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CHAPTER 53
COMMUNITY
ECOLOGY
I. EARLY HYPOTHESES OF COMMUNITY STRUCTURE

SPECIES DIVERSITY WITHIN A COMMUNITY: AN
ASSEMBLAGE OF SPECIES LIVING CLOSE ENOUGH
TOGETHER FOR POTENTIAL INTERACTION, INCLUDES
BOTH THE SPECIES RICHNESS (NUMBER OF SPECIES
PRESENT) AND THE RELATIVE ABUNDANCE OF EACH
SPECIES.
II. INTERACTIONS BETWEEN POPULATIONS OF
DIFFERENT SPECIES
A.

INTERSPECIFIC INTERACTIONS CAN BE STRONG
SELECTION FACTORS IN EVOLUTION
COEVOLUTION: RECIPROCAL INTERACTIONS BETWEEN TWO
SPECIES THAT RESULT IN A SERIES OF ADAPTATIONS AND
COUNTERADAPTATIONS

SECONDARY COMPOUNDS: TOXIC CHEMICALS PRODUCED IN
PLANTS TO DISCOURAGE HERBIVORES.

CAMOUFLAGE: ENABLES AN ANIMAL TO BLEND WITH
ITS SORROUNDINGS.
APOSEMATIC COLORATION: CONSPICOUS PATTERN
THAT WARNS PREDATORS.
MIMICRY: AN ANIMAL BEARS SUPERFICIAL
RESEMBLANCE TO AN UNPALATABLE OR HARMFUL
MODEL.

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MULLERIAN MIMICRY: SEVERAL ANIMALS
BATESIAN MIMICRY: A DEFENSELESS ANIMAL
B. INTERSPECIFIC INTERACTIONS MAY HAVE POSITIVE,
NEGATIVE, OR NEUTRAL EFFECTS ON A POPULATION'S
DENSITY

THESE INTERACTIONS CAN BE REPRESENTED BY PAIRS OF
SYMBOLS (+, -, AND 0) INDICATING THE IDEALIZED
EFFECT OF THE RELATIONSHIP ON EACH OF THE TWO
INTERACTING SPECIES.
C. PREDATION AND PARASITISM ARE (+ /-) INTERACTIONS:

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PREDATION REFERS TO INTERACTIONS IN WHICH
ANIMALS FEED ON OTHER ORGANISMS.
PARASITISM IS A TYPE OF PREDATION IN WHICH A
PARASITE LIVES ON OR IN A HOST , DERIVING
NOURISHMENT FROM IT BUT USUALLY NOT KILLING IT
OUTRIGHT.
D. INTERSPECIFIC COMPETITIONS ARE (- / - )
INTERACTIONS:

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INTERFERENCE COMPETITION: WHEN POPULATIONS OF
DIFFERENT SPECIES IN A COMMUNITY USE SIMILAR LIMITING
RESOURCES, THEY MAY FIGHT OVER RESOURCES.
THE COMPETITIVE EXCLUSION PRINCIPLE: STATES THAT
TWO SPECIES CANNOT COEXIST IN THE SAME COMMUNITY IF
THEIR NICHES ARE IDENTICAL.


INTERSPECIES COMPETITION LEADS TO EXTINCTION OF THE
WEAKER COMPETITOR OR ADAPTATION OF ONE SPECIES TO A
NEW NICHE; THUS, IT CANNOT OPERATE FOR LONG.
EXPLOITATIVE COMPETITION: EACH MAY SIMPLY REDUCE THE
RESOURCES AVAILABLE TO THE OTHER

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RESOURCE PARTITIONING: SPECIES PURSUE SLIGHTLY
DIFFERENT NICHES TO MINIMIZE COMPETITION
CHARACTER DISPLACEMENT (NICHE SHIFT): PHYSICAL
CHANGES IN A SPECIES TO ACCOMMODATE RESOURCE
PARTITIONING.

