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## Populations

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**Populations**Unit 2: Ecology**Populations**• Population—a group of individuals of the same species that live in the same area.**Characteristics**• 3 characteristics of a population: • Geographic Distribution • Density • Growth Rate**Geographic Distribution**• Geographic Distribution is often determined by availability of resources.**Population Density**• Population Density—the number of individuals per unit of area.**Factors Affecting Population Size**• Number of births • Number of deaths • Immigration and Emigration**Migration**• Immigration—movement of individuals into an area that has an existing population • Emigration—movement of individuals out of a population**Types of Growth**• Exponential • Logistic**Exponential Growth**• Exponential Growth—(also called J curve) occurs when population grows at a constant rate.**Exponential Growth**• Under ideal conditions with unlimited resources, a population will grow exponentially. • Exponential growth does not continue in natural populations for long because resources are limited.**Logistic Growth**• Logistic Growth— (also called S curve) occurs when a population’s growth slows or stops following a period of exponential growth.**Carrying Capacity**• The maximum number of organisms of a particular species that can be supported by the environment. • If the population is larger than the carrying capacity, the death rate may rise.**Resources**• Renewable Resources—a resource that can regenerate quickly and that is replaceable. • Examples: sunlight, trees, etc… • Nonrenewable Resources—a resource that cannot be replenished by natural processes. • Example: fossil fuels**Limiting Factors**• Limiting Factor—a factor that causes population growth to decrease. • Most of the time the limiting factor is a resource in insufficient supply.**Limiting Factors**• Density-dependent limiting factor • factor that only limits growth of a population when the population density reaches a certain level (overcrowded). • Examples: competition, predation, parasitism, and disease**Limiting Factors**• Density-independent limiting factors • these limit growth regardless of the population size. • Examples: unusual weather, natural disasters, seasonal cycles, and human activities