LECTURE 10: POPULATION DYNAMICS
Population dynamics – the ways in which populations change
in abundance over time
Population size changes as a result of four processes: birth,
death, immigration and emigration
Patterns of Population Growth
Populations exhibit a wide range of growth patterns, including
exponential growth, logistic growth, fluctuations, and regular
These four patterns are not mutually exclusive. A single population can experience each of them
at different times
o Population increases by a constant proportion at each point in time.
o When conditions are favorable, a population can increase exponentially for a limited
time. When a species reaches a new area, exponential growth can occur if conditions are
o The population may grow exponentially until density-dependent factors regulate its
o Species such as the cattle egret colonize new regions by long-distance or jump dispersal
o Local populations then expand by short-distance dispersal events.
o These populations first increase, then fluctuate by a small amount around the carrying
o Plots of real populations rarely match the logistic curve exactly.
o “Logistic growth” is used broadly to indicate any population that increases initially, and
then levels off at the carrying capacity.
o For K (carrying capacity) to be
constant, birth rates and death
rates must be constant over time
at any given density.
o This rarely happens in nature.
Birth and death rates do vary over
time, thus we expect carrying
capacity to fluctuate
o In all populations, numbers rise and fall over time.
o Fluctuations can be deviations from a growth pattern, e.g., the Tasmanian sheep
o Or erratic - In Lake Erie phytoplankton populations, fluctuating abundance could reflect
changes in environmental factors such as nutrient supplies, temperature, or predator
Population outbreak – number of individuals increases rapidly
o An ongoing outbreak of the mountain pine beetle has killed hundreds of millions of trees
across British Columbia. This has altered forest composition, and CO 2s released as the
trees decay—17.6 megatons every year. Population cycles
o Some populations have alternating periods of high and low abundance at regular
o Populations of small rodents, such as lemmings and voles, typically reach a peak every
o Different factors may drive population cycles in rodents.
o For collared lemmings in Greenland, field studies and modeling indicated that the 4-year
cycle is driven by predators, such as the stoat
o In other studies, predator removal had no effect on population cycles. Factors driving
population cycles may vary by place/species.
o Some population cycles may stop if certain environmental factors change.
o Warmer winter temperatures affect snow conditions at high latitudes, making it more
difficult for lemmings to feed and avoid predators. Their populations have stopped
cycling every 3 to 4 years
Delayed density dependence – delays in the effect that density has on population size
o Commonly, the number of individuals born in a given time period is influenced by
population densities that were present several time periods ago
o Delayed density dependence can cause populations to fluctuate in size
o Example: A predator reproduces more
slowly than its prey.
If predator population is small,
prey population may increase,
then the predators will
increase, but with a time lag.
Many predators may reduce
the prey population, and then
the predator population will
A. J. Nicholson studied density dependence in
sheep blowflies using laboratory
In the first experiment, adults were given
unlimited food, but larvae were restricted to
50 g of liver per day
With abundant food, females laid
enormous numbers of eggs, but when the
eggs hatched, most larvae died because of
lack of food. This resulted in an adult
population size that fluctuated
In the second experiment, both adults and
larvae were given limited food.
The adult population size no longer
showed repeated fluctuations.
The risk of extinction increases greatly in
Fluctuations in growth rate, population size,
and chance events can affect this The geometric growth equation can include random
variation in the finite rate of increase (λ).
Random variation in environmental conditions can
cause λ to change from year to year (good years and
bad years for growth).
Computer simulations for three populations allowed
λ to fluctuate at random.
Two of the populations recovered from low
numbers, but one went extinct.
Fluctuations increase the risk of extinction, and the
degree of fluctuation is important
The range of variation in λ is measured by the
standard deviation (σ).
o σ = 0.2, 0.3% of populations went extinct in 70 years.
o σ = 0.4, 17% of populations went extinct.
o σ = 0.8, 53% of the populations went extinct.
When variable environmental conditions result in large fluctuations in growth rate, the risk of
Small populations are at greatest risk.
If the simulations are repeated using larger initial population sizes, there are fewer extinctions
These patterns have been observed in real populations.
In bird populations on the Channel Islands in California, 39% of populations with less than 10
breeding pairs went extinct.
No extinctions occurred in populations with over 1,000 breeding pairs (Jones and Diamond