Community Biological communities...G. fortis Alone G. fuliginosa and G. fortis Sympatric Los Hermanos Islets Daphne Major Island San Cristobal and Individuals in each size class (%)

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1

Biosphere

Ecosystem

Community

Population

Organism

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Community Ecology

Chapter 54

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Community• A biological community refers to all the

species that occur together at any particular locality.

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Biological communities

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Gleason & Clements

Individualistic Holistic

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Individualistic

Holistic

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Individual species in a community respond independently to changing environmental conditions.

400

200

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Moisture gradientWet Dry

Num

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ecta

re

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A synchronous change of species in communities exist only when sharp changes of environmental conditions occur.

Normalsoil

Ecotone Serpentinesoil

Black oakPoison oak

IrisDouglas fir

HawkweedCalifornia fescue

SnakerootCanyon live oak

Collomia

RagwortYarrow

Buck brush

Small fescueFireweedKnotweed

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– Biodiversity —The number and evenness of species.

– The niche concept—The role of interspecific interactions

– Food web structure —The feeding relationships among species.

– Succession—The change of communities through time.

Attributes of ecological communities

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kingfisher

great blue heronmerganser otter

dipper

steelhead roach stickleback newt caddis fly larva

snailfrog tadpolewater scavengerbeetle larva

tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Biodiversity: number & evenness

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• Species richnessmay be equal,but relativeabundance may be different.

Biodiversity measures the number of species and their evenness

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Most species in a community are rare

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Biodiversity: community 1 > 2

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kingfishergreat blue heron

merganser otter

dipper



tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Exercise: What factors determine the occurrence of a species in a community?

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First, a species needs to be able to tolerate the local physical-chemical environment.

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Second, the environment must provide required resources, such as food.

+ required resources

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~ the fundamental “niche”

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A niche

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Joseph Grinnell (UC Berkeley)

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

High tide

Low tide

Chthamalus

Semibalanus

The distribution patterns of Chthamalus & Semibalanus

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Joseph H. Connell (UC Santa Barbara)

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High tide

Low tide

Chthamalus

Semibalanus

Exercise: Is this range the fundamental niche of Chthamalus? How do you know?

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[1] removing Chthamalus …

[2] removing Semibalanus …

Let’s do a removal experimentHigh tide

Low tide

Chthamalus

Semibalanus

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Results: What does the removal experiment tell you?

High tide

Low tide

Chthamalus

Semibalanus

After removing Chthamalus …

After removing Semibalanus …

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Realized Niches

– Actual niche utilized by an organism is influenced by competition, predation, parasitism, mutualism, etc..

– Biological interactions change niche space, thus contribute to the structure of a community.

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Fundamental vs Realized Niches

Fundamentalniches

Realizedniches

High tide

Low tideChthamalus

Semibalanus

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Predation often reduces “niche” space

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~ the realized “niche” Biotic interactions limiting

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Ghost of competition past …Ghost of predation past …

The fundamental niche of a species may shift due to consistent biological interactions over evolutionary time scale. Thus, the effects of competition or predation are no longer exist in the present day.

Species as we see them today are products of biological interactions over evolutionary times.

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merganser otter

dipper



tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Exercise: What’d happen if 2 species had overlap niches, e.g. steelhead & roach?

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– If niche overlap extensively, then, to coexist, the shared resources must not be limiting. Otherwise, …..

– One species may be driven to extinction (competitive exclusion).

– Or, one or both species has to shift their niches by changing their behavior (resource partitioning).

– Sometimes, morphology change as well (character displacement) to enhance resource partitioning.

Outcomes of interspecific competition

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Principle of Competitive Exclusion

• When resources are limiting, no two species utilizing the same niche can coexist indefinitely.– one will eventually eliminate the other

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Georgyi F. Gause (Moscow Univ.)

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050

100150200

50100150200

Days40 8 12 16 20 24 40 8 12 16 20 24

0

Days

P. aureliaP. caudatum

Popu

latio

n de

nsity

200

150

100

50

00 4 8 12 16 20 24

Days

P. caudatumP. aurelia

Popu

latio

n de

nsity

Competitive Exclusion Among Paramecium

Alone

Together

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00

20161284Days

P. caudatumP. bursaria

50

75

25

Popu

latio

n de

nsity

50100150200

50

100150200

40 8 12 1620 24 40 8 12 1620 240

Days

0

Days

P. caudatumP. bursaria

Alone

Together

If competitive exclusion did not occur,resource partitioning must exist.

