Population Dynamics
Populations are dynamic
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Size
Density
Age distribution
Dispersion (spatial pattern)
These changes (population dynamics) occur
in response to environmental stress
Dispersion patterns within populations
Why clumping?
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Resources vary from place to place
Moving in groups allow better chance finding resources
Living in groups protects some animals from predators
Hunting in packs give some predators an advantage of finding
and capturing prey
• Some species form temporary groups for mating and caring
for young
Raft of sea otters
Four variables which govern
population size
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Births
Immigration
Death
Emigration
Population Change = (Births + Immigration) – (Deaths + Emigration)
Age Structure
• How fast a population grows or declines depends on its
age structure.
– Prereproductive age: not mature enough to
reproduce.
– Reproductive age: those capable of reproduction.
– Postreproductive age: those too old to reproduce.
Baby hippos stay with their
mother for 4 years and
reach their reproductive
age at about 5 – 6 years
What would you say about this population of fish if they are capable of
reproducing between the ages of 10 and 20 years?
What would you say about this population if they did not reach reproductive age
until the age of 26 years?
Factors affecting population size
• Intrinsic rate of increase
• Environmental Resistance
• Carrying Capacity
• Minimum Viable Population
Limits on population growth
• No population can increase its size
indefinitely.
– The intrinsic rate of increase (r) is the rate at
which a population would grow if it had unlimited
resources.
– Carrying capacity (K): the maximum population of
a given species that a particular habitat can
sustain indefinitely without degrading the habitat.
Factors affecting carrying capacity
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Competition for resources
Immigration and emigration of other species
Natural and human-caused catastrophes
Seasonal fluctuations
Lake Lanier, GA in Spring 2008
What happens when a population
exceeds its carrying capacity
• Dieback (crash)
• Easter Island
• Cause damage – reduce
area’s carrying capacity
• Overgrazing by cattle
– Reduces grass cover in some
areas
– Allows nonnative species to
inhabit the area (opportunists)
Sagebrush invaded after overgrazing
occurred by cattle (the cattle do not eat
the sagebrush)
Minimum viable population
The smallest possible size at which a population can exist
without extinction from natural disasters, genetic changes,
human influence, or environmental factors
If a population falls below minimum viable population
(MVP) needed to support a breeding population:
• Individuals may not be
able to locate mates
• Genetically related
individuals may
interbreed
• Genetic diversity may
be too low
California condor
The intrinsic rate of increase falls and extinction is likely
Genetic diversity can affect the size of
a population
• Founder effect
• When only a few individuals in a population colonize a new
habitat that is geographically isolated
• Demographic bottleneck
• When only a few individuals in a population survive a
catastrophe (i.e. fire or hurricane) and this lack of genetic
diversity may limit ability to rebuild population
• Genetic drift
• Random changes in gene frequencies where some
individuals breed more than others and their genes may
eventually dominate the gene pool
• Inbreeding
• Individuals in a small population mate with one another
Under some circumstances population
density affects population size
• Density-independent population controls
• Their effect is not dependent on the density of the pop.
• Floods, hurricanes, severe drought, seasonable
weather, fire, habitat destruction, and pesticide
spraying
• Density-dependent population controls
• Competition for resources, predation, parasitism, and
disease
Density-dependent population controls
• Higher density may help find mates, but can lead
to increased competition for mates, food, living
space, water, sunlight, and other resources
Density-dependent population controls
• Can help shield members from predators, but it can also make
large groups such as schools of fish vulnerable to human
harvesting methods
Density-dependent population controls
• Close contact among individuals in a dense population can
increase the transmission of parasites and infectious disease
April 2010: Connecticut deer population is 64 deer/ square mile – the deer population’s
density is contributing to the spread of Lyme disease to humans (they would like to get
the deer population down to 12 deer/ square mile
Several different types of population
change occur in nature
• Population sizes may stay the same, increase,
decrease, vary in regular cycles, or change erratically.
– Stable: fluctuates slightly above and below
carrying capacity.
– Irruptive: populations explode and then crash to a
more stable level.
– Cyclic: populations fluctuate and regular cyclic or
boom-and-bust cycles.
– Irregular: erratic changes possibly due to chaos or
drastic change.
Cyclic fluctuations
(boom-and-bust cycles)
Top-down population control
• Predators (high on trophic pyramid) regulate
prey population size
• Example of top-down effects
– Increase in number of predators
– Reduces size of herbivore population
– Increases size of plant population
– Reduces nutrients in soil
Bottom-up population control
• Each trophic level derives its energy from the level
below it
• Productivity of the species on the lower layer sets
the bounds on the population sizes of the species on
the higher layer
• Example of bottom-up effects
– Increase nutrients in soil
– More nutrients increases size of plant population
– More plants increases herbivore population
– More herbivores increases size of predator
population
Lynx-Hare Population Cycle
Is it top-down or bottom-up?
How does productivity influence top-down
or bottom-up population control?
• With high primary productivity
– Higher primary productivity results in herbivore
populations large enough to support prey
populations
– Predators suppress the herbivore populations =
top-down regulation
• With low primary productivity
– When primary productivity is low, there is not
enough production of biomass to sustain large
herbivore populations = bottom-up regulation
Reproductive Patterns
Survivorship Curves
K-selected species
r-selected species