1Ch. 47 – Community Ecology

2Community Ecology

Concept of Community

A community is an assemblage of populations interacting with one another within the same environmentComposition is a thorough listing of various species in the community

Species Diversity includes: 1. Species Richness – total number of different species in the community 2. Relative Abundance – proportion of the total population represented by each species

The various animals and plants surrounding this watering hole are all members of a savanna community in southern

Africa

Figure 53.1

4Community Structure

Coniferous Forest Rain Forest

5Two different communities cTwo different communities can have the same species an have the same species

richness, but a different relative abundancerichness, but a different relative abundance

Community 1

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

Community 2

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

D

C

B

A

Figure 53.11

A community with an even species abundanceis more diverse

than one in which one or two species are abundant and the remainder rare

6Community Ecology

Structure of the Community

CompetitionWhen two species compete, the abundance of both species is negatively impacted Predation (Herbivory – plant predation)● One organism, the predator, eats another called the prey.Parasitism ● One organism, the parasite, feeds ON another called the host.These tend to increase the abundance of the predator (or parasite) and reduce the abundance of the prey (or host)

7

8Community Ecology

Habitat and Ecological Niche

HabitatThe area an organism lives and reproduces in

Ecological nicheThe role a species plays in its community

Includes its habitat, and

Its interactions with other organisms

Fundamental niche - All conditions under which the organism can survive & reproduce

Realized niche – part of the fundamental niche that the species actually occupies

9Feeding niches for Wading Birds

10Community Ecology

Competition Between Populations

Interspecific competitionWhen members of different species try to use a resource that is in limited supply (food, light, etc.)

Competitive Exclusion PrincipleNo two species can indefinitely occupy the same niche at the same time

Resource Partitioning decreases competitionCan lead to character displacement. This is the tendency of a characteristic to become more divergent when species are together.

11

Competition betweentwo laboratory populations of Paramecium

Both grow fine separately but only P.

aurelia survives when they are grown

together

12

Character Displacement in

Fincheson the

Galápagos Islands

13

Niche Specialization

AmongFive Species of

Coexisting Warblers

14 Competition BetweenTwo Species of Barnacles

Connell removed the

larger Balanus individuals.

The smaller Chthamalus barnacles

moved down & survived equally

well in both places

15 Competition BetweenTwo Species of Barnacles

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

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

RESULTS

CONCLUSION

Ocean

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

EXPERIMENT

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.

Low tide

High tide

Chthamalusfundamental niche

Chthamalusrealized niche

Low tide

High tide

Chthamalus

Balanusrealized niche

Balanus

Ocean

16Community Ecology

Predator-Prey Interactions

PredationOne living organism, the predator, feeds on another, the prey Predator is frequently larger Predator population is usually smaller than the prey population

Predator has lower reproductive rate Prey is usually consumed in its entirety

Presence of predators can decrease prey densities, and vice-versa

17 Predator

s

18 Predator-prey Interaction BetweenParamecium caudatum and Didinium nasutum

Paramecium & Didinium placed in

same culture.

Didinium ate all the Paramecium and

then died of starvation

19

Hares given adequate food but with predators; they still cycle

Predators excluded but no food was given to hares; cycling stopped

When hares have adequate food & there are no predators, the cycling stops.

Predator-prey InteractionBetween a Lynx and a Snowshoe Hare

20Boom-and-bust cyclesBoom-and-bust cycles are are

influenced by complexinfluenced by complex interactions interactions between biotic and abiotic factors.between biotic and abiotic factors.

Predator populations may be Predator populations may be influenced by influenced by availability of preyavailability of prey - - as more prey is available, they as more prey is available, they reproduce more and their reproduce more and their population increases. When the population increases. When the predator population gets too high predator population gets too high and they eat all the prey, their and they eat all the prey, their population falls.population falls.

21Prey populations may be influenced byPrey populations may be influenced by 1.1.PredationPredation 2. Fluctuations in the availability of 2. Fluctuations in the availability of the the plants they feed onplants they feed on

Prey may not be regulated by Prey may not be regulated by predators. Their population may predators. Their population may increase until they run out of increase until they run out of resources, then their population resources, then their population crashes. crashes.