Eat bacteria

Eat yeasts

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Either way, competition reduce population sizes, thus, change relative abundance and community structure.

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Consequences of long-term competition

• The fundamental niche of a species may shift under long-term interspecific competition through 2 processes:

– Resource partitioning

– Character displacement

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Resource Partitioning

• Species living in the same area (sympatricspecies), over time, evolve behavioral mechanisms that partition available resources to avoid direct competition.

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RESOURCE PARTITIONING OF INSECT-EATING WARBLERS

Cape Maywarbler

Bay-breastedwarbler

Myrtlewarbler

Resource Partitioning – the sympatric species have subdivided the niche

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Robert MacArthur (Princeton Univ.)

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Resource partitioning of insect-eating lizards

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Character displacement

• Sympatric species tend to exhibit greater differences in morphology than allopatric species (species that live apart from each other) to avoid direct competition.

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5025

05025

05025

07 9 11 13 15 Finch beak depth (mm)

G. fuliginosaAlone

G. fortisAlone

G. fuliginosaand

G. fortisSympatric

Los HermanosIslets

Daphne MajorIsland

San Cristobal andFloveana Islands

Indi

vidu

als

in e

ach

size

cla

ss (%

)

Character Displacement

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Does the diversification of Darwin’s finches involve resource partitioning & character displacement?

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merganser otter

dipper



tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Exercise: Use an analogy to describe “niche”to your friend.

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Mode of interspecific competition

• Exploitative competition– e.g. Paramecium compete for food– the 2 species consume shared resources– The 2 species don’t need to see each other

• Interference competition– e.g. barnacles compete for space– the 2 species fight over resources (space)– there are face-to-face confrontations

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Exploitative competition – Hey, don’t drink too fast !

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Interference competition – Hey, you pinch my straw !

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Kangaroo rats and other desert rodents

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Removal experiments are the strongest tests of the existence of interspecific competition

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15

5

0

199019891988

Kangaroo rats removedKangaroo rats present

Num

ber o

f oth

er ro

dent

s

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Detecting Interspecific Competition

• Negative effects of one species on another do not automatically indicate competition.

– Presence of one species may attract a predator that consumes both, causing one species to have a lower population size than the other. (apparent competition)

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merganser otter

dipper



tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Exercise: So, how does competition affect community structure?

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merganser otter

dipper



tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Predation also affects community structure.

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merganser otter

dipper



tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Exercise: What’d happen to the community when heron reduce roach?

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Predators reduce the population size of prey, and often induce top-down trophic cascades.


1200

800

400

Paramecium

Didinium

0 1 2 3 4 5 6 7Days

Num

ber o

f ind

ivid

uals

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Trophic cascades are a key feature in community dynamics.

We will talk about it in ecosystem ecology.

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The interdependent evolution of two or more species.

Particularly, “arms races” between predatorsand prey drive evolutionary changes and remarkable adaptations in both predator and prey species, and promote species diversity.

Coevolution

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Plant defenses against herbivores-- Morphological defenses

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Plant defenses against herbivores-- secondary compounds

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Some herbivores can breakdown secondary compounds

Mustard oils – cabbages – cabbage butterfly

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Herbivores make use of secondary compounds

Cardiac glycoside – milkweed – Monarch butterfly 62

Herbivores make use of secondary compounds

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Prey defenses against predators

• Chemical defenses• Morphological defenses

– Cryptic (camouflage) coloration.

– Warning (aposematic) coloration.

– Mimicry

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Chemical defenses

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Camouflage! Can you see them?

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Warning! Dare you eat them!

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Batesian mimicry is where a harmless species mimics a harmful one.

Danaus plexippus Limenitis archippus

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Müllerian mimicry is where two or more unpalatable species resemble each other.

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Müllerian mimicry & Batesian mimicry

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In addition to competition, predation & herbivory,there are other types of species interactions

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Cannibalism

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CommensalismEpiphytes Barnacles on whales

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Mutualism

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Parasitism -- ectoparasites

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Parasitism -- endoparasites

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Insect Parasitoid

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Parasites with complex life cycles often manipulate host behavior

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Mutualism turns parasitism

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Interactions Among Ecological Processes

• Predation may reduce competition• Parasitism may alter competition• Indirect effects

– Presence of one species may affect a second species through interactions with a third species.

