Chapter 27 Community Interactions

Learning Goals for Ch. 27

27.1 Why Are Community Interactions Important?

27.2 What Is the Relationship Between the Ecological

Niche and Competition?

27.3 What Are the Results of Interactions Between

Predators and Their Prey?

27.4 What Is Parasitism?

27.5 What is Mutualism?

27.6 How Do Keystone Species Influence Community

Structure?

27.7 Succession: How Do Community Interactions

Cause Change Over Time?


An ecological community consists of all the

interacting populations within an ecosystem

– A community can encompass the entire biotic,

or living, portion of an ecosystem

– Interactions between populations in a community

help limit their size

Copyright © 2011 Pearson Education Inc. Biology: Life on Earth, 9e

Ecological hierarchy

Biosphere

Ecosystem

Community

Population

Organism

Table 27-1

Type of

Interaction Effect on

Species A Effect on

Species B

Competition between A and B Harms Harms

Predation by A on B Benefits Harms

Parasitism by A on B Benefits Harms

Mutualism between A and B Benefits Benefits


An ecological community consists of all the interacting

populations within an ecosystem (continued)

– The process by which two interacting species act as

agents of natural selection on one another is called

coevolution

Co-Evolution Bull Thorn Acacia and Ants


The most important community interactions are:

– Competition, which harms both species

– Predation, which benefits the predator but harms

the prey

– Parasitism, which benefits parasite but harms the

host

– Mutualism, which benefits both species

Table 27-1

Type of

Interaction Effect on

Species A Effect on

Species B

Competition between A and B Harms Harms

Predation by A on B Benefits Harms

Parasitism by A on B Benefits Harms

Mutualism between A and B Benefits Benefits

27.2 What Is the Relationship Between the

Ecological Niche and Competition?

Each species occupies a unique ecological niche that encompasses all aspects of its way of life

– These include:

– Its physical home or habitat

– The physical and chemical environmental factors necessary for its survival, such as nesting sites, climate, and the type of nutrients it needs

– The role that the species performs within an ecosystem, such as what it eats and the other species with which it competes

– Although different species share aspects of their niche with others, no two species ever occupy exactly the same ecological niche within a community



Competition occurs whenever two organism

attempt to use the same, limited resources

– Interspecific competition occurs between

members of different species, if they feed on the

same things or require similar breeding areas

– Ex. Cattle and deer

– Ex.Wild horses and Elk

– Ex. Zebra mussels and native mussels



Adaptations reduce the overlap of ecological

niches among coexisting species

– The competitive exclusion principle states that

if two species occupy exactly the same niche

with limited resources, one will outcompete the

other



The competitive exclusion principle was formulated by

microbiologist G. F. Gause, who performed laboratory

experiments using two species of protists, Paramecium

aurelia and P. caudatum

– Both species thrived on the same bacteria and fed in the

same region of their laboratory flasks

– When put into the same flask, P. aurelia always

eliminated P. caudatum

– Gause repeated the experiment, replacing P. caudatum

with P. bursaria, which fed in a different part of the flask

– In that case, both species could coexist because they

occupied different niches

P. aurelia P. caudatum

(a) Grown in separate flasks

(b) Grown in the same flask

Competitive Exclusion

Fig. 27-1

Resource Partitioning

Adaptations reduce the overlap of ecological

niches among coexisting species (continued)

– When species with largely similar ecological

niches coexist and compete, each species

occupies a smaller niche than it would by itself, a

phenomenon called resource partitioning


Ecologist Robert MacArthur explored the competitive

exclusion principle by carefully observing five species

of North American warbler

– These birds all hunt for insects and nest in the same type

of eastern spruce tree

– MacArthur found that each species concentrates its

search for food in specific regions within spruce trees,

employs different hunting tactics, and nests at a slightly

different time


Fig. 27-2

Blackburnian

warbler

Black-throated

green warbler

Cape May

warbler

Bay-breasted

warbler

Yellow-rumped

warbler



Interspecific competition may reduce the

population size and distribution of each species

– Although natural selection can reduce niche

overlap, interspecific competition may still restrict

the size and distribution of competing

populations

Intraspecific Competition

Competition within a species is a major factor controlling population size

– Intraspecific competition, competition between individuals of the same species, is the most intense form of competition

