Ch. 5 Populations

Ch. 5 Outline 5-1: How Populations Grow Characteristics of Populations Population Growth Exponential Growth Logistic Growth 5-2: Limits of Population Growth Density-Dependent Factors Density-Independent Factors 5-3: Human Population Growth Patterns of Population Growth

Characteristics of Populations Geographic Distribution Density Growth Rate Age Structure Geographic Distribution (or range) is the area inhabited by a population Population density: the number of individuals per unit area

Population Growth Three factors can affect population size: The number of births The number of deaths The number of individuals leaving or entering the population A population is growing when its birthrate is greater than its death rate.

Population Growth The movement of Individuals also affects population size. Immigration: The movement of individuals to an area Emigration: The movement of individuals out of an area What might cause the movement of individuals either to or from and area? Food Supply Finding a mate

Exponential Growth Exponential growth: occurs when individuals in a population reproduce at a constant rate A population will grow exponentially under ideal conditions: No predation Plenty of Food and Space The graph of a population growing exponentially: J-shaped Curve Bacteria often grow exponentially

Exponential Growth Curve

Logistic Growth Logistic Growth: occurs when a population’s growth slows or stops following a period of exponential growth Takes into account a limited amount of resources Graph of Logistic Growth: S-shaped Curve The largest number of individuals that a given environment can support is called the carrying capacity.

Limits to Population Growth What causes population size to decrease? Limiting Factors: a factor that causes population growth to decrease Examples of Limiting Factors: Competition Predation Parasitism Drought (or other climate extremes) Human Disturbances

Density-Dependent Factors A limiting factor that depends on population size is called a density-dependent factor. They only become limiting when the population density reaches a certain level. (They don’t affect small, scattered populations)

Examples of Density-Dependent Factors Competition When populations are crowded, organisms begin competing for food, space, mates, sunlight, etc… Competition between two different species also exists  leads to adaptations and natural selection (EVOLUTION!!!) Predation The predator-prey relationship is one of the best known mechanisms for population control. Ex. Moose-wolf population near Lake Superior

Examples of Density-Dependent Factors Parasitism and Disease Parasites take nourishment at the expense of their host, often weakening them (leading to death).

Density-Independent Factors Density-Independent Factors: affect all populations in similar ways, regardless of the population size. Examples: Weather/seasonal changes Natural disasters Human Activities

Historical Overview For most of history, the human population has grown very slowly About 500 years ago, agriculture and industry made life easier and the population growth rate increased. With the Industrial Revolution came improved medical care and increased sanitation, which lead to exponential human population growth.

Patterns of Population Growth The Earth cannot sustain exponential population growth forever. It DOES have a carrying capacity. (Resources are limited!) Demography: the study of human populations Attempts to explain how they will change over time Birthrates, death rates, and age structures of populations help predict why some countries have high growth rates while others have low growth rates.

Patterns of Population Growth In the last century, Japan, the United States, and much of Europe have slowed their population growth. Why? With improved nutrition, sanitation, and medical care, the death rate was slowing and the birthrate was still high (cause an increase in the population). BUT, as societies become more modern, families are tending to have less children (as education level and standards of living increase).

Patterns of Population Growth However, the worldwide population is still growing exponentially. Population growth also depends on age structure. How many people of different ages make up a population? Age-Structure Diagrams: Graph the number of people in different ages in a population Ex: USA vs. Rwanda.