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–

–

–

+ +

Rodents

Large seedsSmall seeds

Ants

What’d happen to antsif you removed rodents?

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Fig. 54.22(TE Art)Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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40

20

Sampling periodsOct 74 May 75 Sep 75 May 76 Aug 76 Jul 77

Rodents removedRodents not removed

Num

ber o

f ant

col

onie

s

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Fig. 54.23(TE Art)Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

–

–

–

+ +

Rodents

Large seedsSmall seeds

Ants

Complex indirect interaction

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Keystone speciesA species whose absence would bring about a significant change in the community. Top predators in a community are often keystone species (e.g. sea stars affect the diversity of tidal pool communities).

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A tidal pool community

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85

After sea star removal

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Before After

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Sea stars as a keystone species

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merganser otter

dipper



tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Exercise: Who do you think is the keystone species in this community?

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Ecosystem engineeringSome keystone species modify their habitats to produce a particularly strong effect on community structure, e.g. beavers.

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Succession

• Biological communities change with time, usually, but not necessarily, from a simple to a more complex structure.

– primary succession – secondary succession

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Primary succession - occurs on bare substrates

Glacial retreatYear 1

Year 100

Year 200

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Soil nutrients change as succession progressCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Pioneermosses

Invadingalders

Alderthickets

Spruceforest

Year 1 Year 100 Year 200

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100

150

200

250

300

b

c

Nitrogenin mineralsoil

Nitrogenin forestfloor

Nitr

ogen

(g/m

2 )

a

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Primary succession - occurs on bare substrates

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Secondary succession occurs where an existing community has been cleared, but the soil is left intact.

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Why succession occurs?• Pioneer species are able to tolerate harsh

conditions during early succession.

• Settled species alter the habitat which facilitate the invasion of subsequent colonizers.

• The habitat may be further altered that inhibitthe growth of original colonizers.

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Why succession occurs?• Seed dispersers also aid the succession

process

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97

Most communities are in a state of nonequilibriumowing to frequent disturbances

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Marine and island communities are subject to disturbance by tropical storms.

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We often think that disturbances have a negative impact on communities, but some communities are maintained by disturbance.

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Exercise: Do disturbances increase or decrease species diversity in a community? How?

Disturbance

Spec

ies n

umbe

r

?

101

Intermediate disturbance theoryCommunities experiencing moderate levels of disturbance will have greater species richness than communities experiencing either smaller or larger levels of disturbance.

simultaneous successional stagesdominant competitors kept at bay

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merganser otter

dipper



tuft midge

diatomsgreen algae

blue-green algae

crayfish

garter snake

Exercise: What factors influence the biodiversity of this community?

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103

Causes of biodiversity, so far

Physical-chemical environmentBiological interactionDisturbanceOthers?

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Biosphere

Ecosystem

Community

Population

Organism

105

Dynamics of Ecosystems

Chapter 55

Chapter 55

106

Ecosystem ecology

• An ecosystem includes all the organisms living in a particular place, and the abioticenvironment in which they interact.

• Ecosystem ecology is about …– Biogeochemical cycles of chemical

elements through ecosystems.– Energy flows through ecosystems (a

one-way process).

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Eugene Odum (Univ. of Georgia)

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Biogeochemical Cycles

• The water cycle• The carbon cycle• The nitrogen cycle• The phosphorus

cycle

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109

Physical processes move elements around

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Biological processes move elements around

Photosynthesis

Decomposition

Feeding

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cycle

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The water cycle

Organisms

Ice

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The Water Cycle

• By area, oceans receive ¾ and lands ¼ of precipitation. Both lands and oceans evaporate much of water back to the atmosphere.

• On lands, transpiration from plants account for 90% of water back to the atmosphere.

114

Reservoirs of water

Atmosphere

Organisms

Ice

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115

Water cycling processes

Organisms

Ice

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Groundwater

• Groundwater (stored in aquifer) is the largest reservoir of water on lands.

• It is recharged via percolation of rainfall and water seeps down from lakes and streams through soil, very slowly. The flow is also very slow.

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Mining and pollution of groundwater

• Human mine groundwater, thus many underground aquifers have much higher withdraw rates than recharge rates. Agriculture will be in trouble.

• Pesticides, herbicides, and fertilizers accumulate in groundwater are virtually impossible to be removed (b/c slow turnover rate). Yet, groundwater supplies about 50% of drinking water (in U.S.).

