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Population Biology
Definition

A population is a
group of organisms
of the same species,
interbreeding or
closely related
through
interbreeding and
evolving as a unit.
Review

Community: a group
of interacting plants
and animals forming
an identifiable group
 Organisms of
different species
interacting together
Review

Biosphere: the
entire part of the
earth where
organisms are
found
Review

Habitat: the place
where an organism
naturally lives or
grows
Review

Niche: position or
function of an
organism in its
community – its
occupation
Population Size Calculations

Growth rate (gr) is
involve changes in
populations (N) over
a time period (t) in a
defined space.

gr=
N/ t
Ex) What is the growth rate?
Initial Population
20 gulls move in
(immigration)
0 gulls move out
(emigration)
32 chicks hatch
(natality)
10 chicks die
2 adults die
Time
200
20
0
32
-10
-2
1 year
Total
240
So gr= (240-200)/1
Annual growth rate increase of +40 birds per year
Density

Formula: D = N/A or N/V

Density equals number of organisms
divided by area (or volume or space)

Eg. 200 bison in a 100 acre pasture is a
density of 2 bison per acre

(D= 200bison/100acres)
Rate of Change

Often uses density

R=

Rate of density change equals change in
density over change in time.
D/
t
Example
In 1996 there were 10 Grizzly Bears in a
10 000 ha forest. In 2005 there are only 8.
What is the rate of density change?
R=
D/
t
R = 0.0008 – 0.001 / 2005 – 1996
R= - 0.0000222 bears/ha/y
Factors that cause populations
to change

A) Density independent factor: affect a
population regardless of size


Ex. Climate, oxygen, natural disasters, etc
b) Density dependent factors: affect a
population only if it’s a big population

Ex) food, space, parasitism
Per Capita Growth Rate
Or cgr
 The amount that a population changes
per individual over a set period of time

Cgr =
N /N
 Per capita growth rate = change in
number divided by initial population size

CGR (per capita growth rate)
A lynx population was 19 per 10 000 sq. km in
1991. In 1993 it was 3 per 10 000 sq. km.
What was the cgr of this population from
1991 to 1993?
Cgr = N/ N
= -16 / 19
= - 0.84 per lynx
Distributions of Populations

Can be clumped – more individuals
together than apart

Often involves cooperation among group
members (eg. Herd, pack)
Distribution of Populations

Can be random – not seen often in
nature

Organisms have no effect on each other
Distribution of Populations

Can be uniform: evenly distributed


Usually due to competition between
individuals
Territories, etc.
Open Populations
Are those where organism can enter or
leave
 Often have S-shaped curves (sigmoid
curve) (logistic growth curve)

Closed Populations
True closed populations are rare
 On islands, isolated communities
 Often show a J-shaped curve
(exponential growth)
 Ex. Mesocosm (aquariums
terrariums)

Population Growth Graphs

typically
have
numbers on
vertical axis
and time on
horizontal
axis
Logistic Growth
S shaped
curves are
typical of
stable
populations

Eg. Wild
Horses on
reserve land
in AB
Exponential Growth

J shaped curve (initially) occurs with
short-lived populations that rapidly
deplete their environment

Eg. Flies on a carcass
Overshoots

Result when k is
greatly exceeded
and the
environment
deteriorates
Carrying Capacity

Is the realistic
number of
organisms a habitat
can sustain over
the long term
 “k”
 Influenced by Biotic
Potential and
Environmental
Resistance
Biotic Potential

maximum number of offspring produced,
usually not achieved naturally due to
environmental factors (lack of food, space,
and mates, predation, climate, etc)
Biotic potential is determined by:
capacity of offspring to survive to reproduce
 number of times per year an organism
reproduces
 age at which offspring are reproductively
mature


