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Population Ecology. Chapter 52. Ecology. Def – Study of the interactions of organisms with their physical environment & with each other Population – Group of individuals of one species living in one area who are able to interbreed and interact with each other

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Population ecology

Population Ecology

Chapter 52


Def – Study of the interactions of organisms with their physical environment & with each other

Population – Group of individuals of one species living in one area who are able to interbreed and interact with each other

Community – All organisms living in one area

Ecosystem – all the organisms in a given area & the abiotic (non-living) factors with which they interact

Abiotic (non-living) – temperature, water, sunlight, wind, rocks, & soil

Biosphere – global ecosystem


Ecosystem 1


Ecosystem 2

Pop 1

Abiotic Factors

Pop 3

Pop 2

Ecosystem 3

Population properties
Population Properties

  • 5 Properties

    1. Size– Total number of individuals in a population

    -- Typically represented by N

    2. Density – The number of individuals per unit area/volume

    -- Here we are measuring not population size but how closely

    packed they are into an area

    -- For example: which population is more dense: Manhattan’s or


    -- Sampling techniques used to estimate the number of

    organisms living in an area

    -- One sampling technique is called mark & recapture

Mark recapture method
Mark & Recapture Method

  • Sampling technique used to estimate population size

  • Organisms are captured, tagged, then released

  • Then at some future time, the process is repeated

  • Example: 1st catch: 50 whippets tagged

    2nd catch: 100 whippets captured, but only 10 tagged

Population Estimate = 500 whippets

Population properties page 2
Population Properties (Page 2)

3. Dispersion – Pattern of spacing of individuals within the area the population inhabits

a) Clumped

-- Most common pattern of dispersion

-- Pack animals

b) Uniform

-- Animals that defend their territories

-- Certain plants which secrete toxins that keep away other plants that compete for the same resources

c) Random

-- Spacing occurs in absence of any spatial factors

-- Uncommon pattern in nature

-- Tree spacing in a forest

Population properties page 3
Population Properties (Page 3)

4. Survivorship Curve (Mortality Curve)

-- Show the size & composition of a population

-- 3 basic types

a) Type I

-- Low death rates in young & middle age

-- High mortality in old age

-- Humans

b) Type II

-- Constant death rate over entire life span

-- Hydra, reptiles, & rodents

c) Type III

-- High death rate among young, but then slows

-- Fish & Invertebrates (external fertilization)

Population properties page 4
Population Properties (Page 4)

5. Age Structure Diagrams

-- Shows relative number of individuals at each age

Age structure diagrams
Age Structure Diagrams

  • Curve I – Afghanistan

    • Pyramidal shape

    • Bottom heavy or majority of population is young

    • May indicate future population explosion

    • Alternatively, may indicate population pressure

      • The majority of population will die young

  • Curve II – USA

    • Stable population

    • Experiencing little or zero population growth

    • Birth & death rates are equal = numbers in each age group is the same

Population growth
Population Growth

  • Biotic Potential

    • Maximum rate at which a population could increase under ideal conditions

    • Maximum reproductive capacity of a population under optimum environmental conditions.

  • As should be evident, this is probably never true

    • If true, it will only exist briefly

    • Used more for comparison to the actual situation

Biotic potential factors
Biotic Potential Factors

1. Age of reproductive opportunity onset

2. Reproductive Life span

3. Number of reproductive periods during lifetime

4. Number of offspring produced in each reproductive event

  • Also called Variables of Life History

    -- The variables that affect an organism’s schedule of reproduction and survival

Exponential population growth
Exponential Population Growth

  • Simplest model for population growth is exponential growth

    • Represents unrestrained growth

    • Unlimited resources

    • No predation/parasitism

    • No competition

    • No immigration/emigration

  • Example: introduction of a foreign organism into a stable ecosystem

    • Humans have experienced exponential growth for ~300 yrs.

Exponential growth is constrained
Exponential Growth is Constrained

  • Populations grow exponentially at first, but then grows logistically

    • S-shaped curve

    • Highest growth rate is at intermediate population size

  • An example of constrained optimization

  • There is an environmental limit to population size

    • Carrying Capacity – the population size that an environment will permit

    • Referred to as K

Actual Population Growth Models my growth curve”

Characteristic Overshot

Factors influencing population growth
Factors influencing Population Growth my growth curve”

  • 2 main factors

    • Density-dependent factors – factors that increase directly as the population density increases

      • K-selection (K = carrying capacity)

      • Associated with logistic growth

    • Density-independent factors – factors that are independent of the density of a population

      • r-selection (r = growth rate)

      • Associated with exponential growth

Different life history strategies
Different Life History Strategies my growth curve”

1. r-strategists

  • Reproduce rapidly

  • Small but numerous offspring

  • Little or no parenting

  • Insects

    2. K-strategists

  • Large but few offspring

  • Intensive parenting

  • Mammals (Chimps)

Regulation of population growth
Regulation of Population Growth my growth curve”

  • Density-dependent factors

    • Competition for resources

    • Territoriality

    • Predation

    • Infectious Disease

  • Density-independent factors

    • Naturally occurring disasters, such as fires, floods, earthquakes, & hurricanes (not The “U” type of hurricane, though they were a disaster last year)

  • Typical populations
    Typical Populations my growth curve”

    • Boom-and-bust cycles are common in populations over time

      • Due to complex interactions between biotic and abiotic factors

    Predator prey relationships
    Predator-Prey Relationships my growth curve”

    --Exponential Growth then crash

    -- Lynx population follows the hare pop.

    -- Cycles in hare due to food availability (grass overgrazing), disease, or predation

    -- Cycles in Lynx due to hare availability

    Human populations
    Human Populations my growth curve”

    • Until 1650, human populations were very low growth

    • Humans have experienced high growth rates since about 1650

    • Since 1970s, population growth has decreased over time

    Demographic transition
    Demographic Transition my growth curve”

    • Individual country populations vary widely in size and growth rates, but economic development leads to a demographic transition

    • From: high birth rates – high death rates = 0 growth

    • To: Low birth rates – Low death rates = 0 growth

    • Typically, death rate falls  Fast population growth

      • Due to medical care & Santiation

    • Too many people, so birth rates fall as well

    Demographic transition1
    Demographic Transition my growth curve”

    Ecological footprint
    Ecological Footprint my growth curve”

    • Proxy measure of carrying capacity

    • Total land + water area needed for all the resources 1 person consumes in a population

    • 1.7 hectares per person = sustainable usage

    • In the US, 10 hectares per person is typical