Download Ch. 4 Populations and communities

CHAPTER 4
Populations and
Communities
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Dynamics of natural populations
• Population: a group of members of the same
species living in an area
• Community: populations of different species living
together in an area
• Populations grow with births and immigration
• They decline with deaths and emigration
(Births + Immigration) – (Deaths + Emigration)
= Change in population number
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Review Question-1
(Births plus ________) minus (________ plus
Emigration) = Change in population number
a. Emigration; Births
b. Immigration; Deaths
c. Immigration; Births
d. Emigration; Deaths
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Three models of population growth
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Interpreting Graphs and Data-2
According to Fig. 4-8, the approximate date that
this population reached its greatest level of
environmental resistance was
a. 1944.
b. 1955.
c. 1963.
d. 1991.
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Real-life growth
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Thinking Environmentally-1
A K-strategist’s population size typically
fluctuates around
a. the midpoint of the J-curve.
b. carrying capacity.
c. density-dependent factors.
d. density-independent factors.
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Biotic potential and environmental
resistance
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Reproductive strategies: r-strategists
• The interplay of environmental resistance and
biotic potential drives the success of two
reproductive strategies
• r-strategists (r-selected species): produce lots
of young, but leave their survival to nature
•
•
•
•
•
•
•
Results in low recruitment
Rapid reproduction, rapid movement, short life span
Adapted to a rapidly changing environment
“Boom-and-bust” populations
“Weedy” or “opportunistic” species
For example, housefly
LIST 3 EXAMPLES………
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Reproductive strategies: K-strategists
• K-strategists (K-selected species): lower biotic
potential
• Care for and protect young
• Live in a stable environment already populated by
the species
• Larger, longer lived, well-adapted to normal
environmental fluctuations
• Their populations fluctuate around carrying capacity
• Also called equilibrial species
• LIST 3 EXAMPLES
• Now do Class Activity
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Population Controls
A sudden hard freeze that kills members of an
ecosystem is an example of
a. a critical number.
b. a density-dependent factor.
c. a density-independent factor.
d. density-driven resistance.
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Major types of interactions between
species
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Predator-prey relationships
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Plant-herbivore interactions
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Plant-herbivore dynamics, Territoriality
and Predator-Prey Relationships on
Campus – Class Activity
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Natural selection and evolution
• Natural selection: the process of specific traits
favoring survival of certain individuals
• Charles Darwin’s On the Origin of Species by Means
of Natural Selection (1859)
• Biological evolution: modification of the gene pool
of a species by natural selection over generations
• Our understanding of DNA, mutations, and genetics
supports the theory of evolution by natural selection
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Adaptations to the environment
• Fitness: features (traits) that adapt an organism for
survival and reproduction
• A population’s gene pool is tested by selective
pressures exerted by environmental resistance
• All traits adapt an organism to survival and
reproduction
• Adaptations needed for coping with abiotic factors
• Obtaining nutrients, energy, defense against
predation
• Finding and attracting mates
• Migrating and dispersal
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Impact on the species
• Survival of the fittest: one of the forces in nature
leading to evolutionary changes in a species
• Those individuals in a competing group that can
survive and reproduce demonstrate superior fitness
to the environment
• Every factor of environmental resistance is a
selective pressure
• Individuals who survive and reproduce have the
genetic endowment to better cope with their
environment
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Adaptation for survival and
reproduction
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The limits of change
• Does the trait increase survival and reproduction?
• If “yes,” the trait is maintained through natural
selection
• Various organisms have evolved different traits
• To accomplish the same function (e.g., avoid
predation)
• A species has three alternatives when faced with a
new selective pressure
• Adaptation: through natural selection
• Migration: move to an area with suitable conditions
• Extinction: inevitable if the first two options are not
possible
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Factors that determine whether a
species can adapt
• Some individuals have traits (alleles: variations or
new combinations of genes) that allow them to
survive and reproduce under the new conditions
• There must be enough survivors to maintain a viable
breeding population
• Natural selection should lead to increased
adaptations over successive generations
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Humans are responsible for
biodiversity loss
• Human activities are responsible for the decline and
extinction of species
• Humans change habitats, introduce alien species,
pollute, hunt, etc.
• Human activities are not density dependent
• They can even intensify as numbers decline
• The Endangered Species Act calls for recovery of
• Threatened species: populations are declining rapidly
• Endangered species: populations are near the critical
number
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Critical number
• Critical number: the minimum population base
allowing the survival and recovery of a population
• A pack of wolves, flock of birds, school of fish
• The group is necessary to provide protection and
support
• If a population falls below this number
• Surviving members become more vulnerable
• Breeding fails
• Extinction is almost inevitable
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Implications for human management
• Ecologists study populations and communities
• To increase our understanding of the world
• To better manage natural resources by protecting
declining species and controlling pest species
• To study our impacts on populations
• Keystone species are such an integral part of the
ecosystem
• Removing them can cause the ecosystem to collapse
• For example, removing beavers (by overhunting and
habitat destruction) hurt wetland meadow species
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Keystone species
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Introduced species
• Humans have introduced species from foreign
ecosystems, changing community and population
relationships
• Thousands of species have been accidentally or
deliberately introduced
• Economic losses in the U.S. of $138 billion/year
• Rabbits were introduced into Australia for sport
shooting
• Without natural enemies, the population exploded,
devastating the environment
• After being eradicated on Philip Island, the island’s
vegetation was dramatically restored
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Introduced plant species
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