Chapter 5:

Populations

Section 5.1: How Populations Grow

*population – a group of organisms of a single species living in the same area at the same time.

A. Describing Populations

*researchers study population’s geographic range, density and distribution, growth rate, and age structure.

1. Geographic Range

a) geographic range (definition) – the area inhabited

by a population.

b) Geographic range can vary in size

ex) bacteria in a rotting pumpkin – does not have a large geographic range, but hydrilla that was

tossedinto a canal in Florida have run rampant

2. Density and Distribution

a) population density – the number of individuals per

unit area.

b) distribution – how individuals in populations are spread out in their area.

1) random – individuals aredistributed randomlythroughout their area.

Ex) purple lupines

2) uniform – individuals are spaced fairly evenlythroughout their environment

ex) King penguins

3) clumped – individuals are intight packs or clusters

ex) striped catfish

3. Growth Rate

a) a population’s growth rate determines whether the

size of a population:

1) increases2) decreases3) stays the same

b) GR = Birth Rate – Death Rate

1) GR > 0 = more birth than death2) GR < 0 = less birth than death

4. Age Structure

a) age structure – the # of males and females of EACH

AGE in a population.

b) some animals cannot reproduce until they are a certain age, therefore it is good to know how

many reproducing organisms there are in a

population to determine if the population will grow.

B. Population Growth

*a population will increase or decrease in size depending on how many individuals are added to it

or removed from it.

*4 key factors affect population size:1) birth rate2) death rate3) immigration4) emigration

1. Birth rate and Death rate

a) growth – populations will grow if Birth > Death

b) 0 growth – Birth rate = Death rate

c) decline – populations will decrease if Death > Birth

2. Immigration & Emigration

a) immigration – causes populations to grow; occurs when new individuals move into an area from elsewhere.

b) emigration – causes populations to decrease; occurswhen organisms leave an area.

C. Exponential Growth*if you provide a population with all the food and space

it needs, protect it from predators and disease, and remove its waste products, the population will grow indefinitely.

1. exponential growth – the larger a population gets, the faster it grows.

*under ideal conditions w/ unlimited resources a population will grow exponentially, with a graph that is shaped like a “J.”

Exponential Growth

a) Organisms that reproduce rapidlytend to:

1) be small in body size2) start to reproduce at a young age3) have many offspring, but do not usually

care for them

4) have a short lifespan5) examples – flies, mosquitoes, numerous

insects

Exponential growth, the bacteria example:

*bacteria are capable of reproducing every 20 minutes

after 20 minutes – 2 cellsafter 60 minutes – 8 cells (23)after 2 hours - 64 cells (26)after 3 hours - 512 cells (29)after 24 hours -4, 720, 000, 000, 000, 000, 000, 000

b) Organisms that reproduce slowlytend to:

1) be larger in body size2) reproduce at a later age3) have few offspring at a time and tend to

care for their young

4) have a relatively long life span5) examples – humans, dogs, elephants

Exponential Growth, the elephant example:

*produce only 1 offspring every 2-4 yrs, takes about 10 years for a baby elephant to mature and be able to have babies of their own:

If they had unlimited resources, their population size wouldincrease dramaticallyover time.

c) Organisms in new environments

1. sometimes when an organism is moved to a newenvironment it may grow exponentially for a time.

2. example – Hydrilla, Gypsy month, European rabbit

D. Logistic Growth

*natural populations do not grow exponentially for long; sooner or later something inhibits exponential growth from continuing.

1. Phases of Growth

a) Phase 1: Exponential Growth1) initial growth period2) resources are unlimited so individuals can

grow and reproduce rapidly3) few individuals die and many offspring

are produced

b) Phase 2 : Growth Slows1) population is still growing, but at a much slower rate

2) resources are becoming limited

c) Phase 3: Growth Stops“leveling off stage”

1) occurs when growth rate = 0

2) birth rate = death rate

2. The logistic growth curve (S shaped curve)

a) logistic growth – occurs when a population’s growthslows, then stops after a period of exponential growth.

b) S-shaped curve – graph has the shape of an “S.”

c) may occur due to:

1) decrease in birth rate2) increase in death rate3) decrease in immigration4) increase in emigration

3. Carrying Capacity – the maximum number of individuals of a particular species that an environment can support

*once a population reaches the Carrying Capacity of it’s

environment a variety of factors stabilize it at that size.

a) If populations are under the carrying capacity, the birthrate will exceed the death rate until carrying capacity is reached.

b) If populations are OVER the carrying capacity, thendeath rate will exceed the birth rate until the population

drops back down to the carrying capacity line.

Section 5.2Limits to Growth

*why does a species that is “well-behaved” in one environment grow out of control in other environments?

A. Limiting Factors

1. Limiting Factor – a factor that controls the growth of

a population.

Ex) competition predation

parasitism/ disease weather natural disaster

2. Acting separately or together, limiting factors determine the carrying capacity of a species.

3. Limiting factors keep most natural populations somewhere between extinction and overpopulation.

B. Density-dependent Limiting Factors

1. density-dependent limiting factors – depend on the

“density” (# of organisms in an area). As a population’s density goes up, these factors will

begin to play a role in limiting population growth.