As a result the predator population As a result the predator population soon declines because of lack of soon declines because of lack of food availability.food availability.

22

Feeding adaptationsFeeding adaptations of predators of predators include:include:cclaws, teeth, fangs, stingers, and laws, teeth, fangs, stingers, and poisonpoison

Some predator-prey interactions Some predator-prey interactions lead to lead to coevolutioncoevolution: a series of : a series of reciprocal adaptations in two reciprocal adaptations in two speciesspecies

23Predator Adaptations

24Community Ecology

Prey Defenses

Mechanisms that thwart the possibility of being eaten by a predator. Some examples:- Spines

- Tough Epidermis - Poisonous Chemicals - Camouflage – ability to blend into the background. Have cryptic coloration.

- Bright Coloration - Flocking Behavior

25

Spines as protection for plants against herbivory

26Chemical Defenses in Plants

Poison Oak and its rash

27 Camouflage in the Anglerfish

28 Cryptic Coloration

29 Cryptic Coloration

30Cryptic Coloration

31 Anti-predator Defenses

Poisonous skin

+

Warning coloration

False eyespots

Large false head

32Aposematic (warning) coloration

All these snakes

are poisonous

33Community Ecology

MimicryOne species (mimic) resembles another species (model) that possesses an overt anti-predator defense.

Two main types: 1. Batesian Mimicry - Mimic lacks

defense of the organism it resembles 2. Müllerian Mimicry - Mimic shares

same protective defense as its model

34Mimicry Among Insects with

Yellow and Black Stripesa, b, & c are examples of

Batesian mimicry

because they do not have the

capability to sting.

d & e are Mullerian mimics since they both

use stinging as a defense

Fly Beetle

Moth Yellow jacket

Bumblebee

35In In Batesian mimicryBatesian mimicryA palatable or A palatable or harmless species mimicsharmless species mimics an unpalatable or an unpalatable or harmful modelharmful model

(a) Hawkmoth larva

(b) Green parrot snake

Figure 53.7a, b

36Batesian Mimicry (flies, beetles & ants mimic wasp)

Wasp

37Batesian Mimicry

Poisonous models on

left

Harmless mimics on

right

38In In Müllerian mimicryMüllerian mimicryTwoTwo or more or more unpalatable speciesunpalatable species resemble each otherresemble each other

(a) Cuckoo bee

(b) Yellow jacketFigure 53.8a, b

39Community Ecology

Symbiotic Relationships

Interactions in which there is a close relationship between members of two species. Frequently one species lives in or on another.

Three main types of symbiosis are:

- Parasitism - Commensalism - Mutualism

40Community Ecology

Symbiotic Relationships

Parasitism - Parasite derives nourishment from a

host, and may use host as habitat and mode of transmission

- Endoparasites - live inside host - Ectoparasites - live on outside of

host Natural selection favors parasites that infect but don’t

kill their hosts.

41Parasites

Mosquito

Leech

Ringworm

Flea

42The Life Cycle of a Deer Tick

Human may end up with Lyme Disease

43Community Ecology

MutualismA symbiotic relationship in which both members

of the association benefit

Need not be equally beneficial to both species

- Cleaning Symbiosis - one animal cleans another

- E. coli in human intestines

- Protozoans in termite intestines

- Mycorrhizae between roots & fungi

- Ants & bullhorn acacia trees

Often help each other obtain food or avoid predation

44Cleaning Symbiosis

45

More Cleaning Symbiosis

46Mutualism Betweenthe Bullhorn Acacia Tree and

Ants

Figure 53.9

47

Pollination of Plants

by Animals is an

example of Mutualism

48Community Ecology

Commensalism

A symbiotic relationship in which one species benefits and the other is neither benefited or harmed

- Remoras attach to sharks & get a free ride - Clownfish living inside of sea anemone’s

tentacles - Epiphytes, like Spanish moss, live on trees

Many supposed examples may turn out to be mutualism or parasitism

Inferred amount of harm or benefit that 2 species do to one another is subject to investigator bias

