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Populations
Population– all the individuals of a
particular species that occupy a certain
geographical area
Stable and Sustainable Population – a
population of a species that meets the needs of
present and future generations. Over time the
population survives and functions, despite
constant changes.
Example#1 -trees in a forest
(continuous cycling:
thrive/burn/regrow)
Example#2 – Rabbit and fox
populations
Factors that increase a population
Natality - births
Immigration –
movement into the area
Factors that decrease a population
Mortality- deaths
Emigration – movement
out of the area
population growth = (births + immigration) – (deaths + emigration)
Limits on Population
A. Biotic Potential (...limits population growth) – the maximum
number of offspring that a species could produce if resources were
unlimited.
But resources are limited, so biotic
potential is limited by 4 factors
1. Birth potential – number of
offspring per year
Ex. Whooping cranes lay 2 eggs per
year...only one may survive
2. capacity for survival – the number
of offspring that reach reproductive age
ex. female sea turtles lay many
eggs…only a few reach the sea…fewer
still reach maturity (so many chances to
be something else’s prey)
3. procreation – the number of times
that a species reproduces each year
ex. elk mate only once per year, during
the fall
4. length of reproductive cycle – the
age of sexual maturity and the # of
years the individual can reproduce
ex. African elephants reach sexual maturity at
about 15 years, but may reproduce until they are
90. (the young can’t make babies, but once
mature, they can reproduce for many years to
ensure their genes are passed on)
...also limits population growth
Factors that increase populations:
Abiotic
 favourable light
 favourable temp.
 favourable chem. environment
Biotic
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Factors that decrease populations:
 too much or too little light
 too cold or too warm
 unfavourable chem. environment
sufficient food
 insufficient food
low number or ineffective predators  high number of highly effective
few or weak parasites or diseases
predators
ability to compete for resources
 many or strong parasites or
diseases
 inability to successfully compete for
resources
Examples
Ferns
-produce greater than 50,000 spores in a year
-fern population is limited by moisture (abiotic) and
grazing animals (biotic)
-without limiting factors: in two generations would
cover N.America!!
People
-produce only 1 offspring per year (but all
or most survive)
-population is limited by food resources
and disease (biotic) and available water,
etc (abiotic) (but technology has greatly
removed these limits)
-no natural predators (biotic)!
Pet cat, dandelion, rabbit, mosquito, spruce trees...
Density-Dependent Factors
Density-Independent Factors
(factors affecting populations that depend
on the density of the pop.)
(factors affecting populations that occur
regardless of the size of the population)
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food shortage
competition for mates, breeding areas
(habitat)
disease caused by micro-organisms or
parasites
introduction of an exotic species
increased predation
competition for water and other resources
ex. dwarf-mistletoe (in pines) spreads more quickly in Belair
Forest than in the city
ex. illness spread more easily in highly populated areas than
in the country
ex. colds spread more easily in school than during summer
holidays
ex. A disease that damages the bark of willow trees would
affect an already crowded population of deer in the bioreserve
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flood
fire
spraying with pesticides
change in climate or temperature
destruction of habitat
drought
ex. waste water dumped into a lake by industry changes the temp.
and chem. composition of the lake and kills the fish (no matter how
dense the fish pop. was to begin with)
ex. floods will wipe out a farmer’s crops (whether the crop was a
good one to begin with or not)
ex. insecticide will kill all the insects present…whether there were few
or many to begin with
ex. A very cold winter in the bioreserve will affect the whole deer
population, no matter how many there are that year
Stress
Limiting factors do not function independently of each other!! For
example, if a bird population is already stressed because of human
pollution and habitat removal, insecticide spraying (for their food
source) could cause further stress on the population, and possibly
wipe it out completely. (domino effect)
1. resources and energy (sun, water, and other essential materials)
2. food chains (populations are limited by their food supply and
predators)
3. competition – the demand for resources (food, water, mates and
space) results in competition among individuals (same and different
species)
ex. field mouse
low # of predators…# of mice grows
as available food drops, mice become diseased and easy prey
mouse pop. declines to re-establish equilibrium