– If resources are limited, this is a major factor controlling population size



Predator–prey interactions shape evolutionary adaptations

– Predators eat other organisms; these include herbivores (animals that eat plants) as well as carnivores (animals that eat other animals)

–Predators include a grass-eating pika, a bat hunting a moth, and the more familiar example of a hawk eating a bird

– Predators tend to be less abundant than their prey

Forms of Predation

Fig. 27-3



Predator–prey interactions shape evolutionary

adaptations (continued)

– Predator and prey populations exert intense

selective pressure on one another, resulting in

coevolution

–As prey become more difficult to catch,

predators must become more adept at hunting



Some predators and prey have evolved counteracting behaviors

– Bat and moth adaptations provide excellent examples of how body structures and behaviors are molded by competition

– Bats emit high-pitch sound pulses that bounce off their surroundings, allowing them to navigate and detect prey

– Moths (their prey) have evolved ears sensitive to the pitch of sounds the bats emit, and they take evasive actions in response

– The bats, in turn, counter by switching the frequency of their sound pulses away from the moth’s sensitivity range

Bat-Moth Coevolution



Camouflage conceals both predators and their

prey

– Camouflage renders animals inconspicuous

even when in plain sight

–Predators and prey have evolved colors,

patterns, and shapes that resemble their

surroundings

Camouflage by Blending In

Fig. 27-4

Camouflage




prey (continued)

– To avoid detection by predators, some animals

have evolved to resemble objects, such as

leaves, twigs, seaweed, thorns, or even bird

droppings

– Some plants have evolved to resemble rocks to

avoid detection by herbivores

Camouflage by Resembling Specific Objects

Fig. 27-5a, b

Camouflage by Resembling Specific Objects

Fig. 27-5c, d




prey (continued)

– Camouflage also helps predators ambush their

prey

–Examples include the cheetah blending with

tall grass and the frogfish resembling a rock

Camouflage Assists Predators

Fig. 27-6



Bright colors often warn of danger

– Some animals have evolved bright warning

coloration that attracts the attention of potential

predators

–Warning coloration advertises that the animal

is bad-tasting or poisonous before the

predator attacks

–Examples include poison arrow frogs, coral

snakes, and honey bees

Warning Coloration

Fig. 27-7



Some prey organisms gain protection through mimicry

– Mimicry refers to when members of one species have

evolved to resemble another species

– Two or more distasteful species may each benefit from a

shared warning coloration pattern (Müllerian mimicry)

– Predators need only experience one distasteful

species to learn to avoid all with that color pattern

– For example, toxic monarch and viceroy butterflies

have similar wing patterns; if a predator becomes ill

from eating one species, it will avoid the other

Mullerian Mimicry

Fig. 27-8

Some prey organisms gain protection through

mimicry (continued)

– Some harmless organisms can gain a selective

advantage by resembling poisonous species

(Batesian mimicry)

–For example, the harmless hoverfly avoids

predation by resembling a bee

–The harmless mountain king snake is

protected by a warning coloration that

resembles the venomous coral snake

Batesian Mimicry

Fig. 27-9a, b

Batesian Mimicry

Fig. 27-9c, d



Some prey organisms gain protection through mimicry (continued)

– Some animals deter predators by employing startle coloration

–These animals may have spots that resemble the eyes of a larger animal

– If a predator gets close, the prey will flash its eyespots, startling the predator and allowing the prey to escape

