Unit 3: Populations

Chapter 5

Populations

Sea otters Sea urchins kelp

• Long ago, hunters hunted the sea otters almost to extinction

• This made the sea urchin population grow • Which in turn, made the kelp population do

what? __________________

Populations

• After sea otters were put on the endangered species list, hunters were not allowed to hunt them anymore

• With hunters out of the picture, sea otter populations recovered

• Which greatly decreased the sea urchins• Which greatly ___________ the kelp

Characteristics of Populations

• Three important characteristics of population are:

1. Its geographic distribution2. Its density3. And its growth rate

Geographic Distribution

• Geographic Distribution, or range, is a term that describes the area inhabited by a population

Population Density

• Population Density is the number of individuals per unit area

Growth Rate

• Growth rate is the number of births minus the number of deaths in a given area, also taking into affect immigration and emigration.

Factors that affect Growth Rate

• Three factors can affect population size

1. The number of births 2. The number of deaths3. The number of individuals that enter (immigrate

into) or leave (emigrate out of) the population

Populations

• Generally, populations grow if more individuals are born than die in any period of time

Birthrate > Death Rate

Populations

Birthrate > Death Rate **population growth**

Birthrate = Death Rate **population remains the same**

Birthrate < Death Rate **population shrinks**

Moving in and out

• Immigration – the movement of individuals into an area– can make a population increase

• Emigration – the movement of individuals out of an area– can make the population decrease

Exponential Growth

• If a population has abundant space and food, and is protected from predators and disease, then the organisms in that population will multiply and the populating size will increase.

• Exponential growth – occurs when the individuals in a population reproduce at a constant rate (i.e. bacteria, humans)

Exponential Growth

• Under IDEAL CONDITIONS with UNLIMITED RESOURCES, a population with grow exponentially

• Will show a “J-Curve” graph

Logistic Growth

• Obviously, bacteria, elephants, and humans cannot increase in population until we cover the planet

• We reach a carrying capacity, or a maximum quantity that the planet (or on smaller terms, the ecosystem) can handle

Logistic Growth

• As resources become less available, the growth of a population slows or stops– Run out of space– Run out of food– Not enough clean water

• Logistic Growth occurs when a population’s growth slows or stops following a period of exponential growth.

What is CARRYING CAPACITY???

Carrying Capacity

• Carrying Capacity- The largest number of individuals of a population that a given environment can support

Checkpoint

• What factors can change a population’s size?

• What is the difference between exponential growth and logistic growth?

Do Fruit Flies and Rabbits Have Similar Population Growths?

Fruit Fly Population Growth

Days Number of Fruit Flies

5 10

10 50

15 100

20 200

25 300

30 310

35 320

40 320

Rabbit Population Growth

Generations Number of Rabbits

1 100

2 105

25 1,000

37 1,600

55 2,400

72 3,350

86 8,000

100 13,150

• Graph the data provided (make 2 graphs)

QUESTIONS1. What type of

growth pattern is the fruit flies?

2. Is it the same as the rabbit’s growth pattern? Explain.

Fruit Flies vs. Rabbits

3. Does either graph indicate a carrying capacity?4. If so, when does the population reach this carrying

capacity? 5. What is the maximum number of individuals that can be

supported at that times?

6. Predict: Animals such as foxes and cats often prey on rabbits. Based on the growth curve of the rabbit population, what might happen if a group of predators move into the rabbits’ habitat during the tenth generation and begin eating the rabbits?

PDN Review

1. List three characteristics that are used to describe a population

2. What factors can change a population’s size?3. What is the difference between exponential

growth and logistic growth?4. What is meant by population density?5. Define carrying capacity.

Limits to Growth

• Back to the sea otters,– When their population declines, something has

changed with their birth rates or death rates– Or between that rates of immigration and

emigration

• What caused the otter population to decrease to greatly??

Factors that impact populations

• The hunters are considered a limiting factor

• A Limiting Factor is a factor that causes population growth to decrease

– A limiting factor that is caused by or made worse by high population (density) is known as a density-dependent factor

– A limiting factor that is NOT caused by a high population is known as a density-independent factor

Density-Dependent Factors

• A limiting factor that depends on population size is called a density-dependent limiting factor

• These factors become limited only when the population density reaches a certain level

• Some examples: (YOU MUST KNOW THESE!!) competition, predation, diseases, and parasitism

Competition

• When populations become crowded, organisms compete with one another for food, water, space, sunlight, and other essentials

• Competition WITHIN the population• Competition among members of the SAME

species is a density-dependent limiting factor!!

Completion with others

• Competition can also occur between members of different species

• This type of competition is a major force behind evolutionary change

• When two species compete for the same resources, both species are under pressure to change in ways to win and thus decrease their competition

Predation

• Populations in nature are often controlled by predation

• A predator-prey relationship is one of the best mechanisms for population control

• Ex: the sea otters controlled the sea urchin population… what controlled the kelp?

Parasitism and Disease

• Parasites can also limit the growth of a population

• Parasites can be microscopic bacteria to larger organisms like tapeworms and leaches

• These organisms obtain nutrients from the host similar to predation, but do not kill the PREY… why not??

Density-Independent Factors

• Density-Independent Factors affect all populations in similar ways BUT DO NOT depend on the population (meaning they will affect small groups and large)

• Unusual weather, natural disasters, seasonal cycles, and certain human activities– What kind of human activities do you think??

Human Activities

• Some human activities that affect populations are:– Damming of rivers– Clear cutting forests– Local pollutions– Habitat destruction

• Environments are always changing, and most populations can afapt to a certain amount of change

• Populations often grow and shrink in response to these changes

Checkpoint !!

1. List three density-dependent factors and three density-independent factors that can limit the growth of a population

2. What is the relationship between competition and population size?

3. If an entire fox population disappears, what is likely to happen to the hare population?? How about the grass in the rabbits’ ecosystem?

Human Population Growth

• How quickly is the world’s human population growing?

• Like the populations of many other living organisms, the size of the human population tends to increase with time

Human Population

• About 500 years ago, the human population began growing much more rapidly.

• WHY???

A Growing Population

• Agriculture and industry made life easier and safer

• Food supplies became more reliable• Essential goods were able to be shipped

around the globe• Improved sanitation• Improved health care

Demography

• The scientific study of human populations is called demography

• Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly

Age Structure

• Population growth depends on how many people of different ages make up a given population

• Demographers can predict future growth using models called age-structure diagrams

Age-Structure Diagrams

• Equal numbers of people in each group – predicts a slow but steady growth rate in the near future

• Larger number of children than adults (fat bottom) predicts a sharp population increase

• Fat top, or a larger number of adults than children, predict a decrease in population

Future Population Growth

• To predict the overal human population growth (the whole world) demographers need to consider the following:– Age structure of each country– Prevalence of life threatening diseases • AIDS, malaria, cholera

Human Growth Rate

• It is predicted that the human growth rate will level off or even decrease by 2050

• A lower growth rate means that the human population will be growing more slowly over the next 50 years

• Because this growth rate is still above zero, the human population will still rise

Review

1. What is the difference between immigration and emigration?

2. How can a predator-prey relationship serve as a population control?

3. Sketch the exponential growth curve of a hypothetical population• How is this different from a logistic?

4. How does age structure of a population affect its growth rate?