environmental science chapter 8 understanding populations 8.1 how populations change in size

ENVIRONMENTAL ENVIRONMENTAL SCIENCESCIENCE

Chapter 8 Understanding Chapter 8 Understanding PopulationsPopulations

8.1 How Populations Change 8.1 How Populations Change in Sizein Size

8.1 How Populations Change in 8.1 How Populations Change in Size ObjectivesSize Objectives

Describe the three main properties of Describe the three main properties of a population.a population.

Describe exponential population Describe exponential population growth.growth.

Describe how the reproductive Describe how the reproductive behavior of individuals can affect the behavior of individuals can affect the growth rate of their population.growth rate of their population.

Explain how population sizes in Explain how population sizes in nature are regulated.nature are regulated.

IntroductionIntroduction

While researching population growth, While researching population growth, Charles Darwin calculated that one pair of Charles Darwin calculated that one pair of elephants could theoretically produce elephants could theoretically produce about 19 million descendants in just 750 about 19 million descendants in just 750 years!years!

The actual number of individuals is The actual number of individuals is controlled by their environment.controlled by their environment.

In order to understand species better, they In order to understand species better, they are often studied at the population level.are often studied at the population level.

What is a Population?What is a Population? All the members of a All the members of a

species that live in a species that live in a particular place at a particular place at a particular time is particular time is referred to as a referred to as a population.population.

A population makes up A population makes up an interactive gene an interactive gene pool.pool.

The term The term populationpopulation can also be used to can also be used to refer to the number of refer to the number of individuals that make individuals that make up the population.up the population.

Properties of PopulationsProperties of Populations

Three major properties of Three major properties of populations may be measured to populations may be measured to describe them.describe them.• Density – the number of individuals per Density – the number of individuals per

unit area or volumeunit area or volume• Dispersion – the relative distribution or Dispersion – the relative distribution or

arrangement of individuals such as arrangement of individuals such as even, clumped, or randomeven, clumped, or random

• Size – the total number of individuals in Size – the total number of individuals in the gene pool at a given timethe gene pool at a given time

How Does a Population Grow?How Does a Population Grow?

As offspring are produced, a As offspring are produced, a population gains individuals.population gains individuals.

As individuals perish, the population As individuals perish, the population declines.declines.

Growth rate is the change in size of a Growth rate is the change in size of a population over a given period of population over a given period of time.time.

Births – Deaths = Change in Births – Deaths = Change in Population SizePopulation Size

How Does a Population Grow?How Does a Population Grow?

Birth rates and death rates can fluctuate, Birth rates and death rates can fluctuate, thus causing growth rates to change.thus causing growth rates to change.

Growth rates can be positive, negative, or Growth rates can be positive, negative, or zero.zero.

For a growth rate to be zero, the average For a growth rate to be zero, the average number of births must equal the average number of births must equal the average number of deaths for the given time.number of deaths for the given time.

If adults lost from a population are not If adults lost from a population are not replaced by new offspring, the growth rate replaced by new offspring, the growth rate will be negative, resulting in a shrinking will be negative, resulting in a shrinking population.population.

How Fast Can a Population How Fast Can a Population Grow?Grow?

Over her lifetime, a female sea turtle can Over her lifetime, a female sea turtle can lay 2,000 eggs.lay 2,000 eggs.

If all of those eggs produced turtles that If all of those eggs produced turtles that survive to adulthood, the turtle population survive to adulthood, the turtle population would explode exponentially.would explode exponentially.

They do not all survive.They do not all survive. The population of any species tends to The population of any species tends to

remain stable from year to year due to remain stable from year to year due to environmental factors.environmental factors.


Environmental factors have an impact on Environmental factors have an impact on how a species evolves.how a species evolves.

The fastest rate at which a species can The fastest rate at which a species can grow is called it’s biotic potential.grow is called it’s biotic potential.

The biotic potential is determined by the The biotic potential is determined by the reproductive potential of the species – the reproductive potential of the species – the maximum number of offspring that each maximum number of offspring that each member of a species can produce.member of a species can produce.

The reproductive potential of species can The reproductive potential of species can vary greatly.vary greatly.


In contrast to the biotic potential of a pair of In contrast to the biotic potential of a pair of elephants, a bacterium can produce 19 elephants, a bacterium can produce 19 million descendants in a matter of a few days.million descendants in a matter of a few days.

Generation time is the average time it takes a Generation time is the average time it takes a member of the population to reach the age member of the population to reach the age when it reproduces.when it reproduces.