THE ECOLOGICAL NICHE: THE SUM TOTAL OF
THE ORGANISM’S USE OF THE BIOTIC AND
ABIOTIC RESOURCES IN ITS ENVIRONMENT.
 FUNDAMENTAL NICHE: THE NICHE THAT
AN ORGANISM OCCUPIES IN THE ABSENCE
OF COMPETING SPECIES.
 REALIZED NICHE: THE NICHE THAT AN
ORGANISM OCCUPIES IN THE PRESENCE OF
COMPETING SPECIES.
COMPETITION IN LABORATORY POPULATIONS OF PARAMECIUM
E. COMMENSALISM AND MUTUALISM ARE (+ / 0) AND (+ / +)
INTERACTIONS, RESPECTIVELY

SYMBIOSIS: IN WHICH A HOST AND A SYMBIONT
MAINTAIN A CLOSE ASSOCIATION (PARASITISM,
COMMENSUALISM, MUTUALISM)

COMMENSALISM: REFERS TO SYMBIOTIC INTERACTIONS
IN WHICH ONE SPECIES BENEFITS AND THE OTHER IS NOT
AFFECTED
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EX. BIRDS BUILDING NESTS IN TREES
MUTUALISM: REFERS TO SYMBIOTIC INTERACTIONS IN
WHICH BOTH SPECIES BENEFIT.

EX. THE INTERACTIONS OF POLLINATING ANIMALS WITH
FLOWERING PLANTS.
III. INTERSPECIFIC INTERACTIONS AND COMMUNITY
STRUCTURE

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THE TROPHIC STRUCTURE OF A COMMUNITY REFERS TO
ALL OF THE FEEDING RELATIONSHIPS IN THE
COMMUNITY.
FOOD-WEB ANALYSIS EMPHASIZES THE COMPLEXITY OF
TROPHIC CONNECTIONS IN A COMMUNITY.
A. PREDATORS CAN ALTER COMMUNITY STRUCTURE BY
MODERATING COMPETITION AMONG PREY SPECIES
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PREDATORS MAY REDUCE OR INCREASE DIVERSITY IN A
COMMUNITY.
KEYSTONE SPECIES HAVE A DISPROPORTIONATELY
STRONG IMPACT ON COMMUNITY STRUCTURE RELATIVE
TO THEIR OWN ABUNDANCE.
KEYSTONE PREDATORS INCREASE SPECIES DIVERSITY BY
LIMITING THE DENSITIES OF THE MOST COMPETITIVE
SPECIES IN A COMMUNITY, ALLOWING MORE SPECIES TO
COEXIST.
B. INTERSPECIFIC COMPETITION INFLUENCES
POPULATIONS OF MANY SPECIES AND
CAN AFFECT COMMUNITY STRUCTURE

EXOTIC SPECIES INTRODUCED INTO A COMMUNITY
OFTEN OUTCOMPETE NATIVE SPECIES, BUT COMPETING
SPECIES SOMETIMES COEXIST AT REDUCED DENSITIES.

COMPETITION MAY BE IMPORTANT IN STRUCTURING
MANY COMMUNITIES, SERVING TO REGULATE RELATIVE
ABUNDANCE AND PERHAPS SPECIES RICHNESS.
IV. DISTURBANCE AND NONEQUILIBRIUM
A.

HUMANS ARE THE MOST WIDESPREAD AGENTS OF
DISTURBANCE
AMONG ALL ANIMALS, HUMANS CREATE THE GREATEST
DISTURBANCES IN COMMUNITIES, USUALLY REDUCING
SPECIES DIVERSTITY.
B. SUCCESSION IS A PROCESS OF CHANGE THAT RESULTS
FROM DISTURBANCE IN COMMUNITIES

SUCCESSION: INVOLVES CHANGES IN SPECIES (NOTED BY
PLANTS) COMPOSITION OF A COMMUNITY OVER
ECOLOGICAL TIME.
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PRIMARY SUCCESSION: OCCURS WHERE NO SOIL
PREVIOUSLY EXISTED;
SECONDARY SUCCESSION: BEGINS IN AN AREA WHERE
SOIL REMAINS AFTER A DISTURBANCE
CLIMAX COMMUNITY: FINAL SUCCESSIONAL STAGE OF
CONSTANT SPECIES COMPOSITION.
PIONEER SPECIES: PLANTS AND ANIMALS THAT ARE FIRST
TO COLONIZE A NEWLY EXPOSED HABITAT