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Warm-Up #38
• Complete Study Guide page 124
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Populations
How Populations Grow
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Objectives—Section 1
• List the characteristics used to describe a population.
• Identify factors that affect population size• Differentiate between exponential and logistic
growth.
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Characteristics of Populations
• Three important characteristics of a population:– Geographic Distribution—the area inhabited by a
population (also known as range)– Density—The number of individuals per unit area.– Growth Rate
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Population Growth
• Populations can stay the same, grow or decrease.
• Four factors affect population size and growth:– Natality (birth rate)– Mortality (death rate)– Immigration
• Movement into a population
– Emigration• Movement out of a population
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Growth vs Decline
• When do populations grow?– Birthrate higher than death rate
• When do they decrease?– Death rate higher than birth rate
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Exponential Growth
• Under ideal conditions with unlimited resources, a population will grow exponentially—J-shaped curve.
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Logistic Growth
• At first, as populations begin to reproduce, population grows slowly.
• Then, because resources are unlimited, the population grows exponentially.
• Eventually, the rate of population growth begins to slow down. (this does not mean population is getting smaller, just growth is slowing.
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Carrying Capacity
• Carrying capacity is the largest number of individuals that a given environment can support.
• This is where the logistic growth curve will level off, and growth will slow down.
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Analyzing Data page 123
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Summary—Section 1
• What characteristics are used to describe a population?
• What factors affect population size?• What is exponential growth?• When does it occur?• What does the curve look like?• What is logistic growth?• What does the curve look like?
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Populations
Limits to Growth
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Objectives—Section 2
• Identify the factors that limit population growth.
• Differentiate between density-dependent and density-independent limiting factors.
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Limiting Factors
• A limiting factor is a factor that causes population growth to decrease.
• For example—a limiting nutrient is an example of a limiting factor.
• Some limiting factors include– Competition– Predation– Parasitism and Disease– Drought and other climate extremes– Human Disturbances
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Density-Dependent Factors
• A limiting factor that depends on population size is called a density-dependent limiting factor.
• These only become a factor when the population density reaches a certain level.
• They operate most strongly when the population is large and dense (not small and scattered)
• Include:– Competition —Predation– Parasitism —Disease
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Competition
• Competition among members of the same species is a density-dependent limiting factor.
• The more individuals living in an area, the more they use up the resources and the more they must compete.
• Competition between two different species is a major driving force for evolutionary change.
• Both species are under pressure to change in ways that decrease their competition.
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Predation
• Populations are often controlled by predation• Isle Royale• Why is It density-dependent?
Predator Prey on Isle Royale
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Parasitism and Disease
• Parasitic organisms range from microscopic (disease causing bacteria) to tapeworms more than 30 cm in length.
• Similar to predators—take nourishment at the expense of the host causing disease and sometimes even death.
• Why is it density-dependent?
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How does a parasite serve as a density dependent limiting factor?
• Parasites are more likely to spread through a population (or ecosystem) if the population density is high.
• Higher density suggests organisms are– Closer to each other– Likely to interact– Likely to come in contact with feces, carcasses or
other “leftovers”
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Density-Independent Factors
• Density-independent limiting factors affect all populations in similar ways, regardless of the population size.
• Include– Unusual weather– Natural disasters– Human disturbances (damming of rivers or clear-
cutting forests, for example)
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Summary—Section 2
• What factors limit population growth?• Which factors are density-dependent limiting
factors?• Which are density-independent factors?
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Population Growth Lab—SG 125
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Homework
• Complete Population Growth Lab• Read Text 129-132• Complete Study Guide 126-127