Download Populations in Ecosystems

Populations in Ecosystems
• Def’n: A population is the sum of the members of a
particular species that lives in a particular
ecosystem. (so what’s an ecosystem?)
• An ecosystem is made of communities of living
animals and plants that coexist, influence and
depend on each other within their environment.
The ecosystem includes living (biotic) and non-living
(abiotic) parts.
• To study ecosystems, we need to study the
individual populations that exist within them.
• We look at population size, density and distribution.
Population Size
• By monitoring the size of a population (how many
individuals are in it), we can determine if it is
endangered or is endangering other species. (For
example, moose were introduced to Newfoundland a hundred
years ago and are now over-running the province, destroying the
vegetation and endangering motorists. A cull has just been
announced.)
Populations are influenced by;
•
•
•
•
Births
Deaths
Immigration – individuals entering the area
Emigration – individuals leaving the area
• A balance of these four factors determines
whether a population increases, decreases or
stays the same.
To measure population size (three ways)
1 Count every individual (census). This is tough to
do in a big population or area.
…but you can do it with
elephants (aerial count).
2 Sample. Count within a sample area (quadrat), then
extrapolate (good for plants or slow animals).
For example, count the flowers in 1 m2 of a field, then multiply
by the area of the field.
Ex. 2 – turtles are counted in five quadrats of 1 km2 each in a wildlife
preserve that measures 100 km by 300 km. Give an estimate to the
population size.
quadrat 1 – 37 turtles
quadrat 2 – 59 turtles
quadrat 3 – 1 turtles
quadrat 4 – 233 turtles
quadrat 5 – 76 turtles
turtles counted
sample area
=
pop size
total area
what’s the population size?
solution
406 turtles = population size
5 km2
30 000 km2
Population size = 406 * 30000 = 2 436 000 turtles
5
Mark and recapture
Good for fast moving animals
with a large range.
The idea here is to tag a certain
number of individuals, then
release them. The proportion of
tagged: untagged animals
observed can give a population
estimate.
Example – 1000 robins from a
population are tagged and
released. Three months later, 500
individuals are captured, of which
135 had been tagged. Estimate
the population size. (Hint: X-multiply)
solution
# tagged birds in capture =
Population size
1000
Pop size
=
# tagged in recapture
# recaptured
135
500
Pop size = 500*1000 = 3703 robins
135
Population Density is..
The number of individuals per unit area (or volume)
of habitat.
(what’s habitat? – the specific environment that
a population lives in).
It is simply determined by dividing the number of
individuals by the area (or volume) of habitat.
Example: 300 000 sunfish live in 120 000 000 m3 of
water in a pond. What’s the population density?
solution
• Sunfish population density =
=
300 000 fish
120 000 000 m3
.0025 sunfish/m3 of water
The population density can give useful information about
the health of a population.
Population Distribution
• Populations of animals or plants can be
uniformly distributed (spread out evenly),
grouped in clumps (ex., flocks of birds, schools
of fish), or randomly distributed (many plants)
Ecological Factors
• Within an ecosystem, each species has a habitat,
the environment in which it lives. The habitat
includes living (biotic) and non-living (abiotic)
factors. Examples of each are below;
• Biotic factors – predators,
prey, competitors for space
and food,
‘cooperative’ species, etc.
• Abiotic factors – sunlight, rain, soil quality, pH, etc.
Limiting Factors
• These are factors that, if there is not
enough, the population density will
decrease.
ex., the density of plants at the floor of
a tropical forest is low because
sunlight, a limiting factor, is very low
there.
ex. 2, Many plants need to be
pollinated by bees to reproduce. This
means that bees are a limiting factor.
Almond trees are threatened by the
collapse of the bee population due to
environmental conditions
Biological Cycles
• The population size
of a species can
fluctuate over time.
This is common in
seasonal species
like mosquitoes or
in predator-prey
relationships like
wolves and deers,
or lynx and
snowshoe hares.
• When lynx are few, the snowshoe hare increase
in number. This causes lynx to become more
numerous. The lynx then reduce the hare
population by predation until the lynx population
runs out of food and crashes, thus starting the
cycle again.