Population Ecology

What is a Population?• Population - A group of individuals of the

same species that live together and interbreed

• Populations:o Share resourceso Are influenced by the same environmental factorso Breed with one another

Density and Distribution• Two important characteristics of a

population are the density, and spacing of individuals

Population Density• Population Density – Number of

individuals per unit or area of volume• Examples:

o Number of oak tree’s per acreo Number of deer per square mileo Number of plankton per liter of water

Measuring population density• One of the most popular method of

calculating population density is the mark-recapture method

• Here’s how it is done…

Steps to the Mark-Recapture Method1.Boundaries are set for the study2.Traps are set out to capture animals3.Animals are marked with tags, dies, collars,

or bands, then released4.Traps are set again at a later date5.Recaptured animals are used to estimate

population density of an area

FormulaNumber of Recaptures in Second Catch

Total number in Second CatchNumber Marked in First Catch

Total Population (N)

Number Marked in First Catch X Total number in second catch

Number of Recaptures in Second Catch

Example• Scientists capture 50 rabbits, and put ear

tags on them

• Two weeks later, 100 rabbits are capturedo 10 of these rabbits were recaptures from the first

capture

FormulaNumber of Recaptures in Second Catch

Total number in Second CatchNumber Marked in First Catch

Total Population (N)

The estimated population would

be 500 rabbits

Number Marked in First Catch X Total number in second catch

Number of Recaptures in Second Catch

Other Methods• Scientists will sometimes use indexes for

population size, by counting things such as:o Number of Nestso Number of Burrowso Trackso Droppings

Dispersion• Population Dispersion – The pattern of

spacing among individuals within the geographic boundaries of the population

• Three main types of population Dispersion:o Clumpedo Uniformo Random

Clumped• Clumped Dispersion –

When individuals are grouped into patches

• Why would they be clumped?o Food Sources for animalso Soil Conditions for plants

• This is the most common type of dispersion

Uniform• Uniform Dispersion – When individuals

are evenly spaced

Random• Random Dispersion – Unpredictable

distribution of individuals• The most rare of the three types of dispersion

Demography• Demography – The study of the vital

statistics that affect population size• Some Demographic Variables:

o Birth Rateo Mortality Rateo Immigration Rateso Emigration Rates

Life Tables• Life Tables – An age-specific summary of the

survival pattern of a population• Life Tables Show:

o Probability of surviving a particular ageo Life expectancyo Fertility at each age

Uses of Life Tables• Originally created for life insurance agencies• Biologists use them to study population

dynamics• See if mortality rates increase due to

environmental factors• See at what age a species is most fertile

Survivorship Curves• Survivorship Curves – A plot of the

proportion or numbers in a cohort still alive at each age

• There are 3 types of survivorship curveso Io IIo III

Type I• Little mortality at early ages• Mortality rate increases as the organisms

get older• An example of this is with humans• We have few offspring, but give them great

care

Type III• This curve shows high mortality rates during

the early stages of life• The line flattens out once the organisms

reach a ‘critical age’ of survival• This is for animals that produces many

offspring, but little to no care for themo Example – plants, clams, fish

Type II• This is a constant death rate• This falls in-between I and III type curves• Examples here are squirrels, lizards

Population Growth• Biotic Potential – the maximum growth rate

of a population under ideal conditions, and unlimited resources (no restrictions)

• Factors affecting biotic potential:

• Age of Reproductive Maturity

• Clutch Size• Frequency of

Reproduction

• Reproductive Lifetime• Survivorship of offspring

to reproductive maturity

Carrying Capacity• The maximum number of individuals of a

population that can be sustained in a particular habitato How many rainbow trout could this creek

support?o How many wolves can survive in Isle Royale

National Park?

Limiting Factors• Limiting factors are things that prevent

populations from reaching their biotic potential (2 types)

• Density-Dependent Factors• Density-Independent Factors

Density-Dependent Factors• Factors whose effects become more intense as

the pop. density becomes greater• Examples:

o Resource Competition (Food, Space, Sunlight, O2)o Toxic Levels of Waste (CO2 levels)

Density-Independent Factors• Factors that are not effected by pop. density • Examples:

o Natural Disasterso Climate extremeso Pollution

Population Formulas• Population Growth Formula:

r = reproductive rateN = population size

Growth Patterns• There are 2 types of pop. Growth patterns:• Exponential – Occurs when growth rate is

above zero• Logistical – When limiting factors restrict

size of population