118

Deforestation breaks the local water cycle

• In forest ecosystems, plants take up much moisture and transpire it back into the atmosphere, forming clouds. When forests are cut down, water drains from the local area instead of forming clouds.

• The lack of rains further prevents reforestation, and may create semi arid desert (positive feedback).

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Deforestation also causes erosions

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cycle

21

121

The carbon cycle

122

Carbon Cycle

• Carbon cycle is based on CO2 which makes up 0.03% of the atmosphere, dissolved in water, and those C locked up in organic compounds (live or dead organisms) and sediment (carbonates).

• Most organic compounds formed as a result of CO2 fixation (photosynthesis). About 70% of CO2 in the atmosphere is fixed by photosynthesis annually.

123

Reservoirs of Carbon

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Carbon Cycle

• Carbon in the ecosystems cycles through photosynthesis and respiration processes. Most organic compounds are ultimately broken down, and C is released back into the atmosphere.

• Under natural condition, photosynthesis and respiration are proximately balanced. But we have been tipping the balance.

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Carbon cycling processes

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Tipping the balance

• Increasing fuel consumption, and deforestation by burning are liberating carbon at an increasing rate.

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127

Green house effects

128

CO2 and global warming

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cycle

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The nitrogen cycle

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The Nitrogen Cycle I

• Only a few kind of prokaryotes can fix atmospheric nitrogen into forms that can be used for biological processes. Only symbiotic bacteria fix enough nitrogen to be of major significance

• Nitrogen fixation: N2+ 3H2 2NH3

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Nitrogen-fixing symbionts

• Nitrogen fixation: N2+ 3H2 2NH3

• Nitrogen-fixing bacteria form symbiotic relationship mainly with the roots of legumes (the pea family).

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133

Reservoirs of nitrogen

Organisms

134

The Nitrogen Cycle II

• After organisms die, nitrogen in organisms become soil nitrates through 2 processes carried out by decomposers.

• Ammonification – NH4+ (ammonium) are released during the metabolic processes of decomposers.

• Nitrification -- NH4+ are converted by bacteria into NO2- (nitrite) and NO3- (nitrate) which can be taken up by regular plants.

135

The Nitrogen Cycle III

• Nitrogen fixed in soil (NO3- , nitrate) may return to atmosphere through Denitrification, carried out by bacteria under anaerobic condition.

• Denitrification: Bacteria get O2 from NO3- and release N2 or N2O back to the atmosphere.

136

Nitrogen cycling processes

Feeding

137

Tipping the balance

• The use of fertilizers, thus large amount of nitrogen, in agriculture alter the natural cycles of nitrogen.

138

Eutrophication

24

139



cycle

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Phosphorus Cycle

141

The Phosphorus Cycle

• The largest reservoir of phosphates exist in rocks.

• Phosphates weather from rocks and soils into water. They may enter plants and animals, or are transported to oceans and accumulate in sediments.

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Reservoirs of phosphate

Organisms

143

Sediments & up-welling

• The nitrates and phosphates locked up in sediments, particularly those in the deep ocean floor, can be brought back only by up-welling.

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Guano deposits

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145

Tipping the balance

• Three times more phosphates than crops required are added to agricultural lands each year.

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Eutrophication

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Exercise: What would happen to nutrient cycles after deforestation?

Hubbard Brook Experimental Forest

148

When the watershed is intact …

• The amounts of nutrient inputs from rains and snows are about equal to the amounts that flow out.

When the forest is cut down …

• The nutrient inputs from rains drain from the local area.

149

Con

cent

ratio

nof

nitr

ate

(mg/

l )

1965 1966

2

0

4

40

80

1967 1968

Deforestation

Consequences of deforestation

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Nutrients cycle, but energy doesn’t

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151

Energy flows through ecosystems

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Different types of producers

– Photo-autotrophs –CO2+ H2O C6H12O6(Energy provided by solar energy)

– Chemo-autotrophs–CO2 or CH4 C(H2O)n(Energy provided by H2S+ O2

Energy)

153

Different types of consumers

– Primary consumers – herbivores, grainivores, frugivores,

– “Higher” consumers – carnivores, insectivores, parasites,

– Detritivores -- break down dead organic material

Scavengers – Vultures and crabsDecomposers – Bacteria and fungi

154

Trophic level refers to the feeding level of an organism

155

Primary Productivity

• Green plants capture about 1% of the solar energy that falls on their leaves.