Includes:a) competitionb) predationc) herbivoryd) parasitism/diseasee) stress from overcrowding

2. Competition

a) when populations becomeovercrowded organisms fight for food, water, space,and sunlight.

b) can lower birthrates and increase death rates.

c) the more individuals living in an area, the sooner the

resources get used up, and thus competitionoccurs.

3. Predation and Herbivoryex) wolves, moose, plants on Isle Royale

a) Predator-prey relationships1) moose increases; wolves increase2) wolves increase; eventually moose

decreases3) moose decreases; wolves begin to starve4) as wolves decrease; moose can rebuild

b) Herbivore Effects

*on Island Royale, if moose population is high they will eat many of the balsam firs.

1) increase in moose; decrease in firs2) decrease in firs then results in starving moose3) as moose decrease, firs replenish

c) Humans as Predators

*in some situations human activity limits populations

ex) overfishing for codfish in New England has caused

codfish populations to decline

4. Parasitism and Disease

a) Parasites and disease causing organisms feed at the expense of their hosts; weakening them and causing disease or death.

b) The more dense or “overcrowded” an area, the easier it is to spread disease and parasites.

Ex) CPV outbreak on Island Royale killed all but 13 wolves, this lead to an over population of moose which easily passed winter ticks to one another causing hair loss and weakness

5. Stress from overcrowding

a) some species fight amongst themselves if overcrowding occurs

b) fighting leads to stress

c) stress weakens the body’s immune system making it

easier to get sick

d) stress can:1) increase death2) decrease birth3) increase emigration

C. Density – Independent Living Factors

1. density-independent factors – affect ALL populations

regardless of population size and density.

2. examples:a) hurricanesb) droughtc) floodsd) fires

3. True density independence?

*sometimes these factors can vary with populationdensity

ex) Island Royaleharsh winter killed many plants, therefore

moosepopulation decreased, but if moose

population wassmaller they may have been ok

*hard to say if a limiting factor acts ONLY in an independent way.

4. Controlling Introduced Species

It is often hard to control introduced species b/c they often have no natural predators or diseases in that area, thus they run rampant and can actually cause native species to decline or even become extinct.

Ex) European rabbits introduced to Australia

Section 5.3

Human PopulationGrowth

*How quickly is the human population growing?*Worldwide there are more than 400 births/second

I. Historical Overview

*The human population, like populations of other other organisms, tends to increase. The rate of that

increase has changed dramatically over time.

*For most of human existence the population grew slowly because life was harsh – food was hard to

find, predators and disease were common; therefore

humandeath rates were high.

A. Exponential Human Population Growth

1. As civilization advanced, life became easier, and the

human population began to grow more rapidly.

2. Industrial Revolution – 1800’s; food became morereliable and goods could be shipped around the

world.

3. reduction in death rates – death rates went downb/c of better nutrition, sanitation, medicine, and improved healthcare.

4. birth rates remained high

5. Increase in birth rates + decrease in death rates = POPULATION EXPLOSION

B. The predictions of Malthus

1. Thomas Malthus – English economist; suggested only

war, famine, and disease could limit population growth.

*all were “density-dependent” factors

2. Malthus’ work was crucial to Charles Darwin

C. World Population Growth Slows

*What is happening to human population growth today?

1. 1962-1963 – human population growth reached a peak, then began to drop.

2. The size of global human population is still growingrapidly, but the rate of growth is slowing down.

3. “Doubling”

a) it took 123 billion years for human population growth to double from 1 billion to 2 billion in

1927

b) Then it took just 33 years to grow another billion

c) But today it takes longer for the population to grow

by a billion than it did 20 years ago. WHY?

II. Patterns of Human Population Growth

* Why do population growth rates differ among countries?

1. demography – the scientific study of humanpopulations; attempts to explain how these

populationschange over time.

2. Birth rates, death rates, and the age structure of a

population help predict why some countries have high

growth rates while others grow more slowly.

A. The Demographic Transition

1. demographic transition – a dramatic change from high birth rates and death rates to low birth rates anddeath rates.

2. 3 stages:

a) stage I – BR and DRhigh

b) stage II – BR high, DR low

c) stage III – BR down, DR down

3. US, Japan, and Europe have completed the demographic transition.

4. Parts of S. America, Africa, and Asia are in stage II

5. India and China are still in stage I

*J shaped human growth curve may become more like an S

B. Age Structure & Population Growth

1. age structure – graphs that depict the number of males and females in particular age groups.

2. United States vs. Guatemala

a) In US there are equal numbers of people in each

age group.

b) Guatemala has many more young children and

teenagers than adults.

United States vs Guatemala Age Structure Pyramids

C. Future Population Growth

1. To predict how the world population may grow, demographers consider many factors such as:

a) age structure of each countryb) effects of disease on death rates ex) AIDS

2. 9 billion – current predictions suggest that by 2050

world population will = 9 billion.

*are we at, below, or above our carrying capacity?

What do you think??????????