  • A disturbance is an event that changes a community, removes organisms from it, and alters resource availability
slide53

Characterizing Disturbance

  • Fire is a significant disturbance in most terrestrial ecosystems
  • A high level of disturbance is the result of a high intensity and high frequency of disturbance
slide54

The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance

  • High levels of disturbance exclude many slow-growing species
  • Low levels of disturbance allow dominant species to exclude less competitive species
  • In a New Zealand study, the richness of invertebrate taxa was highest in streams with an intermediate intensity of flooding
figure 54 20
Figure 54.20

35

30

25

Number of taxa

20

15

10

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

Index of disturbance intensity (log scale)

slide56

The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

  • The Yellowstone forest is an example of a nonequilibrium community
figure 54 21
Figure 54.21

(a) Soon after fire

(b) One year after fire

slide58

Ecological Succession

  • Ecological succession is the sequence of community and ecosystem changes after a disturbance
  • Primary succession occurs where no soil exists when succession begins
  • Secondary succession begins in an area where soil remains after a disturbance. Soil was there when succession began.
slide59

Early-arriving species and later-arriving species may be linked in one of three processes

    • Early arrivals may facilitate the appearance of later species by making the environment favorable
    • They may inhibit the establishment of later species
    • They may tolerate later species but have no impact on their establishment
slide60

Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics

  • The exposed moraine is colonized by pioneering plants, including liverworts, mosses, fireweed, Dryas, willows, and cottonwood

Figure 54.22a

1

Pioneer stage, with fireweed dominant

figure 54 22 1
Figure 54.22-1

1941

1907

Pioneer stage, with

fireweed dominant

1

0

5

10

15

Kilometers

1860

Glacier

Bay

Alaska

1760

figure 54 22 2
Figure 54.22-2

1941

1907

Dryas stage

2

Pioneer stage, with

fireweed dominant

1

0

5

10

15

Kilometers

1860

Glacier

Bay

Alaska

1760

figure 54 22 3
Figure 54.22-3

1941

1907

Dryas stage

2

Pioneer stage, with

fireweed dominant

1

0

5

10

15

Kilometers

1860

Glacier

Bay

Alaska

1760

Alder stage

3

figure 54 22 4
Figure 54.22-4

1941

1907

Dryas stage

2

Pioneer stage, with

fireweed dominant

1

0

5

10

15

Kilometers

1860

Glacier

Bay

Alaska

1760

4

Spruce stage

Alder stage

3

slide68

Succession is the result of changes induced by the vegetation itself

  • On the glacial moraines, vegetation lowers the soil pH and increases soil nitrogen content
figure 54 23
Figure 54.23

60

50

40

Soil nitrogen (g/m2)

30

20

10

0

Pioneer

Dryas

Alder

Spruce

Successional stage

slide70

Human Disturbance

  • Humans have the greatest impact on biological communities worldwide
  • Human disturbance to communities usually reduces species diversity
slide72

Concept 54.4: Biogeographic factors affect community diversity

  • Latitude and area are two key factors that affect a community’s species diversity
slide73

Latitudinal Gradients

  • Species richness is especially great in the tropics and generally declines along an equatorial-polar gradient
  • Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate
  • Temperate and polar communities have started over repeatedly following glaciations
  • The greater age of tropical environments may account for their greater species richness & In the tropics, the growing season is longer, so biological time runs faster
slide74

Climate is likely the primary cause of the latitudinal gradient in biodiversity

  • Two main climatic factors correlated with biodiversity are solar energy and water availability
  • They can be considered together by measuring a community’s rate of evapotranspiration – the evaporation of water from soil plus transpiration of water from plants
figure 54 25

180

Figure 54.25

160

140

120

100

Tree species richness

80

60

40

20

0

100

300

500

700

900

1,100

Actual evapotranspiration (mm/yr)

(a) Trees

200

100

Vertebrate species richness

(log scale)

50

10

0

500

1,000

1,500

2,000

Potential evapotranspiration (mm/yr)

(b) Vertebrates

slide76

Area Effects

  • The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species
  • A species-area curve of North American breeding birds supports this idea
figure 54 26
Figure 54.26

1,000

100

Number of species (log scale)

10

1

0.1

1

10

100

103

104

105

106

107

108

109

1010

Area (hectares; log scale)

slide78

Island Equilibrium Model

  • Species richness on islands depends on island size, distance from the mainland, immigration, and extinction
  • The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium point
figure 54 27
Figure 54.27

Immigration

Immigration

Extinction

Extinction

Immigration

Extinction

(near island)

(small island)

(far island)

(large island)

Extinction

Immigration

(far island)

(large island)

Extinction

Rate of immigration or extinction

Rate of immigration or extinction

Rate of immigration or extinction

Immigration

(near island)

(small island)

Equilibrium number

Small island

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

(b) Effect of island size

(c) Effect of distance from mainland

slide80

Studies of species richness on the Galápagos Islands support the prediction that species richness increases with island size

figure 54 28
Figure 54.28

RESULTS

400

200

100

Number of plant species

(log scale)

50

25

10

5

10

100

103

104

105

106

Area of island (hectares)

(log scale)

slide82

Concept 54.5: Pathogens alter community structure locally and globally

  • Ecological communities are universally affected by pathogens, which include disease-causing microorganisms, viruses, etc.
  • Pathogens can alter community structure quickly and extensively
slide83

Pathogens and Community Structure

  • Pathogens can have dramatic effects on communities
    • For example, coral reef communities are being decimated by white-band disease
  • Human activities are transporting pathogens around the world at unprecedented rates
slide84

Community Ecology and Zoonotic Diseases

  • Zoonotic pathogens have been transferred from other animals to humans
  • The transfer of pathogens can be direct or through an intermediate species called a vector
  • Many of today’s emerging human diseases are zoonotic
  • Identifying the community of hosts and vectors for a pathogen can help prevent disease
    • For example, recent studies identified two species of shrew as the primary hosts of the pathogen for Lyme disease
slide86

Avian flu is a highly contagious virus of birds

  • Ecologists are studying the potential spread of the virus from Asia to North America through migrating birds
slide92

Why bother?

  • We can measure and monitor an area’s productivity:
  • In terms of whole community changes.
  • To compare the energy output of plant species.
  • To compare the demand for nutrients by various species.
  • To determine whether the health of a site is changing.
  • To estimate the hydrological (water based) properties of the area (e.g. run-off, uptake flooding, or landslides).
slide93

Measuring Biomass

Estimate the population of each species to be included in the area.

Take a numbered sample of individuals.

Dry them until the reach a stable weight.

Multiply the mean mass of the individuals by the estimated population.

  • Ethical considerations:
  • Some individuals must be killed.
  • Dried organic matter should be returned to the sample site to be recycled for nutrients and energy.
  • More accurate data may require more samples