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Lecture 10

Biology 2483A Lecture Notes - Lecture 10: Metapopulation, Eutrophication

Course Code
BIOL 2483A
Lisa Henry

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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
: population size at time t
B: number of births
D: number of deaths
I: number of immigrants
E: number of emigrants
Patterns of Population Growth
Populations exhibit a wide range of growth patterns, including exponential
growth, logistic growth, fluctuations, and regular cycles
These four patterns are not mutually exclusive  a single population can
experience each of them at different times
Exponential Growth
Population increases by a constant proportion at each point in time
When conditions are favorable, a population can increase exponential for a
limited time  when a species reaches a new area, exponential growth can occur if
conditions are favorable
The population may grow exponentially until density-dependent factors regulate
its numbers
Species such as the cattle egret colonize new regions by long-distance or jump
dispersal events
Local populations then expand by short-distance dispersal events
- Ex. Immigration of birds in South America from Africa is jump dispersal
Logistic Growth
These populations first increase, then fluctuate by a small amount around the
carrying capacity
Plots of real populations rarely match the logistic curve exactly
“Logistic growth” is used broadly to indicate any population that increases
initially, then levels off at the carrying capacity
Ex. Sheep in Tasmania

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For K (carrying capacity) to be constant, birth rates and death rates must be
constant over time at any given density
- This rarely happens in nature, since birth and death rates do vary over time 
so we except the carrying capacity to fluctuate
Population Fluctuations
In all populations, numbers rise and fall over time
Fluctuations can be deviations from a growth pattern (ex.
Tasmanian sheep population)
Or, they can be erratic
- Ex. In Lake Erie phytoplankton populations, fluctuating abundance could
reflect changes in environmental factors such as nutrient supplies,
temperature, or predator abundance
Population outbreak: number of individuals increases rapidly
Ex. Consequences of an insect outbreak
- An ongoing outbreak of the mountain pine beetle has killed hundreds of
millions of trees across BC
- This has altered forest composition, and CO2 is released as the trees away
- Approx. 17.6 megatons every year
Population Cycles
Some populations have alternating periods of high and low abundance at regular
Populations of small rodents, such as lemmings and voles, typically reach a peak
every 3-5 years
Different factors may drive population cycles in rodents  factors driving
population cycles may vary by place/species
- For collard lemmings in Greenland, field studies and modeling indicated that
the 4-year cycle is driven by predators, such as the stoat
In other studies, predator removal had no effect on population cycles
Some population cycles may stop if certain environmental factors change
- 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
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