49

Remoras & Sharks

Spanish MossWhales & Barnacles

50Clownfish AmongSea Anemone’s

Tentacles

51Community Ecology

Community Development

Ecological SuccessionA predictable pattern of change in species replacements following a disturbance Primary Succession occurs in areas where there is no soil formation

•After a volcanic eruption or glacial retreat Secondary Succession begins in areas where soil and life are already present

•Like when a cultivated field returns to naturePioneer Species - first species to colonize an area (frequently lichens & moss)

52Primary Succession occurs after glaciers retreat

0 5 10

Miles

GlacierBay

Pleasant Is.

Johns HopkinsGl.

Reid Gl.

GrandPacific Gl.

Canada

Alaska

1940 1912

1899

1879

1879

1949

1879

1935

1760

1780

1830

1860

1913

1911

1892

1900

1879

1907 19481931

1941

1948

Cas

emen

t Gl.

McB

ride

Gl.

Plateau Gl.

Muir G

l.

Riggs G

l.

53Primary & Secondary Succession at Glacier Bay, Alaska

(b) Dryas stage

(c) Spruce stage

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

Soi

l nitr

ogen

(g/

m2)

Successional stage

Pioneer Dryas Alder Spruce0

10

20

30

40

50

60

(a) Pioneer stage, with fireweed dominant

54Primary Succession

55

Primary Succession

56Primary Succession

57Secondary Succession in a Field

58Secondary Successionin a Forest

59Secondary Successionin a Forest

60Community Ecology

Community Stability

Community stability can be recognized in three ways:Persistence through time – when a community remains just about the same year after year

Resistance to change – when trees are able to regrow leaves after insect infestation

Recovery once a disturbance has occurred – when a community, like chaparral, quickly returns to its normal state after a fire

61Community Ecology

Community Stability

Decades ago, most ecologists favored the traditional view that communities are in a state of equilibrium.

However, recent emphasis on change has led to a nonequilibrium model.This describes communities as constantly changing after being buffeted by disturbances.

Disturbances affect all communities - Floods, fire, glaciers, volcanic eruptions can

change communities greatly

62Fire is Often Necessary to an

ecosystem

(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.

63Yellowstone Fire of

1988

(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.

64Community Ecology

Predation, Competition,and Biodiversity

Keystone species are organisms that play a great role in maintaining function & diversity of an ecosystem. They are not necessarily abundant, but exert strong control on a community by their ecological roles

Keystone predator may help to maintain diversity by reducing the numbers of the strongest competitor in a community

-This helps to prevent exclusion of weaker competitors, and prevents strongest competitor from becoming too dominant

65Effect of a Keystone Species

Pisaster seastars were removed from

experimental tidepools but were

left in control areas.

Diversity decreased in

experimental areas

66Effect of Sea Otters on Ocean CommunitiesObservation of

sea otter populations and their predation

shows the effect the otters have

on ocean communities.

Without sea otters there

might not be any kelp beds

Figure 53.17Food chain beforekiller whale involve-ment in chain

(a) Sea otter abundance

(b) Sea urchin biomass

(c) Total kelp density

Num

ber

per

0.25

m

2

1972 1985 1989 1993 1997

0

2

4

6

8

10

0

100

200

300

400

Gra

ms

per

0.25

m

2

Ott

er n

umbe

r (%

m

ax.

coun

t)

0

40

20

60

80

100

Year

Food chain after killerwhales started preyingon otters

67Community Ecology

Predation, Competition,and Biodiversity

Exotic speciesIntroduction of exotic (alien) species into new areas

Can provide many examples of competition

Can lead to a reduction in biodiversity & even extinction of organisms

68Exotic species – Africanized honey bee

They are replacing the less aggressive honey bees used in agriculture

69Exotic species – Brown tree snake

Spread of Brown tree snake on Guam

Snake eats native birds causing extinction

70Exotic species -

KudzuIt grows on top of trees

& objects. Can kill trees.