Population ecology
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Population Ecology. Study of the factors that affect the population dynamics of any species! Current Population Clock. Overview of Chapter 8. Principles of Population Ecology Reproductive Strategies The Human Population Demographics of Countries Demographics of United States.

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Population Ecology

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Population ecology

Population Ecology

Study of the factors that affect the population dynamics of any species!

Current Population Clock

Overview of chapter 8

Overview of Chapter 8

  • Principles of Population Ecology

  • Reproductive Strategies

  • The Human Population

  • Demographics of Countries

  • Demographics of United States

Principles of population ecology

Principles of Population Ecology

  • Population Ecology

    • Study of populations and why their numbers change over time

    • Important for

      • Endangered species

      • Invasive species

  • Population

    • Group of individuals of same species living in the same geographic area at the same time

Population dynamics

Population Dynamics:

  • Several factors are included:

  • Population density

  • Birth rate

  • Death rate

  • Growth rate

  • Age structure

  • Resources (quantity, type, quality)

How a population responds to the environment depends upon several interactions

How a Population Responds to the Environment Depends Upon Several Interactions:

  • How individuals compete for food

  • How disease, predation, and other environmental pressures affect population

  • Reproductive success (or failure)

  • Management of populations for ecosystem health and/or human needs

  • forestry management – tree populations,

  • agronomy – pest and crop populations

  • wildlife management – animal and fish populations

Population density

Population Density

  • the number of individuals of a species per unit of area or volume at a given time

  • Population density can change dramatically in different habitats, or in the same habitat over a short distance

  • Population density can indicate a potential pollution source in a river or stream

Population density1

Population Density

  • Population density

    • The number of individuals of a species per unit area or volume at a given time

    • Ex: minnows per Liter of pond water

  • Ovals below have same population, and different densities

Birth rate

Birth Rate

  • the rate at which individuals produce offspring

  • Birth rate in humans: (b) = the number of births per 1000 people per year (human terms)

Death rate

Death Rate

  • = the rate at which organisms dieDeath rate in humans: (d) = the number of deaths per 1,000 people per year

Growth rate

Growth Rate

  • the rate of change (increase or decrease) in a population;

  • Formula: Growth rate (r) = b-d Growth rate in humans is also called the “natural increase” in the population

Example growth rate problem

Example Growth Rate Problem:

  • Population of 40,000 people; 800 births per year; 400 deaths per year

    • b = 800/40,000 = 0.02

    • d = 400/40,000 = 0.01

    • r = 0.02 – 0.01 = 0.01 or 1.0% per year population growth rate

  • r is positive if people are born faster than they die; r is negative is people die faster than they are born



  • the movement of individuals from one location to another, which affects the population at either location

  • Immigration (i) – individuals entering a population

  • Emigration (e) – individuals leaving a population

Global populations vs local populations

Global Populations vs. Local Populations…

  • Global populations depend purely on birth and death rates

  • Local populations depend on birth and death rates plus immigration and emigration rates

Example growth rate problem 2

Example Growth Rate Problem 2:

  • Population of 250,000; 1400 births per year; 900 deaths per year; 100 immigrants and 200 emigrants yearly

  • b = 1,400/250,000 = 0.0056

  • d = 900/250,000 = 0.0036

  • i = 100/250,000 = 0.0004

  • e = 200/250,000 = 0.0008

  • r = b – d + i –e = 0.0056 – 0.0036 + 0.0004 – 0.0008 = 0.0016

  • **Note: This is equivalent to 0.16% growth

Birth rate death rate calculations points to consider

Birth Rate & Death Rate Calculations…Points to Consider

  • NOTE: Sometimes birth and death rates are simply given as percentages, which indicates how many people were born (or died) out of 100. Either way of reporting (rate or number per 1,000 living) is acceptable.

  • Example: Birth rate of 1.1% means 1.1 people were born for every 100 people living in area under consideration.

  • Typically reported as 11 births per 1,000 people living.

Example growth rate problem 3 note not on your packet

Example Growth Rate Problem 3: Note: NOT on your packet…

  • Birth rate = 1.1% (NOTE: this would be 1.1 per 100; or 11 per 1,000)

  • Death rate = 0.8%

  • Immigration = 0.08%

  • Emigration = 0.04%

  • Growth rate = 1.1 – 0.8 + .08 - .04

    = 0.34%

    This means that 0.34 people are added to the population for every 100 living; or 3.4 people per 1,000 living.

