Population Biology

Rainforest Movie Questions1. Describe what happens in the rainforest when a

large tree falls.2. Describe the mating rituals of jungle birds.3. Explain why some of the earth’s largest rivers are

found in rainforests.4. Describe the importance of decomposers in the

rainforest.5. Explain how the “pitcher plant” gets its nutrients

in very nutrient poor soil. 6. Explain why most animals in the rainforest are

small.

Population Growth

• Population growth is the change in the size of a population over time.

• Populations of organisms do not experience linear growth, instead they grow exponentially, when the number of organisms rise at an ever-increasing rate.

• It results in a population explosion and a J-shaped curve on a graph.

Graph of Exponential GrowthPOP

U LATION

SIZE

TIME

Environmental Limits• Populations can not grow forever.• There are limiting factors, such as food availability

and space, that limit the size of a population. • Eventually, all populations will level off then they

reach carrying capacity- the number of organisms of a population that a particular environment can support over an indefinite period of time.

• The leveling off of the population results in a s-shaped graph.

Population growth when carrying capacity is reached

Carrying capacity

TIME

POPULATION

SIZE

Estimating populations• Scientists find that obtaining an exact number for

a population of organisms is very difficult if not impossible.

• Why? • Most animals do not just line up and wait to be

counted!!! They wander or migrate. • Plants and non-mobile species are easier. • Scientists use 2 different techniques to help them

to estimate the size of a population…….

Sampling

• Sampling- organisms in a few small areas are counted and projected for the entire area.

• Ex. If there are 5 deer living in a 200 square foot area, how many would there probably be living in the surrounding 2000 square foot area?

Mark and recapture• Mark and recapture- animals are captured alive,

marked or tagged, and then returned unharmed into their natural environment.

• Over time, the animals from the population are continued to be trapped and data is recorded on how many are captured with tags.

• Then the population size is estimated mathematically……..

Total # captured X Total # marked/tagged = Estimate of Total PopulationTotal # recaptured with mark/tag

Bellwork1. Describe what type of growth do populations

exhibit.2. Explain carrying capacity. How do you know

when a population exceeds the carrying capacity?

3. Explain why is it difficult to find out an exact number of a population.

4. Describe the 2 techniques that scientists use to estimate population size.

Patterns of Population Growth• All population levels do not reach carrying capacity

and remain stable.• It depends on the most important factor that

determines how a population grows- the organisms reproductive pattern.

• There are 2 growth patterns…..• Rapid reproduction pattern- Some species

reproduce rapidly, produce many offspring, have a small body size, mature rapidly, and have a short life span.

• These organisms are successful in environments that are unpredictable.

• These populations increase rapidly and then decline rapidly as environmental conditions suddenly change and become unsuitable.

• Ex. Mice, insects, rabbits……• Slow reproduction pattern- Some species have a

slow rate of reproduction, have relatively few young over their lifetimes, live in stable environments, are large in size, reproduce and mature slowly.

• Ex. Elephants, whales, California Redwood…..

Environmental Limits to Population Growth

• Limiting factors (last chapter) can also regulate the size of a population.

• There are 2 main kinds……1.Density-dependent factors- have an increasing

effect as the population size increases.• Ex. Disease, competition for

food/shelter/space, parasites……

2. Density-independent factors- have the same affect on all populations, regardless of the density or size.

• Ex. Temperature, floods, storms, drought, habitat destruction……..

Other influences on population growth and size

• Predator and prey relationships will affect population size.

• Competition for resources can affect a population’s size.

• Crowding and stress can negatively affect a population and cause a decrease in population size.

Bell Work1. Explain why insects need to mature quickly.2. Explain in your own words how disease can

be a density -dependent factor.3. Explain in your own words how cutting down

a forest is a density-independent factor.4. Compare the care given by the mothers who

fall into a rapid reproductive pattern and care given by mothers who fall into a slow reproductive pattern.

Bell Work1. Describe the pattern of population growth of

a predator and prey relationship. You can use a fox/rabbit example if necessary.

2. Explain why a flood is a density-independent factor.

3. Describe an environmental factor that might have an effect on a population of individuals who reproduce rapidly. What effect will it have on the population?

Human Population Growth• Demography is the study of population growth

characteristics.• Scientists can study……• Birth rates- the amount of children being born into a

population in a period of time• Fertility rates- number of offspring a female produces

during her reproductive years• Death rates- the amount of individuals dying in a

population in a period of time.• Immigration- movement of individuals into a

population.• Emigration- movement of individuals out of a

population

YEARS

POPULATIONINBILLIONS

• The birth rates, fertility rates, and death rates are directly linked to…..

• Education• Health care• Sanitation• Births and immigration add to a population’s

size.• Deaths and emigration subtract from a

population’s size.

Bell Work1. Compare immigration and emigration.2. Describe what can increase the size of a

population.3. Explain why Kenya has higher birth rates and

death rates than the Unites States does.4. Describe the type of population growth that

humans have had in the past 150 years.

Age Structures

• An age structure is a type of graph that shows the proportion of a population that is in a certain age range.

• By looking at an age structure, you can tell if a population is growing rapidly, growing slowly, or not growing at all.

Answer the following questions for each age structure, making sure to

label the countries.1. Calculate the total population for this

country.2. Describe the type of growth that this country

is currently experiencing. Explain why!!3. Using your answer to number 2, describe the

education, health care and sanitation likely in this country.