Bell Ringer

1. What critical function do nitrogen fixers serve?2. What would happen to an ecosystem if the

nitrogen fixers died out?3. State whether primary or secondary

succession will occur after:a. A glacier melts.b. A wildfire.c. Nature reclaims abandoned farmland.d. A volcanic eruption.

Population GrowthWhy do populations grow and shrink?

Populations

• Recall from chapter 1 that a population is the entire number of one organism.• Populations do not stay static; instead, the size of

populations changes with the availability of resources and the presence of predators.• Because of this, the size of one population is

often dependent on the health of other populations!

Populations

• For example, take the following food chain:• Kelp →Sea Urchin → Sea Otter → Killer Whale

• At one point in time, otters were almost hunted to extinction. As a result, sea urchins grew out of control, and kelp was being overconsumed.• As otters became a protected species, they

managed to bring the urchin population under control. Kelp also was restored to healthy levels.• However, killer whales are also protected, and as

their numbers recover, they eat more and more otters. Urchin numbers are expanding as well.

Populations• Populations have

four important characteristics:• Geographic

Distribution• Density• Growth Rate• Age Structure

Geographic Distribution• A population’s

geographic distribution is the relative area that a population inhabits.• For instance, if we

look at whales in the Pacific Ocean, the entire ocean would be the geographic distribution.

Population Density• Population density is

the total number of one kind of organism per unit area.• For example, saguaro

cactus in the Mojave desert has a low density, because there are very few saguaro cacti in that desert.

Growth Rate• A population’s growth

rate is the rate of change in the number of organisms that make up that population.• It can be positive, if

the population is growing, or negative, if the population is shrinking.

Growth Rate• Growth rate is itself

affected by three important factors:

1. The total number of births

2. The total number of deaths

3. The overall number of organisms that migrate in or out of the population.

Growth Rate

Population Increase• If more births than

deaths occur, then the population will increase.• If organisms

immigrate – move into the area – then the population will increase.

Population decrease• If more deaths than

births occur, then the population will decrease.• If organisms

emigrate – move out of the area – then the population will decrease.

Growth Rate• Note that both

immigration and emigration occur as organisms search for new resources, like food, water and mates.• Also, those two events

happen as organisms enter and leave to establish new territories.

Exponential Growth• Exponential growth

is a level of growth that occurs under ideal conditions.• For exponential

growth to occur, resources MUST be unlimited, and conditions MUST be ideal.

Exponential Growth• Note the graph to the

right. This is a good illustration of exponential growth.• Growth happens at a

constant rate.• At first, growth occurs

slowly, but over time, growth speeds up, until the population’s size approaches infinity.

Logistic Growth• In reality, exponential

growth is unsustainable. As growth increases, resources are used up at faster levels.• Once resources begin

to grow scarce, population growth slows or stops.

Logistic Growth• Note the graph to the

right. This is a good illustration of logistic growth.• The population grows

only up to a certain point. After reaching that point, growth tapers off.

Logistic Growth• We call this point the

carrying capacity. It is the maximum number of organisms that this ecosystem can viably support.

Exit Ticket

Select the correct word to finish each sentence.1. If the birth rate is greater than the death rate,

the growth rate will be [positive, negative].2. [Immigration, Emigration] happens when

organisms move out of their old habitat. This migration [raises, lowers] the growth rate.

3. Exponential growth occurs when resources are [limited, unlimited] and conditions are ideal.

4. Exponential growth is [sustainable, unsustainable].

Age Structure• The age structure of

a population refers to the relative age of all members in the population.• This is important

because age structure can predict future growth!

Age Structure

General Notes:• Each half of the

graph relates to males and females.• The bottom of the

graph relates to young individuals, and the top to older individuals.

Age Structure

General Notes:• The wider the

bottom, the greater the birth rate.• The narrower the

bottom, the lower the birth rate.

Age Structure

General Notes:• The wider the top,

the lower the death rate (also called mortality).• The narrower the

top, the greater the death rate.

Age Structure• If the curve is

bottom-heavy, then the overall population is young.• This pattern predicts

population growth.

Age Structure• If the curve is top

heavy, then the overall population is old.• This pattern predicts

a shrinking population.

Exit Ticket1. Is the birth rate high or low?2. Is the mortality rate high or low?3. Does this age curve predict positive population

growth? Yes or no?

Limiting Factors• Population growth is

opposed by many factors, such as disease, predation and human activity.• We call these limiting

factors, because they limit the growth of other populations.

Limiting Factors

Density-dependent• Density-dependent

limiting factors are more effective on larger populations.• Predation• Parasitism• Competition

Density-independent• Density-independent

limiting factors are equally effective on small and large populations.• Human activity• Drought and other

climate changes