Calculating future populations

Calculating Future Populations

  • Extending population growth into the future utilizes a physical constant e = 2.7183

  • Formula:

    Population (final) = Population (initial) x e (r * t)

    Pf = Pi x e (r * t)

    Where r is the growth rate IN PERCENT expressed in DECIMAL form, and t is the time (number of years)

    NOTE: On a calculator, you plug in the r x t first, then raise 2.7183 to that number, then multiply result by the initial population!

Calculating future populations1

Calculating Future Populations


    • 2002 Pakistan population = 143.5 million

    • Pakistan population growth rate is 2.1%

    • 2010 population:

      • P = 143.5 million x 2.7183 (.021 x 8 yrs)

      • P = 143.5 million x 2.7183 (0.168)

      • P = 143.5 million x 1.182937939

      • P = 169.8 million

Estimating animal populations

Estimating Animal Populations:

  • Most of the time, determining animal populations in an ecosystem setting is difficult unless the animal is sessile (immobile) and easy to count.

  • Tag & Recapture is often the preferred method used to estimate animal populations

    • Works by catching an initial sample, “tagging” with some identification mark or tag, releasing them, and recapturing them later.

    • A simple ratio is used to determine final population.

Biotic potential

Biotic Potential:

  • the maximum rate at which a population can increase

  • Life History Characteristics that Influence biotic potential:

    • Age at which reproduction begins

    • Amount of life that reproduction is possible

    • Number of reproductive periods per lifetime

    • Number of offspring per reproductive event

  • Larger organisms generally have smallest biotic potential, while microorganisms have the highest

Population ecology

Life history traits –characteristics of an individual that influence survival and reproduction

Age at maturity

Atlantic Salmon

African elephant

House Mouse

3-6 years

2 months

11 - 20 years

Population ecology

Life history traits –characteristics of an individual that influence survival and reproduction

Number of offspring produced

Atlantic Salmon

African elephant

House Mouse

1 calf every 3-8 years

5-8 young every month

1,500 to 8,000 eggs once

Population ecology

Life history traits –characteristics of an individual that influence survival and reproduction

Number of reproductive events

Atlantic Salmon

African elephant

House Mouse



~3 - 10

Population ecology

Life history traits –characteristics of an individual that influence survival and reproduction


Atlantic Salmon

African elephant

House Mouse

60 - 70 years

3-6 years

~2 years

Exponential population growth

Exponential Population Growth:

  • Populations with a constant reproductive rate will have an accelerating population growth under optimal conditions

  • doubling of population occurs in successively shorter intervals

Numbers of bacteria during 10 hour period growing exponentially

Numbers of Bacteria During 10 hour Period (growing exponentially!)

Environmental resistance

Environmental Resistance:

  • limits to exponential population growth occur when population reaches a size that allows environmental limits to take effect

  • Environmental limits include:

    • Space

    • Food

    • Exposure to toxins, etc.

    • Increase in population of predators

Carrying capacity

Carrying Capacity:

  • Carrying Capacity (K) = the largest population that can be maintained for an indefinite period of time in a particular environment

  • Carrying capacity changes with changes in the environment, either natural or artificial

    • Droughts, pollution, excess rainfall, etc.

Carrying capacity1

Carrying Capacity

S shaped population curves

S-Shaped Population Curves…

  • Graphs of populations influenced by environmental limitations show a characteristic S shape curve

  • Shows an initial exponential growth, followed by slowed growth and then a flattening of the curve as environmental limits are reached

S shaped growth curve paramecia

S-Shaped Growth Curve (paramecia):

Example 1 bacterial population crash due to toxic waste accumulation

Example 1: bacterial population crash due to toxic waste accumulation

Reproductive strategies

Reproductive Strategies:

  • Nature forces organisms to make tradeoffs in the expenditure of energy

  • Only some energy can be used for reproduction, since each organism must uptake nutrients to grow, hunt for food, etc.

  • “r-selected” and “k-selected” species have developed over time in response to energy requirements

R selected species

r – Selected Species:

  • Those species that have traits that favor growth rate strategies (“r-strategists)

  • Typical r-selected strategies:

    • Small body size

    • Early maturity

    • Short life span

    • Large broods

    • Minimal required parental care

    • Live in unpredictable or temporary environments

    • Opportunists (like mosquitos, insects, weeds, etc.)

