THINK ABOUT IT
People in New England who fish for a living face a problem. Their catch has dropped dramatically, despite hard work and new equipment. The cod catch in one recent year was 3,048 metric tons. Back in 1982, it was 57,200 metric tons—almost 19 times higher! Where did all the fish go? Can anything be done to increase their numbers?
Lesson Overview
How Populations Grow
Describing Populations
How do ecologists study populations?
Researchers study populations’ geographic range, density, distribution, growth rate, and age structure.
Lesson Overview
How Populations Grow
Geographic Range
The area inhabited by a population is called its geographic range.
A population’s range can vary enormously in size, depending on the species.
Lesson Overview
How Populations Grow
Calculating Population Density
Population = # of Individuals
density size of Area
1 km
0.7 km
0.5 km
0.4 km
D = 9 = 9 = 13 trees
1 x 0.7 0.7 sq.km
D = 9 = 9 = 45 trees
.5 x .4 0.2 sq.km
Lesson Overview
How Populations Grow
Distribution
Distribution refers to how individuals in a population are spaced out across the range of the population—randomly, uniformly, or mostly concentrated in clumps.
Lesson Overview
How Populations Grow
Growth Rate
A population’s growth rate determines whether the population size increases, decreases, or stays the same.
Lesson Overview
How Populations Grow
Age Structure
To fully understand a plant or animal population, researchers need to know the population’s age structure—the number of males and females of each age a population contains.
Lesson Overview
How Populations Grow
Population Growth
What factors affect population size?
The factors that can affect population size are the birthrate, death rate, and the rate at which individuals enter or leave the population.
Lesson Overview
How Populations Grow
Exponential Growth
What happens during exponential growth?
Under ideal conditions with unlimited resources, a population will grow exponentially.
In one day, this bacterial population will grow to 4,720,000,000,000,000,000,000 individuals.
If this growth continued without slowing down, this bacterial population would cover the planet within a few days!
Lesson Overview
How Populations Grow
Example of exponential population growth in nature: organisms coming into a new area.
Lesson Overview
How Populations Grow
Logistic Growth
What is logistic growth?
Logistic growth occurs when a population’s growth slows and then stops, following a period of exponential growth.
Time (hours)
Carrying capacity
Lesson Overview
How Populations Grow
Natural populations don’t grow exponentially for long.
Sooner or later, something stops exponential growth. What happens?
Limiting Factors
Density Dependent vs Independent
- Extreme weather events
- Fire
- Flood
- Deforestation
Lesson Overview
How Populations Grow
Carrying Capacity
Carrying capacity is the maximum number of individuals of a particular species that a particular environment can support.
Once a population reaches the carrying capacity of its environment, a variety of factors act to stabilize it at that size.
What factors determine carrying capacity?
Acting separately or together, limiting factors determine the carrying capacity of an environment for a species.
Lesson Overview
How Populations Grow
Review
What limiting factors depend on population density?
Density-dependent limiting factors include competition, predation,
herbivory, parasitism, disease, and stress from overcrowding.
What limiting factors do not typically depend on population density?
Unusual weather such as hurricanes, droughts, or floods, and natural disasters such as wildfires, can act as density-independent limiting factors.
Lesson Overview
How Populations Grow
True Density Independence?
On Isle Royale, for example, the moose population grew exponentially for a time after the wolf population crashed. Then, a bitterly cold winter with very heavy snowfall covered the plants that moose feed on, making it difficult for moose to move around to find food.
Because this was an island population, emigration was not possible. Moose weakened and many died.
In this case, the effects of bad weather on the large, dense population were greater than they would have been on a small population. In a smaller population, the moose would have had more food available because there would have been less competition.
Lesson Overview
How Populations Grow
How has human population size changed over time?
The human population, like populations of other organisms, tends to increase. The rate of that increase has changed dramatically over time.
Lesson Overview
How Populations Grow
The Predictions of Malthus
This kind of exponential growth could not continue forever.
Two centuries ago, English economist Thomas Malthus suggested that only war, famine, and disease could limit human population growth.
Malthus thought that human populations would be regulated by competition (war), limiting resources (famine), parasitism (disease), and other density-dependent factors.
Malthus’s work was vitally important to the thinking of Charles Darwin.
Exponential growth continued up to the second half of the twentieth century, reaching a peak around 1962–1963, and then it began to drop.
The size of the global human population is still growing rapidly, but the rate of growth is slowing down.
Lesson Overview
How Populations Grow
Patterns of Human Population Growth
Why do population growth rates differ among countries?
Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.
Lesson Overview
How Populations Grow
The Demographic Transition
In Stage I, birthrates and death rates are high for most of history.
In Stage II, advances in nutrition, sanitation, and medicine lead to lower death rates.
During Stage III, as the level of education and living standards rise, families have fewer children and the birthrate falls; population growth slows
So far, the United States, Japan, and Europe have completed the demographic transition. Parts of South America, Africa, and Asia are passing through Stage II. A large part of ongoing human population growth is happening in only ten countries, with India and China in the lead.
Lesson Overview
How Populations Grow
Age Structure and Population Growth
In the United States, there are nearly equal numbers of people in each age group.
This age structure predicts a slow but steady growth rate for the near future.
In Guatemala, on the other hand, there are many more young children than teenagers, and many more teenagers than adults.
This age structure predicts a population that will double in about 30 years.
Lesson Overview
How Populations Grow
Future Population Growth
To predict how the world’s human population will grow, demographers consider many factors, including the age structure of each country and the effects of diseases on death rates—especially AIDS in Africa and parts of Asia.
Current projections suggest that by 2050 the world population will reach 9 billion people.
The human population may level out to a logistic growth curve and become stable if countries that are currently growing rapidly complete the demographic transition.
Lesson Overview
How Populations Grow