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:

_______ + ______ = ________ + _________

Birth EmigrationImmigration

Death

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 AreaProvides 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 LynxThe 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

A

Effect on Population

BCompetition - -

Predation and

Parasitism

+ -

Mutualism + +

Commensalism

+ 0