end show 5-1 how populations grow slide 1 of 22 copyright pearson prentice hall chapter 5:...

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5-1 How Populations Grow

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Copyright Pearson Prentice Hall

Chapter 5:Populations(“Population

Biology”)

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Objectives•List the characteristics used to describe a population•Identify factors that affect population growth•Describe exponential and logistic growth

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THINK ABOUT IT

In the 1950s, a fish farmer in Florida tossed a few plants called hydrilla into a canal. Hydrilla was imported from Asia for use in home aquariums because it is hardy and adaptable. The few plants he tossed in reproduced quickly and kept on reproducing. Today, their tangled stems snag boats in rivers and overtake habitats; native water plants and animals are disappearing. Why did these plants get so out of control? Is there any way to get rid of them?

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THINK ABOUT IT

Meanwhile, people in New England who fish for a living face a different problem. Their catch has dropped dramatically, despite hard work and new equipment. The cod catch in one recent year was 3,048 metric tons. Back in 1982, it was 57,200 metric tons—almost 19 times higher! Where did all the fish go? Can anything be done to increase their numbers?

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Characteristics of Populations


The three important characteristics used to study a population are its:

• geographic range

• density and distribution

• growth rate

• age structure


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Recall that a population is a group of organisms of a single species that lives in a given area.

Geographic range describes the area inhabited by a population. This map shows the how the range of the hydrilla population spread each decade from the 1950s to 1990s.

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A populations’s rangecan vary enormously in size, depending on the species.• The range of a bacterial population can be a few centimeters wide. • The range of the mountain gorilla can be in a few isolated areas, as pictured here. • Humans have carried hydrilla to so many places that its range now includes every

continent except Antarctica, and it is found in many places in the United States.


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Population density is the number of individuals found per unit area.

In the desert, for example, large cacti have a low population density compared to smaller plants.

A population of ducks in a pond may have a low density, while fish and other animals in the same pond community may have higher densities.


Density

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Distribution refers to how individuals in a population are spaced out across the range of the population—randomly, uniformly, or mostly concentrated in clumps.


Distribution

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Growth Rate

A population’s growth rate determines whether the population size increases, decreases, or stays the same.

•Hydrilla populations in their native habitats tend to stay more or less the same size over time. These populations have a growth rate of around zero; they neither increase nor decrease in size.

•The hydrilla population in Florida, by contrast, has a high growth rate—which means that it increases in size.

•Populations can also decrease in size, as cod populations have been doing. The cod population has a negative growth rate.

Growth rate is the increase or decrease of the number of individuals in a population over time

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Age Structure

To fully understand a plant or animal population, researchers need to know the population’s age structure—the number of males and females of each age a population contains.

Most plants and animals cannot reproduce until they reach a certain age.

Also, among animals, only females can produce offspring.

Age Structure

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Population Growth

Population GrowthThree factors can affect population size:

• the number of births

• the number of deaths

• the number of individuals that enter (immigrate to) or leave (emigrate from) the population

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A population can grow when its birthrate is greater than its death rate.

A population will stay the same size if the birth rate equals the death rate.

A population will shrink if the death rate is greater than the birth rate. Hunting is a way humans can cause populations to shrink.


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Population Growth

Immigration, the movement of individuals into an area, is another factor that can cause a population to grow.

Populations can increase by immigration as animals in search of mates or food arrive from outside the area.

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Population Growth

Emigration, the movement of individuals out of an area, can cause a population to decrease in size.

Emigration can occur when mature animals leave an area to find mates and establish new territories.

A shortage of food in one area may also lead to emigration.

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Exponential Growth

Under ideal conditions with unlimited resources, a population will grow exponentially.

Exponential growth occurs when the individuals in a population reproduce at a constant rate.

The population becomes larger and larger until it approaches an infinitely large size.

In exponential growth, the larger a population gets, the faster it grows. The size of each generation of offspring will be larger than the generation before it.


Exponential Growth

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Organisms That Reproduce Rapidly

In a hypothetical experiment, a single bacterium divides to produce two cells every 20 minutes.

After 20 minutes, under ideal conditions, one bacterium will divide to produce two bacteria. After another 20 minutes, those two bacteria divide to produce four cells. After three 20-minute periods, we have 2×2×2, or 8 cells.

Another way to describe the size of the bacteria population is to use an exponent: 23 cells (three 20-minute periods).

In another hour (six 20-minute periods), there will be 26, or 64 bacteria.

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If you plot the size of this population on a graph over time, you get a J-shaped curve that rises slowly at first, and then rises faster and faster. If nothing were to stop this kind of growth, the population would approach an infinitely large size.

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Exponential GrowthExponential Growth

Even organisms that grow and reproduce much more slowly than bacteria can experience exponential growth.

For example, even though a female elephant can produce a single offspring only every 2 to 4 years and newborn elephants take about 10 years to mature, if exponential growth were to take place and all descendants of a single elephant pair survived and reproduced, after 750 years there would be nearly 20 million elephants!

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Sometimes, when an organism is moved to a new environment, its population grows exponentially for a time.

Hydrilla are an example of that.


Organisms in New Environments

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Logistic Growth

In nature, exponential growth does not continue in a population for very long.

As resources become less available, the growth of a population slows. At some point, the population growth drops to zero and the population size levels off. The population may or may not remain at or near this size indefinitely.

Logistic growth occurs when a population's growth slows or stops following a period of exponential growth.


Logistic Growth

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Logistic Growth

Logistic growth is characterized by an S-shaped curve.

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Population growth may slow for several reasons.

•Growth may slow if the population’s birthrate decreases or the death rate increases—or if births fall and deaths rise together.

•In addition, population growth may slow if the rate of immigration decreases, the rate of emigration increases, or both.

•When the birthrate and the death rate are the same, and when immigration equals emigration, population growth stops. .


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Logistic Growth

Carrying Capacity

• The largest number of individuals of a population that a given environment can support is called its carrying capacity.

• When a population reaches the carrying capacity of its environment, its growth levels off. The average growth rate is zero.


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On population growth graphs, the point at which a dotted line (through the region where the population growth levels off) intersects the y-axis indicates the carrying capacity of an environment.


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