Reproductive potential varies on three points.Reproductive potential varies on three points.• Number of offspring produced at a timeNumber of offspring produced at a time• Time between reproductive cyclesTime between reproductive cycles• Point in life when reproduction beginsPoint in life when reproduction begins


When graphing population growth, most When graphing population growth, most populations will fit either an “S – curve” or a “J – populations will fit either an “S – curve” or a “J – curve” prior to reaching carrying capacity.curve” prior to reaching carrying capacity.

Carrying capacity for the ecosystem for a Carrying capacity for the ecosystem for a particular species is the maximum population particular species is the maximum population that can be supported without sustainability of that can be supported without sustainability of the environment being compromised.the environment being compromised.

Most large mammal populations fit the “S curve” Most large mammal populations fit the “S curve” pattern where as insects typically fit the “J – pattern where as insects typically fit the “J – curve” pattern.curve” pattern.

Humans are an exception – we fit the “J – curve” Humans are an exception – we fit the “J – curve” pattern.pattern.

What Limits Population Growth?What Limits Population Growth?

Populations do not grow to their reproductive Populations do not grow to their reproductive potential because conditions are not perfect potential because conditions are not perfect or constant.or constant.

Resources are limited, environments change, Resources are limited, environments change, new predators arrive, diseases spread new predators arrive, diseases spread through populations, and other factors place through populations, and other factors place pressure on populations.pressure on populations.

Only some members of a population, typically Only some members of a population, typically those best adapted – or fit – tend to survive.those best adapted – or fit – tend to survive.

Because of this, gene pools and frequency of Because of this, gene pools and frequency of alleles change over time.alleles change over time.


Though a population may increase beyond Though a population may increase beyond carrying capacity, it cannot remain at that carrying capacity, it cannot remain at that level.level.

Carrying capacities for species may be Carrying capacities for species may be difficult to predict because of changing difficult to predict because of changing environmental factors.environmental factors.

It could be said that a species reaches It could be said that a species reaches carrying capacity when it consumes a carrying capacity when it consumes a particular natural resource at the same particular natural resource at the same rate at which the ecosystem replaces that rate at which the ecosystem replaces that resource.resource.


Members of a particular species eventually Members of a particular species eventually have to compete for available resources.have to compete for available resources.

Indirect competition for social dominance Indirect competition for social dominance or for territory also exists.or for territory also exists.

Territory is an area defended by an Territory is an area defended by an individual or group against other individual or group against other individuals.individuals.

The territory provides all of the resources The territory provides all of the resources necessary for survival, therefore, it is necessary for survival, therefore, it is important to defend and maintain.important to defend and maintain.

Two Types of Population Two Types of Population RegulationRegulation

Mortality in a population may be density Mortality in a population may be density dependent or density independent.dependent or density independent.

Mortality that is density dependent occurs Mortality that is density dependent occurs more rapidly in a crowded population than more rapidly in a crowded population than in a sparse population, such as disease.in a sparse population, such as disease.

Density independent mortality affects a Density independent mortality affects a certain portion of the population no matter certain portion of the population no matter how dense or sparse the population, such how dense or sparse the population, such as a winter storm or volcanic eruption.as a winter storm or volcanic eruption.

ReferencesReferences

Orca Population - Orca Population - http://seattlepi.nwsource.com/local/248515_orcas16http://seattlepi.nwsource.com/local/248515_orcas16.html.html

Sea Turtle Hatchlings - Sea Turtle Hatchlings - http://http://www.floridamarine.org/features/view_article.asp?idwww.floridamarine.org/features/view_article.asp?id=21026=21026

Fern Sporangium - Fern Sporangium - http://io.uwinnipeg.ca/~simmons/16labman05/lb3pghttp://io.uwinnipeg.ca/~simmons/16labman05/lb3pg6.htm6.htm

African Elephants - African Elephants - http://www.wallpapers-zone.com/wallpapers-animauhttp://www.wallpapers-zone.com/wallpapers-animaux-elephants-elephants_120.jpg-1-24.htmlx-elephants-elephants_120.jpg-1-24.html

ReferencesReferences

Bacteria - Bacteria - http://www.carnegieinstitution.org/first_light_http://www.carnegieinstitution.org/first_light_case/horn/lessons/imagescase/horn/lessons/images

Growth Curves - Growth Curves - http://www.chem.duke.edu/~bonk/Chem8304/http://www.chem.duke.edu/~bonk/Chem8304/enote1405.htmlenote1405.html

Carrying Capacity - Carrying Capacity - http://online.moraine.cc.il.us/WebSupported/http://online.moraine.cc.il.us/WebSupported/BIO112/plants_and_ecology_notes.htmBIO112/plants_and_ecology_notes.htm

environmental science chapter 8 understanding populations 8.1 how populations change in size

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