Chapter 17 Lecture Outline See PowerPoint Image Slides

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Chapter 17

Lecture Outline

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Population Characteristics

A population is a group of organisms of the same species located in the same place at the same time.

A species is the largest possible population.– Usually the term population is used to describe

portions of a species.


Gene Frequency

A measure of how often a specific gene shows up in the gametes of a population

– Two populations of the same species can have very different gene frequencies.


Gene Flow

The movement of a gene

– From one generation to another

– From one population to another when animals migrate

Both types of gene flow usually occur together.


Age Distribution

The number of organisms in each age in the population

Organisms are grouped by their reproductive status.– Prereproductive juveniles– Reproductive adults– Postreproductive adults


Age Distribution

The age categories of a population may not contain the same number of individuals.– The age structure of some species will change

dramatically over the course of a season.– Birds, flowering plants

Age structure impacts how fast the population will grow.


Age Distribution in the Human Population


Sex Ratio

The number of males compared to the number of females– Birds and mammals

Those that have mate-bonding frequently have a 1:1 ratio of males to females.

Those without mate-bonding usually have more females than males.

– Harems

– Insects usually have far more females than males. Sex ratios do not apply to hermaphroditic species.


Population Distribution

Describes the way individuals within a population are arranged with respect to one another

Three basic arrangements

– Even, random, clumped


Population Density

The number of organisms of a species per unit area Population density varies with

– Success of the individuals– Resources available

Populations that are highly concentrated will begin to experience competition.

– Population pressure describes how the intensity of competition causes dispersal.

Dispersal relieves the pressure and can lead to new populations. If dispersal doesn’t happen, then individuals will begin to die.


Reproductive Capacity

The theoretical number of offspring that could be produced by a population– Also called biotic potential– Usually larger than the number of offspring

needed to maintain the population


Reproductive Capacity

Two strategies for maintaining the species– Have a lot of offspring, but not invest a lot of

energy in them after birth The probability of survival to reproductive age is slim.

– Have fewer offspring, but invest a lot of energy in them after birth

The probability of survival to reproductive age is quite good.


The Population Growth Curve

The growth of a population is dependent on– The rate at which organisms enter the population

Immigration and birth Natality = the # of new individuals added by birth/1000

individuals

– The rate at which organisms leave the population Emigration and death Mortality = the # of individuals leaving by death/1000

individuals


The Population Growth Curve

Population growth curves graph the change in population size over time.– Four phases can be seen in the curve.

Lag phase Exponential growth phase Deceleration phase Stable equilibrium phase


A Typical Population Growth Curve


Lag Phase

A period of time immediately following the establishment of the population– Population is small– Growth is happening slowly

Both natality and mortality are low Occurs slowly because reproduction takes time


Exponential Growth Phase

The period of time when the population is growing rapidly.– As the number of individuals increases, then there are

more individuals available to reproduce.– During this phase, populations increase by the same

percent each year. Natality is greater than mortality.

– Mice population Two mice mate and have 4 Those four mate and have 8 Those eight mate and produce 16


Deceleration Phase

Occurs when population growth rate begins to slow

Population continues to grow until birthrate and deathrate become equal


Stable Equilibrium Phase

The period of time when the population stops growing– Maintains a stable size

Exponential growth cannot continue indefinitely. The availability of resources will limit population growth.

– When a population becomes large enough, resources will be in short supply.

Mortality will increase to the point where it equals natality.

Large organisms that live a long time tend to reach stable equilibrium.


Alternate Population Growth Strategies

K-strategists are large organisms that a live long time and reach a population size that can be sustained (as just described).

r-strategists are small organisms with short lifespans and tend to have fluctuating population that do not reach a stable equilibrium phase

– Characterized by a “crash” after rapid exponential growth


Population Growth Curve for Short-lived Organisms


Limits to Population Size

Factors that determine the size at which a population will reach stable equilibrium– Called limiting factors– Collectively known as environmental resistance


Extrinsic and Intrinsic Limiting Factors

Extrinsic factors come from outside the population and limit population growth.– Predators, availability of nutrients, natural disasters

Intrinsic factors come from within the population and limit population growth.– Crowded conditions leading to

Decreased reproduction Decreased # of offspring produced per reproductive event Decrease in maternal care Increase in death rates


Density Dependent Limiting Factors

These factors become more effective at limiting population size as the population gets bigger.