K selected species

K-Selected Species

  • Called “K-strategists”

  • Species that try to maximize the chances of survival, especially in environments where the number of individuals (N) is near the carrying capacity (K) of the environment

K selected strategies

K-Selected Strategies…

  • Long life span, slow development

  • Late reproduction

  • Large body size

  • Low reproductive rate

  • Examples: redwood trees, animals requiring long parental care (Tawny owls pair for life!)



  • Life tables are constructed by ecologists to indicate the relative chances of survival at any time during the life of the organism

  • Each organism is classified as a Type I, Type II, or Type III Survivorship organism

Population ecology

Survivorship = the probability that a given individual in a population will survive to a particular age.

Type I Survivorship: the young and those at reproductive age have a high chance of living

Type II Survivorship: the probability of survival does not change with age

Type III Survivorship: the probability of death greatest early in life, those that survive have high survival rate until old age

Factors that affect population size

Factors That Affect Population Size

  • Density-Dependent Factors:

  • A change in population density initiates some factor, which subsequently has a counter-effect on the population

  • Examples of D-D Factors:

    • Disease, predation, competition for resources

  • Predator-prey relationships involve dynamics of density-dependent factors.

Predator prey on isle royale

Predator-Prey on Isle Royale…

  • From 1960 to the mid 1980’s…this is a simple predator-prey dynamic relationship

  • Mid 1980’s the wolf population crashed…later confirmed to be the result of a deadly canine parvovirus disease…

  • This allowed moose to flourish…but by mid-1990’s there were so many that they literally overgrazed their main food source (ash and aspen)…which led to a sudden collapse, which then triggered a die-off of the already struggling wolves by 1998 (only 14 individuals were counted!)

Population ecology

Population Cycles – Hares/Lynx Example

Example 2 population cycling due to organism interaction

Example 2: Population cycling due to organism interaction

Start: both have low population density

  • Hares – high food, low predators = pop increase over next generations

  • Lynx – as hares increase, more food = pop increase

  • Hares – when high pop density, increased competition for food and increased predation = low birth rate & high death rate = sharp pop decrease

  • Lynx – when high pop density and few hares, low food = low birth rate & high death rate = sharp pop decrease

  • Back to start

Population ecology

Population Cycles

Density independent factors

Density-Independent Factors…

  • Any environmental factor that affects the size of a population but is not influenced by changes in population density

  • Usually abioticfactors, such as random severe weather, fires, timing & severity of winter season, etc.

  • Cause distress to a population

Human population

Human Population

  • Right now at over 6.6 billion and counting…

  • Scientific advances have enabled us to improve the productivity of the land…so food production has kept pace…not a limiting factor for much of the world

  • However, have we reduced the ability of the land to sustainably feed future generations?

Population ecology

  • Exponential growth so far

  • Should “level out” sometime during the 21st century

  • S-curve may be developing already

  • What will cause the S-curve????

The human population clock

The Human Population Clock…

100 million more people from 2009 to 2010 let s see why

100 million more people from 2009 to 2010…let’s see why:

  • Increase is NOT due to a rise in the birth rate (b). World birth rate has declined slightly.

  • Increase is due to a DRAMATIC decrease in the death rate (d).

    • Greater food production

    • Better medical care

    • Improvements in water quality and sanitation

World growth rate

World Growth Rate

  • Actually has started to decline!

  • Growth Rate (r) was at 2.2% in 1960’s, now estimated at 1.2% (2006)

  • S-curve coming soon…

  • Zero Population Growth: when the birth rate = death rate; should occur by the end of the 21st century (2100)

Role of the u n united nations

Role of the U.N. (United Nations)

  • UN periodically provides estimates of population growth rates in each country, including factors influencing births and deaths.

  • UN estimates that human population will most likely be 9.1 billion by 2050. (range between 7.7 and 10.6 billion)

  • Earth’s “carrying capacity” unknown…but studies by Van Den Bergh and Rietveld (Netherlands) suggest 7.7 billion is our upper limit!

  • Will humans slow down pop. growth, or will there be widespread human suffering and death as Earth becomes uninhabitable????



  • Branch of sociology that deals with population statistics

  • Separates countries into categories based on stage of development, which relates to population growth potential

  • Highly Developed, Developing, Less-Developed nations



  • Highly Developed Countries include:

    • U.S., Germany, Canada, France, Sweden, Australia, Japan, U.K., etc.

