Population and Communities
Chapter 9
Studying Populations
• Definition: a population is a group of individuals of the same species,
living in a shared space at a specific point in time.
– They have a lot of interactions and share the same resources:
• Water
• Food
• Sunlight
Studying Populations
• To describe a population the area that is occupied needs to be
specified.
– Example: population of Canadian geese in North America.
Why do we study populations?
• Ecologists study populations of species to discover if the
species is thriving or in decline.
Population Characteristics
• Three main characteristics used to describe populations:
– A) Size
– B) Density
– C) distribution
A) Population Size
• Definition: population size refers to the number of individuals
in a population.
– Population size is affected by births, deaths, immigration and
emigration.
Factors affecting Population Size
• Birth: increases population size.
• Death: decreases population size.
• Immigration: animal joins the population from other region.
– Increases population size.
• Emigration: animal leaves region and joins different
population.
– Decreases population size.
Stable Population Size
• The population size will remain the same if:
Immigration
Birth
_______ + ______ = ________ + _________
Death
Emigration
Measuring Population Size
• A number of methods exist to measure the size of populations.
The most common methods are counting individuals, counting
by sample area and mark-recapture.
Counting Individuals
• If possible, count all the individuals within the area occupied
by the population.
– Count individuals within the area they occupy.
OR
– Take an aerial picture of the population.
Counting by Sample Area
Provides a good estimate of the population size.
•A) Section off sample area into quadrats (rectangular areas marked off
with rope or ribbon).
•B) The number of individuals are counted in that area and used in the
equation below:
Population size = Average number of individuals per section x Total Area
Area of section
Counting by Sample Area Example
– Example: quadrat is a rectangle (3 m x 4 m = 12 m2), total study
area (100 m x 72 m = 7200 m2), average number of individuals (40)
• Population size = 40 x 7200 m2/ 12 m2 = 24 000 individuals.
Mark and Recapture
• Method used when population moves a lot (sharks, fish etc…)
1. Catch individuals (nets or cages).
2. Count the captured animals and mark them.
3. Release the marked animals back into the population.
4. Recapture individuals (nets or cages).
5. Count the individuals with separate count for marked individuals.
6. Estimate the size of population with the formula (next slide).
Mark and Recapture
Population size = Number of marked animalsxnumber captured 2nd time
Number of marked animals recaptured
•Example: 30 ducks are captured, marked and released. If on the second capture, 10 of 40 captured
ducks are tagged (marked), then the population of ducks is estimated to be:
30 x 40 / 10 = 120 ducks.
B) Population Density
• Definition: population density refers to the number of
individuals per unit area or volume.
– Example: there are 21 white-tailed deer per square kilometer on the
island of Anticosti.
Population density =
Number of Individuals
Space (area or volume) Occupied
Population Density Example
• Area: A population of 10 000 maple
trees is contained within a 300 km2
forest. The density of the maple tree
population is 33.3 maple trees per
km2 .
• Volume: A pond contains 2500 L of
water and 17 lily pads. The density of
lily pads is 0.0068 lily pads per liter
of water.
Population Density Variations
• The density of a population can vary depending on the habitat.
– The presence of predators, parasites or disease can affect the density.
• Increased presence of predators, parasites or disease = lower density
• Decreased presence of predators, parasites or disease = higher density
– Natural or “man-made” disasters can affect the density.
• Deforestation = lower density
– The climate and resources within the habitat can affect the density.
• More resources = higher density
• Fewer resources – lower density
C) Population Distribution
• Definition: population distribution is the way in which individuals are
dispersed within their habitat.
– Three types of population distribution
• Clumped distribution
• Uniform distribution
• Random distribution
Population Distribution
• A) clumped: most common, occurs
when certain areas of the habitat
offer better living conditions
(shelter, food, water …)
– Example: fish moving in schools.
• B) uniform: due to competition for
natural resources.
– Example: certain birds are
uniformly distributed to allow each
bird a certain minimal territory.
Population Distribution
• C) random: least common
– Example: bushes and trees
are randomly dispersed
because they cannot clump
together or spread out
uniformly.
Ecological Factors
• Definition: An ecological factor is an aspect of a habitat that
can affect the organisms living there.
• Different factors of a habitat can affect the individuals of a
population  population density affected.
– 2 types of ecological factors:
• A) abiotic (non-living): factors are ecological factors of physical or
chemical origin.
• B) biotic (living): factors are ecological factors related to the actions of
living organisms.
Abiotic and Biotic Factors
• Abiotic factors: soil or
water pH, amount of
light, depth of snow,
temperature, air
humidity, terrain (land
topography).
• Biotic factors: predation,
birth rate, disease,
amount of food,
competition, human
activity.
Limiting Factor
• If the effect of the factor is dramatic (reduces the population
density or prevents its growth), the ecological factor is called
the limiting factor.
• Definition: a limiting factor is an ecological factor that causes
the density of a population to decrease.
– An ecological factor becomes limiting if:
• Absent from the habitat (no sunlight)
• Insufficient quantities (too cold – not enough heat)
• Excessive quantities (too much water causes cactus root to rot)
Limiting Factor: Temperature
Biological Cycles in Populations
• Definition: Biological Cycles are alternating periods of rise and fall in
population size. These periods are of fixed duration and are repeated
continually.
Growth
Decline
The Hare and the Lynx
The population of the hare and the
lynx rise and fall together.
• The hare population rises 
the lynx have more food 
reproduce a lot  lynx
population increases  more
hares are hunted  hare
population decreases  less
food for the lynx  lynx
population decreases  hare
population increases etc….
Studying Communities
• Definition: a community is a set of populations of different
species sharing the same habitat.
Biodiversity
• Definition: biodiversity describes the variety of species living in a
community.
We must consider the following to measure biodiversity :
1) The number of species in the community (species richness).
2) Relative abundance of each species, meaning the number of
individuals of a particular species in relation to the total number of
individuals in the community.
Biodiversity Example
Factors affecting Biodiversity
• The biodiversity of a community is high when:
– The number of species is high (NOT the number of individuals)
– The relative abundance of different species is similar (percentage of
each species is similar across species)
Interaction between Individuals in a Community
• Four main types of interaction between individuals in a
community are:
– Competition
– Predation
– Mutualism
– Commensalism
Competition
• Definition: competition is the interaction between living
organisms that seek access to the same resource in their
habitat.
– Example of resources: limited precipitation in a
desert allows for competition between plant species.
– Competition cannot exist without a limiting factor
(resource).
2 Types of Competition
• Intraspecific competition
occurs between
individuals of the same
species.
• Interspecific competition
occurs between
individuals of different
species.
Predation
• Definition: predation is the interaction between two living organisms
in which one feeds the other.
– Predator (the one eating) versus prey (the one being eaten)
Parasitism is a form of
predation: parasite versus host.
Parasite obtains food from the
host. The host is harmed in some
way.
Mutualism
• Definition: mutualism is the interaction between two living organisms
that benefit both organisms.
Nemo, the clown
fish, is sheltered
by the sea
anemone.
The sea anemone
uses the clown
fish as bait to
attract its prey.
Commensalism
• Definition: commensalism is the interaction between two
living organisms in which one organism benefits from the
relationship, while the other remains unaffected.
How Interactions Affect the Population Density
Type of Interaction Effect on Population Effect on Population
A
B
Competition
Predation and
Parasitism
Mutualism
+
-
+
+
Commensalism
+
0