What is population?• Group of individuals of the

same species that live in the same area

Does population change?

• Sea otters, kelp, and sea urchins

•Kelp forests offer a habitat for sea otters

•Sea urchins feed on kelp

•Sea otters feed on sea urchins

• What do you think happens when sea otters are hunted?– Sea urchins increase or decrease?– Kelp forest increase or decrease?

• Sea otters are then placed on the endangered species list

• So now what happens to the population of sea otter?– Starts to increase

• How does this affect the kelp and the sea urchins?– Sea urchins start to get eaten again=decrease in #– Kelp increases b/c less sea urchins to eat them

But now we have a new hunter….

What happens to the sea otter, kelp, and sea urchins?

What does this tell us about population?

• Population changes• There are many factors that influence a

population– Natural – Unnatural

• Population density has a great impact on ecosystems

3 important characteristics of Population

• Geographic Distribution• Density• Growth Rate***Population Age structure is also

an important characteristic

Geographic Distribution

• AKA Range• Describes an area inhabited by a

population• Can vary

–Few cubic centimeters–Kilometers of the ocean

Density• # of individuals per unit area• Low density

–Cactus in desert• High density

–Other desert plants and succulents

Math Time

• Formula for calculating population density• Population density=Number of individuals

units area• Problem: Suppose there are 150 bullfrogs

living in a pond that covers an area of 3 square kilometers. What is the density of the bullfrog population?

• 50 bullfrogs per square kilometer

Growth Rate• Many factors affect growth rate

3 Factors that affect population size

• # of births• # of deaths• # of individuals that enter or leave

population

Population will increase or decrease depending on # of

individuals added or removed

• Have more births than deaths?– Population increases

• Have more deaths than births?– Population decreases

• Have equal amounts of births and deaths?– Population remains constant

What happens to the population when we….

Immigration

• “im”= in• Migrate= to move from one place to

another• Immigration is the individual movement

into an area• Animals in search of mates and food in

new areas

Emigration• “E” means ‘out’• Migrate means to move from one place

to another• Emigrate means individuals moving out of

one place and into another• Young wolves and bears leaving as they

mature• Shortage of food

Two types of growth• Exponential growth

– Individuals reproduce at a constant rate• Logistic growth

– Occurs when a populations growth slows or stops after a period of exponential growth

Exponential Growth

• Occurs under ideal conditions with unlimited resources

• Think about exponents in math….• Starts slowly then sky rockets to infinity• Our graph will look like a J• Bacteria

Lets look at bacteria…• Bacteria reproduce by splitting in half• Bacteria have a doubling time of 30 minutes• If you start will one bacterium, how many bacteria will

there be after the first 30 minutes?– 2

• After an hour?– 4

• After an hour and a half?– 8

• After two hours?– 16

• After 15 hours?– Over a billion

Logistic Growth

• As resources become less available, the growth of the population slows or stops

• S-shape curve• What we usually see in nature

Carrying Capacity• The largest number of individuals that a given

environment can support• The part of the logistic graph after the

exponential growth…the flattening out• The point at which this flat line reaches the y-

axis is the size of the population when the growth rate reaches zero

• This doesn’t mean the population stops growing• Many factors slow the growth of plants and

animals…