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Biology Chapter 5 Characteristics of Populations • Geographic Distribution – the area inhabited by a population • Population Density – number of individuals per unit area • Growth Rate – an increase or decrease of the number of individuals in a population over time Population Growth/Decline • A population can grow when its birthrate is greater than its death rate • Immigration – movement of individuals into an area • Populations can increase • Animals maybe searching for mates or food • Emigration – movement of individuals out of an area • Populations can decrease • Animals leave to find mates or food • Exponential Growth • Under ideal conditions with unlimited resources a population will grow exponentially resulting in a population explosion • Exponential growth is characterized by a J-shaped curve • In nature exponential growth does not continue in a population for very long • As resources become less available, the growth of a population slows or stops Exponential Growth Exponential Growth Copyright Pearson Prentice Hall • Logistic Growth • Occurs when a population’s growth slows or stops following a period of exponential growth • Logistic growth is characterized by a S-shaped curve initial carrying capacity new carrying capacity Copyright Pearson Prentice Hall Fig. 46.6, p. 818 • What prevents the world from being overrun with all kinds of living things? • Carrying Capacity – the number of organisms of one species that an environment can support • When a population reaches the carrying capacity of its environment, its growth levels off Limiting Factors • Limiting Factor – a factor that causes population growth to decrease • Density-Dependent Factors – a limiting factor that depends on population size • These factors operate most strongly when a population is large and dense • As populations increase so do these factors • Example: competition, predation, parasitism, disease • Density-Independent Factor – a limiting factor that affects all populations regardless of the population size • Example: natural disasters, storms, floods, drought, pollution • Demography – the study of human population growth • For most of human existence, the population grew slowly • Limiting factors kept population sizes low • About 500 years age, the human population began growing more rapidly • Life was made easier and safer by advances in agriculture and industry • Death rates were dramatically reduced due to improved sanitation, medicine, and healthcare, while birthrates remained high • With these advances, the human population experienced exponential growth Historical Overview Human Population Growth Copyright Pearson Prentice Hall • Age Structure • Age-Structure Diagrams – show the population of a country broken down by gender and age group • Population growth depends, in part, on how many people of different ages make up a given population • Demographers can predict future growth using agestructure diagrams Patterns of Population Growth Rapid Slow Zero Negative Growth Growth Growth Growth Copyright Pearson Prentice Hall • How can you tell if a population is growing? • If the birth rate is more than the death rate • Does it make a difference to population growth if the largest proportion of the population is in one age group? • If a large percent of the population is teenagers and children there is rapid growth • If the percent of people in each category is equal the population is stable