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POPULATION
Dynamics of
Natural Population
Populations of most species remain
more or less constant.
Population Equilibrium: deaths =
births
Dynamics of
Natural Population
Population Growth Curves
Exponential increase results in
Population Explosion
• J-shaped curve (only happens when a
disturbance occurs—foreign species, habitat
change, removal of predator)
• Growth is independent of population density
• Also called geometric growth
Dynamics of
Natural Population
Doubling Time
How long it takes to double a population
Doubling time (years) = 70/annual % growth
Annual % growth = (BR-DR)/10
Example
BR = 20
DR = 5
Annual % growth = (20-5)/10 = 1.5
Doubling time = 70/1.5 = 47 years
Dynamics of
Natural Population
When a population exceeds the carrying
capacity a population crash or dieback occurs
Overshoot: extent to which a population
exceeds the carrying capacity
This pattern of population explosion then
crash is called irruptive or Malthusian
growth
Dynamics of
Natural Population
Population Growth Curves
Logistic
• S-shaped curve (population levels off and
continues in a dynamic equilibrium)
• Growth is not independent of population
density
• A more stable population
BIOTIC POTENTIAL
Number of offspring that a species
may produce
Recruitment: survival through
earthy growth stages to become
part of the breeding population
REPRODUCTIVE
STRATEGIES
Malthusian “Strategies”
Produce massive numbers of young, but
leave survival to nature
High biotic potential, but low recruitment
(high mortality) so population may not
increase
Examples: insects, rodents, marine
invertebrates, parasites and “weeds”
R-strategists
REPRODUCTIVE
STRATEGIES
Logistic “Strategies”
Produce less young, but care for them until
they can compete for resources themselves
Usually are larger, live longer, mature more
slowly and have fewer natural predators
Examples: wolves, elephants, whales and
primates
K-strategists
Factors that Increase or Decrease
Populations
Natality: number of individuals added
through reproduction
Fecundity: physical ability to reproduce
Fertility: measure of the actual number of
offspring produced
Immigration: organisms introduced into a
new ecosystem
Factors that Increase or Decrease
Populations
Mortality: or death rate
Number organisms that die in time period
divided by number alive at beginning of period
Survivorship: percentage of a cohort that
survives to a certain age
Life Expectancy: probable number of
years of survival for an individual of a
given age
Emigration: members leave a population
Life Expectancy
In United States
1900—47.3 years
2004—77.6 years
Often varies with sex and race
Difference due to genetics, access to medical
care, occupations, diet and behavior
Life Span: longest period of life reached by
a given type of organism
Survivorship Curves
Late Loss: K-strategists that produce few young
and care for them until they reach reproductive
age thus reducing juvenile mortality
Constant Loss: typically intermediate reproductive
strategies with fairly constant mortality throughout
all age classes
Early Loss: r-strategists with many offspring, high
infant mortality and high survivorship once a
certain size and age
Can also have early and late loss with high survival during
reproductive years
Factors that Regulate Population
Growth
Various factors regulate population growth,
usually by affecting natality or mortality
Intrinsic: operating within individual
organisms or between organisms in the
same species
Extrinsic: imposed from outside the
population
Factors that Regulate Population
Growth
Biotic Factors:
Ability of animals to migrate or if seeds to
disperse, to similar habitats
Ability to adapt to and invade new habitats
Defense mechanisms
Resistance to adverse conditions and disease
Abiotic Factors:
Light, temperature, nutrients, etc.
Environmental Resistance
Combination of all the biotic and abiotic
factors that may limit a population’s
increase
Replacement Level: when recruitment
(young) = adults that have died
Carrying Capacity
The maximum population a habitat can
support (sustainable)
If population greatly exceeds carrying
capacity, it will undergo a J-curve crash
Population reaches max and then starts to
decline because of lack of resources
Density Dependence/Critical #
Population Density: number of individuals
per unit of area
Density Dependent: factors that change
based on population density
Population increases, environmental resistance
becomes more intense
Density Independent: factors not affected
by population
Abiotic factors
Factors that Regulate Population
Growth
Density-Dependent Factors:
Most are results of interactions between
populations of a community (predation) but some
are within a population
1)Interspecific Interactions
-predation helps prevent population overshoot
-predator/prey populations often oscillate known as
Lotka-Volterra model
-mutualism and commensalism
Factors that Regulate Population
Growth
2) Intraspecific Interactions
-population of a species can increase to
carrying capacity but then start to
compete with each other for resources
-territoriality (usually between members
of the same species)
Factors that Regulate Population
Growth
3) Stress and Crowding
-when population densities get high,
organisms often exhibit symptoms of what
is called stress shock or stress-related
diseases
-physical, psychological and/or behavioral
changes may occur if too much competition
and too close proximity to other members of
the same species
Factors that Regulate Population
Growth
Density-Independent Factors:
Tend to be abiotic components
Examples: weather or climate
Abiotic factors can be beneficial—rainfall
in deserts, fires in grasslands
These factors operate without regard to the
number of organisms involved
Density Dependence/Critical #
Populations may not recover from low
#s
Critical Number: minimum number
of individuals a species needs to stay a
healthy population
If population goes below this it will
probably become extinct.
Mechanisms of Population
Equilibrium
Predator-prey Dynamics
Herbivores and predators, Parasites, Plantherbivore dynamics and Keystone species
Competition
Intraspecific (same species) and Interspecific
(different species)
Introduced Species
Island Biogeography
Diversity in isolated habitats is a balance
between colonization and extinction rates
Smaller islands farther from mainland have
less colonization and fewer individuals of a
species
Larger islands closer to mainland tend to
have greater diversity
Conservation Genetics
Hardy-Weinberg equilibrium
In large populations, if mating is random,
no mutations occur and there is no gene inflow or selective pressure for or against
particular traits, random distribution of gene
types will occur
Different alleles will be distributed in the
offspring in the same ratio they occur in the
parents and genetic diversity is preserved
Conservation Genetics
In small, isolated populations immigration,
mortality, mutations or chance mating
events involving only a few individuals can
greatly alter the genetic makeup of the
population
Gradual changes in gene frequencies due to
random events is called genetic drift
Conservation Genetics
Founder Effect or Demographic
Bottleneck
occurs when just a few members of a
species survive a catastrophic event or
colonize new habitat geographically
isolated from other members of the same
species
Population Viability Analysis
Minimum viable population size
number of individuals needed for long-term
survival of rare and endangered species
Metapopulations
A collection of populations that have
regular or intermittent gene flow between
geographically separate units
Some species exist in several distinct
habitats and individuals occasionally move
among these, mating with existing animals
or recolonizing empty habitats