    • Highly industrialized

    • Low birth rates

    • Very low infant mortality rates

    • Longer life expectancies



  • Developing Countries – includes moderately developed and less-developed

  • Moderately developed countries include:

    • Thailand, Mexico, Turkey, Peru, Brazil, Chile, etc

    • Moderate birth rates

    • Moderate infant mortality rates

    • Moderate level of industrialization



  • Less Developed Nations Include:

    • Cambodia, Bangladesh, Ethiopia, Niger, etc.

    • Highest birth rates

    • Highest infant mortality rates

    • Shortest life expectancies

    • Lowest average per-capita GNI PPP’s (gross national income in purchasing power party divided by midyear population)

Doubling time

Doubling Time:

  • Doubling Time = amount of time it takes for the population to double in size

    • Usually calculated for each country

  • Formula for Doubling Time (td)

    • td = 70/r

  • Shorter the doubling time, the less developed the country likely is

  • Typical Doubling Times:

    • Laos = 30 yrs.; Ethiopia = 28 yrs.

    • France = 175 yrs.;Turkey = 47 yrs.



  • Replacement-Level Fertility = the number of children a couple must produce in order to “replace” themselves (usually considered 2.1, since some infants & children die before reaching maturity)

  • Total Fertility Rate is currently 2.8 worldwide

Demographic stages

Demographic Stages:

  • Four stages are observed, based upon European history

  • Pre-industrial stage

  • Transitional stage

  • Industrial stage

  • Postindustrial stage

Pre industrial stage

Pre-industrial stage

  • High birth and death rates

  • Modest population growth

  • High infant mortality rate

  • Famines, plagues, wars common

Transitional stage

Transitional Stage

  • Lowered death rate due to advances in health care, better food supply, water sanitation measures

  • Birth rate still high

  • Population grows rapidly

Industrial stage

Industrial Stage

  • Decline in birth rates, still relatively low death rate

  • Population growth rate slows a bit

  • Increase in wealth usually accompanies a decrease in birth rates

Postindustrial stage

Postindustrial Stage

  • Low birth and death rates

  • Better educated and more affluent population…tend to desire smaller families

  • Examples: U.S.A., Canada, Australia, Japan, Europe

Demographic stages based on birth and death rates

Demographic Stages based on birth and death rates…

Age structure of countries

Age Structure of Countries

  • Age structure = the number and proportion of people at each age in a population

  • Age structure helps in determining future growth rates

  • Age structure diagrams provide a quick way to look at figures, and determine what is happening to a population

Age structure diagrams

Age Structure Diagrams:

Nigeria united states and germany age structure diagrams

Nigeria, United States, and Germany Age Structure Diagrams:

United states population issues

United States Population Issues:

  • Largest population of all “highly developed” nations

  • U.S. is over-populated due ONLY to immense over-consumption of resources by individuals

  • Immigration rate is among the highest, adding to population growth well beyond birth rate

  • 1 million legal immigrants yearly, plus illegal immigrants estimated at 300,000 yearly (conservative estimate…)

  • No formal population policy exists (as it does in China and other countries, for example)

United states population issues1

United States Population Issues:

  • Immigration and Nationality Act (1952) now called the Immigration Reform and Control Act (IRCA):

    • Abolished national quotas on immigration

    • Gives preference to those with family members already here, those who can fill vacant jobs, and refugees seeking asylum

  • Most immigrants now come from Mexico, Phillipines, Vietnam, Dominican Republic, and China

  • Many immigrants are poor, have few skills

Education of immigrants into us

Education of Immigrants into US

Immigration environmental questions

Immigration & Environmental Questions

  • Arguments/Questions in favor of immigration restrictions:

    • Is population of U.S. already too large?

    • More immigrants means more pollution, less literate public, resource depletion?

    • Immigrants adopt the polluting lifestyle of current U.S. residents?

    • Immigrants generally do not get involved in the political process…creates a more apathetic population

  • Arguments/Questions against immigration restrictions:

    • Morally unacceptable to refuse immigrants?

    • Immigrants typically lead simpler lifestyles, so they contribute only marginally to environmental stress?

    • Creates a global consciousness & more diverse population

    • Immigrants will gladly work in jobs most Americans would prefer not to…so they are necessary to some degree????

  • Where do you stand????

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