How Populations Grow

Learning Objectives Describe how ecologists study populations. Identify factors that affect population growth. Describe exponential growth. Describe logistic growth.

Essential Question

How can I explain the difference between exponential and logistic growth?

Growth Rate

Growth rate = 1

Population size is unchanged.

Growth rate > 0

Population size is growing.

Growth rate < 0

Population size is decreasing. Bacterial populationBacterial population

Population Growth

1. Births 3. Immigration

3. Emigration2. Deaths

# of individuals that enter or leave the population

Exponential Growth

Under ideal conditions with unlimited resources and protection from predators and disease, a population will grow exponentially.

Population is rapidly increasing at a constant rate.

J-shaped curve

Logistic GrowthWhen a population’s growth slows and then stops, following a period of exponential growth

S-shaped curve

Population growth may slow down when birthrate decreases, death rate increases, or both.

Growth rate equals zero at carrying capacity.

Most natural populations follow a logistic growth curve

Carrying Capacity

The maximum number of individuals of a particular species that a particular environment can support

Population stabilizes at carrying capacity.

Limits to Growth

Limiting Factors

Limiting factors determine the carrying capacity of an environment for a species.

Density dependent Density independent

Density-Dependent Factors

• Limiting factors that depend on a population size.

• Density-dependent limiting factors operate strongly when population density reaches a certain level.

Competition

• More individuals use up resources sooner.

• Individuals may compete for food, water, space, sunlight, shelter, mates, territories.

Predator–Prey Relationships

Parasitism and Disease

• Parasites and diseases can spread quickly through dense host populations.

• Stress from overcrowding can lead to lower birth rates, higher death rates, and higher emigration rates.

Density-Independent Factors

Density-independent limiting factors affect all populations regardless of population size and density.

Density-Independent Factors

• Limiting factor that DOES NOT depend on the density of the population.

•Examples: hurricanes, droughts, floods, wildfires

•May sometimes tie in with density-dependent factors

Canyon Lake, TX

Overview: Limits to Growth

Density dependent

Density independent

Flood waters cover a field of wildflowers.

Non-native snakes released into a wetland prey on native rodents.

Flu virus spreads quickly in schools.

Wildfires spread through a grassland.