Population
Ecology
What is a Population?
• Population - A group of individuals of the
same species that live together and
interbreed
• Populations:
o
o
o
Share resources
Are influenced by the same environmental factors
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
o
o
Number of oak tree’s per acre
Number of deer per square mile
Number of plankton per liter of water
Measuring population density
• One of the most popular method of
calculating population density is the markrecapture method
• Here’s how it is done…
Steps to the Mark-Recapture
Method
1.Boundaries are set for the study
2.Traps are set out to capture animals
3.Animals are marked with tags, dies, collars,
or bands, then released
4.Traps are set again at a later date
5.Recaptured animals are used to estimate
population density of an area
Formula
Number of Recaptures in Second Catch
Total number in Second Catch
Number 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 captured
o
10 of these rabbits were recaptures from the first
capture
Formula
Number of Recaptures in Second Catch
Total number in Second Catch
Number 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
o
o
o
Number of Nests
Number of Burrows
Tracks
Droppings
Dispersion
• Population Dispersion – The pattern of
spacing among individuals within the
geographic boundaries of the population
• Three main types of population Dispersion:
o
o
o
Clumped
Uniform
Random
Clumped
• Clumped Dispersion –
When individuals are
grouped into patches
• Why would they be
clumped?
o
o
Food Sources for animals
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
o
o
o
Birth Rate
Mortality Rate
Immigration Rates
Emigration Rates
Life Tables
• Life Tables – An age-specific summary of the
survival pattern of a population
• Life Tables Show:
o
o
o
Probability of surviving a particular age
Life expectancy
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 curves
o
o
o
I
II
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 them
o
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 habitat
How many rainbow trout could this creek
support?
o How many wolves can survive in Isle Royale
National Park?
o
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
o
Resource Competition (Food, Space, Sunlight, O2)
Toxic Levels of Waste (CO2 levels)
Density-Independent Factors
• Factors that are not effected by pop. density
• Examples:
o
o
o
Natural Disasters
Climate extremes
Pollution
Population Formulas
• Population Growth Formula:
r = reproductive rate
N = 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