Populations

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Chapter 16

• Populations

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What is a population?

• Population all the individuals of a species
  that live together in one place at one time.

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How Do We Describe Populations?

• Populations can be described 3 ways
• Size the number of individuals in a population
• Density - the number of individuals that live in
  a given area
• Dispersion the way the individuals are
  arranged.
• Random
• Even
• Clumped

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Population Growth

• Populations grow as more individuals are born
  than die in a given period.
• Growth rate affects population size

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Exponential Growth

• Exponential growth curve a curve that shows
  population size steadily increasing.

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Reproductive Strategies

• r- Selected (maximum growth rate, below carrying
  capacity)
• Early reproduction
• Short life span
• Little or no parental care
• High mortality rate
• Below carrying capacity
• Examples
• Grasshoppers

• K-Selected (maximizes population size near
  carrying capacity)
• Late reproduction
• Long life span
• Low mortality rate
• Extensive parental care
• At or near carrying capacity
• Examples
• Sharks
• Elephants

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R-Strategists

• r-strategists species that grow exponentially
  when environmental conditions allow them to
  reproduce.
• They usually have a short life, mature rapidly,
  and reproduce early.
• Example bacteria, mosquitoes

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K-strategists

• K-strategists populations that grow slowly.
• Usually have a long life, mature slowly, and
  reproduce late.
• Ex whales, tigers, gorillas.

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Logistic Growth

• Logistic growth model a curve that shows
  population growth limited by density-dependent
  factors.

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Population Growth

• Exponential vs. Logistical Growth
  

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What Limits Population Growth?

• Populations do not usually grow without limits.
• Their growth is limited by predation, disease and
  availability of resources.

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Limits on Population Growth

• Density Dependent Limits
• Food
• Water
• Shelter
• Disease
• Density Independent Limits
• Weather
• Climate

Water and shelter are critical limiting factors
in the desert.
Fire is an example of a Density independent
Limiting factor.
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Resources can affect population size

• Limiting factor is an environmental factor that
  prevents an organism or population from reaching
  its full potential of distribution or activity.
• Densitydependent factor is triggered by
  increasing population density, for example a food
  or water shortage. (The rate at which they are
  used up depends on the number of individuals that
  are using them).
• Densityindependent factor reduces a population
  by the same proportion regardless of the
  population size, for example, a forest fire.

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Density-Dependent and Density-Independent Effects
on Populations

• In many habitats, the forces that limit
  population sizes are independent of population
  density. For example, extreme weather events may
  decrease populations.
• For most species, density-dependent factors limit
  birth rates or increase death rates at least some
  of the time. This type of population
  determination often is referred to as
  regulation.
• Disease outbreaks and starvation are two factors
  that may increase with population density.

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Carrying Capacity

• Carrying capacity - the largest population that
  an environment can support at any given time. (k)

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Human Population Growth

• Human population growth does not currently show
  density effects that typically characterize
  natural populations.
• In natural populations, per capita population
  growth rate decreases with population size,
  whereas global human population growth rate has a
  positive relationship.
• Human population growth rate has been growing
  more than exponentially.
• Limited resources eventually will cause human
  population growth to slow, but global human
  carrying capacity is not known.

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Important Vocab Words

• Population
• Population Density
• Population Size
• Population Dispersion
• Carrying Capacity
• Exponential Growth Curve
• R-strategists
• K-strategists
• Density-Dependent Factor
• Density-Independent Factor