–Examples include the peacock moth and the swallowtail caterpillar

Startle Coloration

Fig. 27-10



Predators may use mimicry to attract prey

– In aggressive mimicry, a predator resembles a

harmless animal or part of the environment, to

lure prey within striking distance

–For example, a frogfish dangles a wriggling

lure that attracts a curious fish that is then

eaten

Aggressive Mimicry

Fig. 27-6b

Predators and prey may engage in chemical

warfare

– Predators and prey use toxins for attack and

defense

–The bombardier beetle sprays boiling-hot

chemicals from its abdomen onto its attacker

Chemical Warfare

Fig. 27-12a

Predators and prey may engage in chemical

warfare for attack and defense (continued)

– Many plants have evolved chemical adaptations

that deter their herbivore predators, such as the

milkweed

– In the case of the milkweed, however,

monarch butterfly caterpillars have evolved to

tolerate the toxins and store them in their

tissues as a defense against predation

Chemical Warfare

Fig. 27-12b

Parasites live in or on their prey, which are

called hosts, usually harming or weakening

them but not immediately killing them

– Parasites are generally much smaller and more

numerous than their hosts

–Examples include tapeworms, fleas, ticks, and

many types of disease-causing protists,

bacteria, and viruses

http://upload.wikimedia.org/wikipedia/commons/7/7d/MistletoeInSilverBirch.jpg

http://en.wikipedia.org/wiki/Image:Candiru.png

http://en.wikipedia.org/wiki/Image:Scanning_Electron_Micrograph_of_a_Flea.jpg

http://en.wikipedia.org/wiki/Image:Leech_blutegel.jpg

http://en.wikipedia.org/wiki/Image:Cuscuta_europaea_bgiu.jpg

Social parasites!

Animals that take advantage of the social

behavior of a host to complete their life cycle.

What Is Parasitism?

Parasites and their hosts act as agents of

natural selection on one another (continued)

– Nagana, a disease in cattle caused by a parasitic

protist, kills cattle imported into areas of Africa,

but some African breeds of cattle have evolved

an immunity to it and survive

27.5 What Is Mutualism?

Mutualism refers to interactions between

species in which both benefit

–For example, lichens form a mutualistic

relationship between a fungus and an algae

–The fungus provides support and protection

while obtaining food from the

photosynthetic alga

Mutualism

Fig. 27-13a

27.5 What Is Mutualism?

Mutualism refers to interactions between

species in which both benefit (continued)

– Another example of mutualism is the clownfish

and sea anemones

–The clownfish takes shelter from predators

among the venomous tentacles of an

anemone, while in turn cleaning it, providing it

with scraps of food, and defending it from

predators

Mutualism

Fig. 27-13b

Mutualisms

Obligatory:

Yucca plants and

Yucca moths

27.6 How Do Keystone Species Influence

Community Structure?

In some communities, a keystone species plays a major role in determining community structure

– A keystone species role is out of proportion to its abundance in the community

– If a keystone species is removed from the community, normal community interactions are significantly altered and the relative abundance of other species changes dramatically

– Keystone species need to be identified and protected so that human activities do not lead to the collapse of entire communities and ecosystems

27.6 How Do Keystone Species Influence

Community Structure?

In some communities, a keystone species

plays a major role in determining community

structure (continued)

– An example of a keystone species is the

predatory sea star Pisaster ochraceous from

Washington’s rocky intertidal coast

–When removed from their ecosystem, their

favored prey, native mussels, became so

abundant that they outcompete other

invertebrates and algae

Keystone Species

Fig. 27-14a



Most communities do not emerge fully formed

from bare rock or naked soil

– Instead, they arise through succession, where

the community and its nonliving environment

change structurally over time

– Succession is usually preceded by a

disturbance, an event that disrupts the

ecosystem either by altering the community, its

abiotic (nonliving) structure, or both



During succession, most terrestrial communities

go through stages

– Succession begins with arrival of a few hardy

plants, called pioneers

–The pioneers alter the ecosystem in ways that

favor competing plants, which eventually

displace the pioneers



During succession, most terrestrial communities

go through stages (continued)