• In a given area during a given period of time …

– Gross primary productivity (GPP) is the total organic matter produced by producers.

– Net primary productivity (NPP) is the amount of organic matter produced that is available to consumers.

– Biomass is the total mass of all organisms. 156

Primary productivity of various

ecosystems.

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157

Primary productivity of various ecosystems.

HighLow

158

Exercise:Why do they

have different NPP ?

159

We did not talk about the material covered in the following 20 slides.

But they will show up in the final examine.

160

• The productivity in a given area is ultimately determined by the amount of energy it receives. Thus, NPP often increases as the growing season lengthens.

• In aquatic ecosystems, light and nutrients limit primary production.

• In terrestrial ecosystems, temperature, moisture, and nutrients limit primary production.

Primary Productivity

161

On a local scale, nutrients in the soil can play key roles in limiting primary production.

162

Not all NPP is consumed

• NPP not consumed will eventually become available to decomposers.

• And, not 100% of the NPP consumed by herbivores is assimilated

• Herbivores assimilate about 20% of consumed biomass; carnivores 5%. On average, 10% of consumed biomass is assimilate by consumers.

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163

The assimilation by consumers is only ~10%

17%Growth

33%Cellularrespiration 50%

Feces

= assimilation

= heat loss164

~10% assimilation from one level to the next

165

Tertiaryconsumer

Primary consumer

Primary producer

Secondaryconsumer

Exercise: Why most food chains contain less than 4 trophic levels?

166

Tertiaryconsumer

Primary consumer

Primary producer

Secondaryconsumer

Diminishing energy due to heat loss

One-way energy flow

167

Fig. 54.1

Diminishing energy due to heat loss

168

Energy availability (NPP) affects

the length of a food chain.

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169

Exercise: Why are top

predators rarer than lower

consumers?

170

Ecological Pyramids

Moving up a food chain, you generally find fewer individuals, lower total biomass, and lower total energy at each successive trophic level.

171

Pyramid of numbers

4,000,000,000

111 Carnivore

Herbivore

Photosynthetic plankton

172

Decomposer(5 g/m2)

Herbivore (37 g/m2)

First-level carnivore (11 g/m2)

Second-level carnivore(1.5 g/m2)

Photosynthetic plankton (807 g/m2)

Pyramid of biomass

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Photosynthetic plankton(36,380 kcal/m2/yr)

Decomposer(3890 kcal/m2/yr)

Herbivore(596 kcal/m2/yr)

First-level carnivore(48 kcal/m2/yr)

Pyramid of energy

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Here is a good reason to be a vegetarian

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175

Trophic cascade

• The effect of one trophic level flows up or down to other levels.– Top-down effects– Bottom-up effects

176

Three-level top-down cascadeTrout – Invertebrate -- Algae

No Trout No Trout

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Fish addedNo fish added

before after

300

200

100

06050403020100

50004000300020001000

0

Dam

selfl

ies

(num

ber/m

2 )A

lgae

(g/m

2 )C

hiro

nom

ids

(num

ber/g

alg

ae)

fish addition

Four-level top-down cascade

Predatory fish

Damselfly nymph

Algae-eating insect

Algae

178

Exercise:

In a imaginary state, people love to hunt –deer hunting. They hate the wolves that

take “their” deer. Wolves are bad. So they do everything to get rid of wolves.

What would happen if wolves disappeared?

179

Sand County Almanac

By Aldo Leopold

I have lived to see state after state extirpate its wolves. I have watched the face of many wolfless mountain,

and seen the south-facing slopes wrinkle with a maze of new deer trails.

I have seen every desuetude, and then to death. I have seen every edible tree defoliated to the height of a

saddle horn.

180

Pred

ator

bio

mas

sH

erbi

vore

bio

mas

s Ve

geta

tion

biom

ass

Amount of light

Bottom-up effectsThree levels

Increase the amount of light, increase predator biomass

31

181Productivity

Bottom-up effectsFour levels

Increase productivity, increase food-chain

length

182Productivity

Exercise: Why are there portions of the curves

where vegetation biomass does not

increase as productivity increases?

Documents

Community Biological communities...G. fortis Alone G. fuliginosa and G. fortis Sympatric Los Hermanos Islets Daphne Major Island San Cristobal and Individuals in each size class (%)