# Chapter 20 POPULATIONS.

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Chapter 20 POPULATIONS

20.1 Understanding Populations
As the human population grows, the population of other species declines Population: a group of organisms that belong to the same species and live in a particular place at the same time

Population Size The actual population size can be difficult to measure
Individuals can be too abundant, too widespread, or too mobile to be easily counted

Sampling a Population When it isn’t possible to count every member of the population, a sample is taken From that sample, the population can be estimated

Population Density Measures how crowded a population is
Expressed as the number of individuals per unit of area or volume Estimates are calculated for the total land area

Dispersion The spatial distribution of individuals within a population
Clumped: clustered together Even: separated by the same amount of space Random: no pattern

Clumped Distribution Food or living space may be clumped
Social behavior (herds) Flocking behavior

Even Distribution Social interactions Colonial nesting behavior
V-pattern in flight (waterfowl migration)

Random Distribution Results from seed dispersal by wind or by birds
Example: forests or a field of wildflowers

Population Dynamics Birth rate: the number of births occurring over a certain period of time Death rate/mortality rate: the number of deaths over a period of time Life expectancy: how long an individual is expected to live (the average life span)

Age Structure Shows the distribution of individuals in different age groups in a population Populations with a higher percentage of younger individuals will tend to have rapid growth

Patterns of Mortality Survivorship curves: show the likelihood of survival at different ages throughout the life of an organism Type I, II, or III

Types of Survivorship Curves
Type I: the chance of dying is small until late in life Type II: the chance of dying is the same regardless of age Type III: the chance of dying while young is great

20.2 Measuring Populations
Growth Rate: the amount by which a population changes size in a given amount of time Depends on birth, death, emigration and immigration Birth rate – death rate = growth rate

Immigration vs. Emigration
Immigration: movement INTO a population Emigration: movement OUT OF a population

The Exponential Model Describes a population that increases rapidly after only a few generations The larger the population gets, the faster it grows J-shaped curve Will become limited by not enough food and too much wastes (limiting factors)

The Logistic Model Describes a population that becomes stable at the carrying capacity Carrying capacity: the number of individuals the environment can support over a long period of time S-shaped curve

Population Regulation
Density-independent factors: reduce a population regardless of the population size (weather, natural disasters, fire) Density-dependent factors: the higher the population, the more organisms that are affected (food, territory, nest sites)

Population Fluctuation
Populations can fluctuate due to environmental changes Example: population cycle of the snowshoe hare and the lynx

Perils of Small Populations
Small populations are at risk of extinction Vulnerable to inbreeding, disease, environmental disturbances, etc. Inbred offspring have shorter life spans, decreased genetic variability, and are more susceptible to diseases

20.3 Human Population Growth
Early human populations lived in small nomadic groups and obtained food by hunting and gathering Had a high mortality rate, especially among infants and children

The Agricultural Revolution
10,000 years ago, humans domesticated animals and cultivated plants Agriculture increased the available food supply Human population began to grow faster

The Industrial Revolution
After the Middle Ages, 25% of the human population in Europe died due to the bubonic plague The industrial revolution began around 1780 when there was a shift to using machinery and burning fossil fuels Improvements in hygiene and in health care led to population growth

Developed Countries Developed countries: modern, industrialized areas with better health care, longer life expectancies, but lower population growth Examples: U.S., Japan, Canada, Russia, Germany, Australia Includes only 20% of the world’s population, but uses 75% of all fossil fuels!

Developing Countries Developing countries: poorer areas, lower life expectancy, but higher population growth Examples: Central America, South America, Mexico, Africa Includes 80% of the world’s population, but uses only 25% of the fossil fuels globally

Human Population Explosion
By 2020, the human population could be as high as 12.5 billion Will Earth be able to sustain that amount of people or will the human population reach its carrying capacity?