Warm-Up #38

• Complete Study Guide page 124

Populations

How Populations Grow

Objectives—Section 1

• List the characteristics used to describe a population.

• Identify factors that affect population size• Differentiate between exponential and logistic

growth.

Characteristics of Populations

• Three important characteristics of a population:– Geographic Distribution—the area inhabited by a

population (also known as range)– Density—The number of individuals per unit area.– Growth Rate

Population Growth

• Populations can stay the same, grow or decrease.

• Four factors affect population size and growth:– Natality (birth rate)– Mortality (death rate)– Immigration

• Movement into a population

– Emigration• Movement out of a population

Growth vs Decline

• When do populations grow?– Birthrate higher than death rate

• When do they decrease?– Death rate higher than birth rate

Exponential Growth

• Under ideal conditions with unlimited resources, a population will grow exponentially—J-shaped curve.

Logistic Growth

• At first, as populations begin to reproduce, population grows slowly.

• Then, because resources are unlimited, the population grows exponentially.

• Eventually, the rate of population growth begins to slow down. (this does not mean population is getting smaller, just growth is slowing.

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Carrying Capacity

• Carrying capacity is the largest number of individuals that a given environment can support.

• This is where the logistic growth curve will level off, and growth will slow down.

Analyzing Data page 123

Summary—Section 1

• What characteristics are used to describe a population?

• What factors affect population size?• What is exponential growth?• When does it occur?• What does the curve look like?• What is logistic growth?• What does the curve look like?

Populations

Limits to Growth

Objectives—Section 2

• Identify the factors that limit population growth.

• Differentiate between density-dependent and density-independent limiting factors.

Limiting Factors

• A limiting factor is a factor that causes population growth to decrease.

• For example—a limiting nutrient is an example of a limiting factor.

• Some limiting factors include– Competition– Predation– Parasitism and Disease– Drought and other climate extremes– Human Disturbances

Density-Dependent Factors

• A limiting factor that depends on population size is called a density-dependent limiting factor.

• These only become a factor when the population density reaches a certain level.

• They operate most strongly when the population is large and dense (not small and scattered)

• Include:– Competition —Predation– Parasitism —Disease

Competition

• Competition among members of the same species is a density-dependent limiting factor.

• The more individuals living in an area, the more they use up the resources and the more they must compete.

• Competition between two different species is a major driving force for evolutionary change.

• Both species are under pressure to change in ways that decrease their competition.

Predation

• Populations are often controlled by predation• Isle Royale• Why is It density-dependent?

Predator Prey on Isle Royale

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Parasitism and Disease

• Parasitic organisms range from microscopic (disease causing bacteria) to tapeworms more than 30 cm in length.

• Similar to predators—take nourishment at the expense of the host causing disease and sometimes even death.

• Why is it density-dependent?

How does a parasite serve as a density dependent limiting factor?

• Parasites are more likely to spread through a population (or ecosystem) if the population density is high.

• Higher density suggests organisms are– Closer to each other– Likely to interact– Likely to come in contact with feces, carcasses or

other “leftovers”

Density-Independent Factors

• Density-independent limiting factors affect all populations in similar ways, regardless of the population size.

• Include– Unusual weather– Natural disasters– Human disturbances (damming of rivers or clear-

cutting forests, for example)

Summary—Section 2

• What factors limit population growth?• Which factors are density-dependent limiting

factors?• Which are density-independent factors?

Population Growth Lab—SG 125

Homework

• Complete Population Growth Lab• Read Text 129-132• Complete Study Guide 126-127