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Chapter 53

BIOLOGY 1M03 Chapter Notes - Chapter 53: Biogeography, Lignin, Soltyrei

Course Code
Jon Stone

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Biology Chapter 53: Community Ecology
Species Interaction
A biological community consists of interacting species, usually living within a
defined area
To study species interaction, biologists focus on analyzing the effects on the
fitness of individuals involved
Four categories of interaction
o-/- relationship known as competition
o+/- relationship known as consumption and parasitism
o+/+ relationship termed mutualism
o+/0 relationship termed commensalism
Ex. Birds that follow moving army ants in the tropics
As ants march along forest, they hunt insects and small vertebrates
As they do, birds follow and pick off prey species that fly or jump
out of the way of the ants
The birds are commensals that benefit from the association (+), but
have no measurable impact on the ants (0)
Section focuses on three key themes
oSpecies interaction may affect the distribution and abundance of a
particular species
Changes in species interactions often explain short-term changes
in population size and distribution
oSpecies act as agents of natural selection when they interact
Deer are fast and agile in response to natural selection exerted by
major predators
Speed and agility of deer, in turn, promote natural selection
that favours wolves and cougars that are fast and have
superior eyesight and senses of smell
Changes in species interactions lead to long-term changes
in characteristics of populations, a phenomenon called
coevolution, in addition to having short-term impacts on
population size

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oOutcome of interactions among species is dynamic and conditional
Consider relationship between army ants and bids that follow them,
which is usually commensal
If bird attacks start to force other insects into path of ants, then both
benefit and relationship becomes mutualistic
If birds begin to steal prey that would otherwise be taken by ants,
relationship becomes parasitic
oCompetition is a -/- interaction that occurs when different individuals use
the same resources and when those resources are limiting
oCompetition that occurs between members of same species is called
intraspecific competition
Intraspecific competition for space, sunlight, food and other
resources intensifies as population’s density increases
A major cause of density-dependent growth
oInterspecific competition arises when individuals from different species
use the same limiting resources
oThere are many types if interspecific competition
Consumptive competition occurs when individuals consume the
same resources (ex. trees competing for same water and nutrients)
Pre-emptive consumption exists when one species makes space
unavailable to other species
Overgrowth competition happens when one species grows above
another (ex. large fern overgrown other individuals and is shading
Chemical competition takes place when one species produces
toxins that negatively affect another species
Territorial competition arises when a mobile species protects its
feeding or breeding territory against other species (ex. grizzly bears
drive off black bears)
Encounter competition occurs when two species interfere directly
for access to specific resources (ex. spotted hyenas and vultures
fight over kill)

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oUsing the Niche Concept to Analyze Competition
A niche can be thought of as the range of resources that the
species is able to use or the range of conditions it can tolerate
Interspecific competition occurs when niches of two species overlap
oWhat Happens When One Species Is a Better Competitor
G.F. Gause claimed it is not possible for species with the same
niche to coexist
This hypothesis is called the competitive exclusion principle and
was inspired by a series of experiments Gause did with
When Gause placed small populations of P. caudatum and P.
Aurelia in separate laboratory cultures, both specie exhibited
logistic growth
But Gause showed that when two species are put in the same
culture together, only the P. Aurelia population exhibits a logistic
growth pattern, P. caudatum is driven to extinction
His results are a product of asymmetric competition
When asymmetric competition occurs, one species suffers a
much greater fitness decline than the other species does
Under symmetric competition, each of the interacting species
experiences a roughly equal decrease in fitness
If asymmetric competition occurs and the two species have
completely overlapping niches, then the stronger competitor is likely
to drive the weaker competitor to extinction
But if the niches do not overlap completely, then the weaker
competitor should be able to retreat to an area of non-overlap
In cases like this, an important distinction arises between a species’
fundamental niche, which is the combination of resources or
areas used or conditions tolerated in the absence of competitors
and its realized niche, which is the portion of resources or areas
used or conditions tolerated when competition occurs
oExperimental Studies of Competition
Joseph Connell observed that there were two species of barnacles
with distinctive distributions in an intertidal rocky shore
Barnacle larvae are mobile, but adults live attacked to rocks
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