– Predators are more likely to capture individuals in a large population.

– Disease epidemics are more common in large populations.– Competition for resources increases as population size

increases.– Intrinsic factors such as decreased reproductive rates occur

as population size increases.

Populations of large, long-lived organisms are usually controlled by these factors.


Density Independent Limiting Factors

These factors control population growth no matter how large the population is.

Usually accidental extrinsic factors– Weather patterns– Natural disasters– Drought/excessive rainfall

Populations of small, short-lived organisms are often controlled by these factors.


Categories of Limiting Factors

Factors that limit population growth can be categorized as– Availability of raw materials

For plants– Nitrogen, water, magnesium

For animals– Water, minerals, materials/sites for housing/nesting, food

(as a source of carbon and nitrogen)

– Availability of energy Availability of light to plants Availability of food to animals



Production and disposal of waste products– Usually not relevant for plant species– Animal species generate more waste.

Waste can be toxic. As waste builds up, reproduction slows, death rates

increase. Accounts for the death phase in bacteria

– Follows stable equilibrium phase


Bacterial Population Growth Curve



Interactions with other organisms– Predation, parasitism and competition limit

population growth.– These interactions usually

Involve more than one population Limit the size of both populations involved Generate population cycles Lemming example

– Mutualistic interactions allow populations to grow at a faster rate than they would have alone.


Population Cycles


Carrying Capacity

Populations at stable equilibrium phase have reached their maximum size for that environment.– This maximum, sustainable population size is

called carrying capacity.– Determined by a set of limiting factors

Environmental changes, succession, disease, and natural disasters can change carrying capacity for a given population.

The size of the organism and the size of its habitat can determine carrying capacity.


Carrying Capacity


The Effect of Biomass on Carrying Capacity


Limiting Factors to Human Population Growth

Human populations follow the same pattern of growth.

– They are limited by the same kinds of factors.

The human population growth curve– Shows the lag and exponential phases

Although the human population has approached carrying capacity for some time,

– Technology continues to increase carrying capacity.

Nevertheless, limiting factors will cause human population growth to level off.


Human Population Growth Curve


Available Raw Materials

Include– Drinking water– Irrigation water– Iron ore– Silicon– Food

Many portions of the global human population do not have enough food.

However, the world can produce enough food to feed everyone. Political and economic barriers prevent equal distribution of food.


Availability of Energy

Ultimately, humans depend on sunlight for energy.– Solar energy can be stored in fossil fuels.

However, limited reserves of petroleum, natural gas and coal exist.

– It is hard to predict how long these will last.– Rate of use of the reserves increases as population size

increases.– It took millions of years to build up these reserves, but we

are using them at a rate that will deplete them in a matter of hundreds of years.


Production of Wastes

Human biological wastes can be decomposed by detrivores.

Technological wastes cannot be decomposed by organisms.– Excess technological wastes cause pollution.– Most of this waste is toxic and is damaging the

environment.


Interactions with Other Organisms

Humans interact with other species in many ways.– We are hosts for a variety of parasites.

Bacteria and viruses

– We are predators of a number of species.– We have mutualistic relationships with

Domesticated and wild animals A number of plant species A variety of bacterial species


Interactions with Other Organisms

We compete with other species for resources.– Because we win, we frequently displace other

species. This has caused a number of extinctions.

We compete with one another for resources.– Evident in an increase in infant mortality


Control of Human Population

is a Social Problem

Humans are able to predict the outcome of a specific course of action.– We have the information to control the human

population and improve health and quality of life.

However, we do not always make the best unbiased decisions.– We tend not to consider what is best for the

population.– We tend to consider what is best for us,

individually.


Control of Human Population

is a Social Problem

Continued population growth will present more social, political and economic problems than biological problems.

– Will likely result in increased famine and starvation– One major factor seems to be the educational status of

women. Women with higher levels of education have fewer children. Improving the education of women may be one social strategy

for limiting population growth.

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