– Succession often progresses to a relatively

stable and diverse climax community

– Recurring disturbances can set back the

progress of succession

–The continuous disturbances maintain

communities in earlier, or subclimax, stages

of succession

Ecological Succession

Change in the

composition of species

over time

Classical model

describes a

predictable sequence

with a stable climax

community

Pioneer

Species

Other

Species

Climax

Community

Succession in Progress

Fig. 27-15b



There are two major forms of succession

– Primary succession

– Secondary succession

Primary succession

Changes an area lacking any community (no plants, animals, seeds, soil) to one with a functioning community of plants, animals, fungi etc.

Pioneer plants: lichen and mosses

Secondary Succession

Follows

disturbance of an

existing

community that

removes or

damages the

vegetation, but

does not remove,

destroy, or cover

the soil.


Pioneer plants of secondary succession start from roots or seeds remaining in the soil or from seeds carried in by wind or animals from surrounding communities.



Primary succession occurs “from scratch,”

where there is no trace of a previous community

– This process may take thousands or even tens of

thousands of years

– The disturbance that sets the stage for primary

succession may be a glacier scouring the

landscape to bare rock, or a volcano



Secondary succession occurs after a disturbance

changes, but does not obliterate, an existing

community, leaving remnants such as soil and seeds

– This type of succession often takes just hundreds of

years

– An example is Mount St. Helens, which erupted in

1980 and left a thick layer of nutrient-rich ash that

encouraged new growth

– Another example is fire, which also produces nutrient-

rich ash and spares some trees and many healthy

roots

Succession in Progress

Fig. 27-15a



Primary succession can begin on bare rock

– Isle Royal, Michigan, is an example of primary

succession

– This island in Lake Superior was scraped down

to bare rock by glaciers

– The bare rock provided a place for pioneer

species, such as lichen and mosses

rock scraped

bare by a

glacier

lichens and

moss on

bare rock

bluebell,

yarrow

blueberry,

juniper

jack pine,

black spruce,

aspen

spruce-fir

climax forest:

white spruce,

balsam fir,

paper birch

0

1,000

Primary Succession

Fig. 27-16

plowed

field

ragweed,

crabgrass,

Johnson

grass

aster,

goldenrod,

Queen Anne's lace,

broom sedge grass

blackberry,

smooth sumac

Virginia pine,

eastern red

cedar

oak-hickory

climax forest:

white and black oak,

bitternut and

shagbark hickory

0

100


Fig. 27-17

Succession in a Small Freshwater Pond

Fig. 27-18



Succession culminates in a climax community

– Succession ends with a relatively stable climax

community, which perpetuates itself if not

disturbed by outside forces, such as fire



Succession culminates in a climax community

(continued)

– Climax species tend to be larger and longer-lived

than pioneer species

– The exact nature of the climax community at a

site reflects the local geological and climatic

conditions, such as temperature, rainfall, and

elevation



Some ecosystems are maintained in a

subclimax stage

– Frequent disturbances maintain subclimax

communities in some ecosystems

– A subclimax community example is the tallgrass

prairies that once covered northern Missouri and

Illinois

–Periodic fires maintained the grasses and

prevented forests from encroaching



Some ecosystems are maintained in a

subclimax stage (continued)

– Another example of a subclimax community is

suburban lawns

–Mowing and use of herbicides keep weeds

and woody species in check

– A further example of a subclimax community is

agriculture

–Plowing and pesticides keep competing

weeds and shrubs from replacing grains



Climax communities create Earth’s biomes

– The climax communities that form during

succession are strongly influenced by climate

and geography

– Extensive areas of characteristic climax plant

communities are called biomes, and include

deserts, grasslands, and forests

Chapter 27 Community Interactions - Linn-Benton Community...

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