CHAPTER 1 / SECTION 2CHAPTER 1 / SECTION 2

•STUDYING STUDYING POPULATIONSPOPULATIONS

Organisms do not generally Organisms do not generally live alone. A live alone. A populationpopulation is a is a group of organisms from the group of organisms from the same species occupying in same species occupying in the same geographical area. the same geographical area.

This area may be difficult to This area may be difficult to define because:define because:

A population may A population may comprise widely dispersed comprise widely dispersed individuals which come individuals which come together only together only infrequently, e.g. for infrequently, e.g. for mating.mating.

Populations may fluctuate Populations may fluctuate considerably over time.considerably over time.

PopulationsPopulations

Migrating wildebeest populationMigrating wildebeest population

Tiger populations comprise Tiger populations comprise widely separated individualswidely separated individuals

Populations are dynamic and Populations are dynamic and exhibit attributes that are not exhibit attributes that are not shown by the individuals shown by the individuals themselves.themselves.

These attributes can be These attributes can be measured or calculated and measured or calculated and include:include:

Population sizePopulation size: the : the total number of organisms total number of organisms in the population.in the population.

Population densityPopulation density: the : the number of organisms per number of organisms per unit area.unit area.

Population distributionPopulation distribution: : the location of individuals the location of individuals within a specific areawithin a specific area..

Features of PopulationsFeatures of Populations

The study of changes in the size and The study of changes in the size and composition of populations, and the composition of populations, and the factors influencing these changes, is factors influencing these changes, is population dynamicspopulation dynamics..

Key factors for study include:Key factors for study include:

Population growth ratePopulation growth rate: the : the change in the total population change in the total population size per unit time.size per unit time.

Natality Natality (birth rate): the (birth rate): the numbernumberof individuals born per unit time.of individuals born per unit time.

MortalityMortality (death rate): the (death rate): the number of individuals dying per number of individuals dying per unit time.unit time.

MigrationMigration: the number moving : the number moving into or out of the population. into or out of the population.

Population DynamicsPopulation Dynamics

Population size is influenced by births…Population size is influenced by births…

……and deathsand deaths

MigrationMigration

MigrationMigration is the movement is the movement of organisms into of organisms into ((immigrationimmigration) and out of ) and out of ((emigrationemigration) a population. ) a population. It affects population It affects population attributes such as age and attributes such as age and sex structure, as well as the sex structure, as well as the dynamics of a population.dynamics of a population.

Populations Populations loselose individuals through individuals through deaths and emigration.deaths and emigration.

Populations Populations gaingain individuals through births individuals through births and immigration.and immigration.

Migrating species may group together to form large mobile

populations

WildebeestWildebeest

Canada geeseCanada geese

The number of individuals The number of individuals per unit area) is termed per unit area) is termed the the population densitypopulation density..

AtAt low population low population densities, densities, individuals individuals are spaced well apart. are spaced well apart. Examples: territorial, Examples: territorial, solitary mammalian solitary mammalian species such as species such as tigerstigers

AtAt high population high population densities, densities, individuals individuals are crowded together. are crowded together. Examples: colonial Examples: colonial animals, such as animals, such as rabbits, rabbits, coralscorals, and , and termites.termites.

Population DensityPopulation Density

High density populations

Low density populations

Environmental FactorsEnvironmental FactorsEnvironmental factorsEnvironmental factors may be categorized may be categorized according to how much according to how much population density population density influences their effect on influences their effect on population growth:population growth:

Density Density independent independent factorsfactors have a have a controlling effect on controlling effect on population size and population size and growth, regardless of growth, regardless of the population the population density. density.

Density dependent Density dependent factorsfactors have an have an increasing effect on increasing effect on population growth as population growth as the density of the the density of the population increases.population increases.

Severe fires can result in high mortality

Humans often live at high density

CompetitionCompetitionEnvironmental resources are finite. Competition within species Environmental resources are finite. Competition within species for resources increases as the population grows. At for resources increases as the population grows. At carrying carrying capacitycapacity, the area can support its largest population., the area can support its largest population.

When the demand for a resource (e.g. water, food, space, When the demand for a resource (e.g. water, food, space, light) exceeds supply, that resource becomes a light) exceeds supply, that resource becomes a limiting limiting factorfactor..

Animals compete for resources such as water (left) or mates (right),

especially when these are in short supply or access to them is restricted.

Physical (or abiotic) Physical (or abiotic) factorsfactors

temperaturetemperatureprecipitationprecipitationAcidityAcidity

Catastrophic Catastrophic eventsevents

floods and floods and tsunamistsunamisfirefiredroughtdroughtearthquake and earthquake and eruptioneruption

Limiting FactorsLimiting Factors

Population Population depends on the depends on the number of individuals added to number of individuals added to the population from births and the population from births and immigration, minus the number immigration, minus the number lost through deaths and lost through deaths and emigration.emigration.

This can be expressed as a This can be expressed as a formula:formula:

Population GrowthPopulation Growth

Population growth =Births – Deaths + Immigration – Emigration

(B) (D) (I) (E)

Calculating Population ChangeCalculating Population Change

Births, deaths, and net migrations determine the numbers of individuals in a population

Emigration (E)

Births (B) Immigratio

n (I)

Deaths (D)

Determining population sizeDetermining population size

• Direct observation - counting all the members within a Direct observation - counting all the members within a

populationpopulation

• Indirect observation – observing signs of organisms ex. Indirect observation – observing signs of organisms ex.

NestsNests

• Sampling – estimating the population size, counting the Sampling – estimating the population size, counting the

number of organisms in a small area and multiply to find number of organisms in a small area and multiply to find

the number in a larger area the number in a larger area

• Mark and Recapture – capturing animals and marking Mark and Recapture – capturing animals and marking

them with a radio collar, or tag and releasing them to be them with a radio collar, or tag and releasing them to be

recaptured laterrecaptured later