Generally speaking, smaller, simpler
organisms have a higher biotic potential than
larger organisms. (r-selected organisms)
Environmental Resistance

limiting factors on a
population





Availability of resources
(food, water, space, etc.)
Competition for resources
with other organisms
Intra-specific – within a
species
Inter-specific – between
a species
Predators
Disease
Climate change
Environmental Resistance
brakes on biotic potential (B) –
maximum reproductive rate
 Puts
Environmental Resistance
Environmental Resistance
B
K
Environmental Resistance
Limiting Factors on Populations

Law of the Minimum: if any one of
many needed nutrients/limiting factors is
reduced below the required levels, the
population growth rate declines
Limiting Factors

Can be density independent – those
that will affect a population regardless of
its size
 Eg. Cold winter
Limiting Factors

Can be density dependant – those that
increase when the population size
increases

Eg. Disease
 Predation
 Food Supply
Gause’s Law

Competitive exclusion

No two species cant remain in
competition for a limited resource
(always a winner, and a loser!)
-Interspecific competition: competition
between 2 or more different species
 -Intraspecific competition: competition
between 2 or more of the same species

Other Graphs

Survivorship curves
Other Graphs

Age distribution pyramids
Population Histograms

are graphs showing the composition by
age and gender of a population at a
specific time. Population histograms have
the following characteristic shapes:
R and K Population Strategies
This is a continuum
 Most populations fall between these two
extremes

K Selection

k-selected species is one that typically
has:
Stable environmental conditions
 Slow growing and aging individuals
 Low reproduction rate (B)
 Low number of offspring
 Long wait period before breeding
 Parental care of offspring
 Tend to be Big organisms
 Ex. Bears, humans, elephants

R- Selection

An R-selected species is one that
typically has:
Unpredictable environment
 Small individuals with short life spans
 Reproduce at a high rate
 Produce a lot of offspring
 Short wait period between breeding
 Little or no parental care
 Ex. Fish, rabbits, frogs

Life History Patterns
Some organisms undergo regular
patterns of growth and decline known as
population cycles
 Small rodents, rabbits, lemmings often
cycle every 1 – 4 years

Population Cycles

Can be due to
fluctuations in food
supply, predation, or
both
Predator - Prey Cycles
Chaos Theory
Used by population biologists to study
and predict the general trends in
populations
 This theory suggest that small
uncertainties in short-term prediction of
individual events may be magnified to
such an extent that complex systems
become quite unpredictable (‘Butterfly
Effect’)
 Chaos is normal!

Chaos
The ‘butterfly effect”
 This is the sensitivity of a system to the
initial conditions
 Change any starting parameter slightly
and the resulting changes magnify until
the result is very different from the initial
prediction

Technologies

Include:

Radio collars
Technologies

Sampling methods

Quadrats –
counting organisms
in defined areas

Transects –
counting organisms
that touch line
Technologies

Mark/recapture
studies
Symbiotic Relationships

Symbiosis – a relationship between two
individuals of different species.

Parasitism – one species lives in or on
another where it obtains food and
resources. The host is usually harmed
by the relationship. Ex. Tapeworm

Many parasites are r-selected

Eg. Tapeworm

Commensalism – one species lives on
or near another, but while one species
benefits, the other is unaffected.
Mutualism – two species that live in
close association with one another,
where both benefit from the relationship.
Ex. honeybee and flower
Ex. shark and remora

Ecological Succession

Ecological succession – is the gradual
and orderly change of a community as it
is either developed from bare land or
replaced by another community.
Succession
Primary Succession – the gradual
colonization of an area that has not
supported an ecosystem before. (from
bare rock)
PrimarySuccession

Secondary Succession – the
colonization of an area that once
supported an ecosystem that was
destroyed by fire, flood, etc.
Exploring Time Gallery Display

Pioneer Community – is the first
species to appear during succession.
 Climax Community – is the final stable
community that results at the end of
succession.

Generalizations about
Succession:




Species composition changes more quickly
at earlier stages.
Total number of species increases
dramatically at early stages, levels off at
intermediate phases and declines at the
climax stage.
Food webs develop in complexity as
succession progresses.
Total biomass increases during succession
and levels off at the climax stage.