PopulationsChapter 5

In Your Notebook

• Look at the picture on page 128

• Identify and Explain three factors that could cause a change in the number of red crabs on Christmas Island.

Chapter Mystery

• Read the chapter mystery on page 129

• Hypothesize why the rabbits turned into a plague in Australia.

How Populations Grow

• Hydrilla in Florida• Used in home aquariums• Farmer threw in canal• Taking over native

plants• Costs $$$$ to control

Describing Populations

• Population – group of organisms of a single species that lives in a given area

• Studied using– Geographic Range– Density and Distribution– Growth Rate– Age Structure

Describing Populations

• Geographic Range–The area inhabited by a population–Can be large•Cod in Western Atlantic ranges from

Greenland to North Carolina–Can be small•Bacteria on rotting pumpkin

Describing Populations

• Density – The number of individuals per unit area

• Distribution – page 131

Describing Populations

• Growth Rate– Determines if the size of the population increases,

decreases or stays the same– If the number of species in a given area remains

basically the same over a large amount of time the growth rate is zero

– Hydrilla in Florida has high growth rate– Cod in Atlantic have negative growth rate

Describing Populations

• Age Structure–The number of males and females of

each age a population contains–Important because:•Must reach a certain age before you

can reproduce•Only females can reproduce

Population Growth• Factors that affect population size:– Birthrate• # born

– Death rate• # that die

– Immigration rate• Moving in

– Emigration rate• Moving out

Exponential Growth

• What happens when you provide a population with everything it needs?– Food, shelter, protection from predators

• Will the population increase, decrease or stay the same?– How?

• Population produces offspring and those offspring produce offspring, etc.

Exponential Growth

• The size of each generation is larger than the one before

• The larger a population gets, the faster it grows

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

Exponential Growth

• Read “Organisms That Reproduce Rapidly” on Page 133

• How are these graphsthe same/different?

In Your Notebook

• Read the Analyzing Data Box on Page 135• Calculate the number of rabbits/generation–Generation 1: 1 pair of rabbits = 6 offspring–Generation 2: 3 pair of rabbits = 18 offspring

• Construct a graph of your data• Answer: What type of growth is the rabbit

population going through after 5 years?

Logistic Growth

• Natural populations don’t grow exponentially forever

• What happens?

Logistic Growth

• Occurs when a population’s growth slows and then stops, following a period of exponential growth

• Reasons:– Birth rate decreases– Death rate increases– Immigration decreases– Emigration increases

Logistic Growth

• Populations eventually reach their carrying capacity– The maximum number of individuals of a

particular species that a particular environment can support

In Your Notebook

• One Minute Response

–How are exponential growth and logistic growth related?

5.2 Limits To Growth

• A situation that causes the growth rate of a population to decrease is called a limiting factor. Some limiting factors depend on the size of the population. Other limiting factors affect all populations in similar ways, regardless of the population size.

In Your Notebook

• Imagine a small island that has a population of five rabbits. How might each of the following factors affect the rabbit population?a. climateb. food supplyc. predation

In Your Notebook

• Now imagine another small island that has a population of 500 rabbits. How would the same factors affect this population?– Climate, food supply, predation

• Which of the factors depend on population size? Which factors do not depend on population size?

Limiting Factors A factor that controls the growth of a population

• Determines the carrying capacity• Some depend on population density, others

do not

Density-Dependent Limiting Factors

• Affect populations when number of organisms per unit area reaches certain level

• Does not affect small, scattered populations• Types– Competition– Predation– Herbivory– Parasitism– Disease– Stress from overcrowding

Density-Dependent Limiting Factors

• Competition – Fight for food, water, space, sunlight and other

essentials– The more individuals in a given area – the faster

the resources are used up– Can also occur among members of different

species that attempt to use similar or overlapping resources

Density-Dependent Limiting Factors

• Predation, Herbivory and Human Predators

Density-Dependent Limiting Factors

• Parasitism and Disease– Parasites weaken or kill their host– If host population is large – parasites easily spread

– CPV virus killed all but 13 wolves, only 3 were female so moose population skyrocketed. But, dense population of moose allowed winter ticks to run rampant so moose began to die

Density-Dependent Limiting Factors

• Stress from overcrowding– Fighting among themselves weakens ability to

resist disease– Can cause females to abandon, kill or eat their

own offspring

In Your Notebook

• Mystery Clue–What factors do you think could

limit the size of a rabbit population?

Density-IndependentLimiting Factors

• Affect all populations, no matter what size– Hurricanes, droughts, floods, wildfires, etc.

• Sometimes hard to tell difference– When moose population got large a snow storm

covered their food source– Might not have had as big of an impact if

population had been smaller

Controlling Introduced Species• Hydrilla population controlled in natural

environment– Plant-eating insects and fish might eat it– May be weakened by pests or diseases

• Those limiting factors aren’t present in US so population grows out of control– Have tried herbicides and mechanical removal– Grass carp introduced to eat it• They must be sterile. Why?

In Your Notebook

• Make a T-Chart listing and describing limiting factors

Density-Dependent Factors Density-Independent Factors