Limits to Growth

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Limits to Growth

• 5-2

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Limiting Factors

• Remember, primary productivity of an ecosystem
  can be reduced by limiting nutrients
• In the context of populations, limiting factors
  can cause population size to decrease

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• A resource base that is limited can also affect
  the long-term survival of a species
• Ex pandas and bamboo

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Density-Dependant Factors

• Limiting factors that depend on population size
• Density-dependent factors become limiting only
  when population density reaches a certain level
• Do not affect small, scattered populations

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• Examples of density-dependent limiting factors
• Competition
• Predation
• Parasitism
• Disease

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Competition

• Occurs when populations become crowded
• All organisms in a population need sunlight,
  food, water, space, etc.
• Competition among members of the same species is
  density dependent

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• The more members in the population, the faster
  the resources are used up
• Competition also happens between members of
  different species
• Major force behind evolutionary change
• When 2 species compete for same resource, they
  are under pressure to change
• Over time, evolve to occupy diff. niches

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Predation

• Populations in nature are controlled by predation
• Known as predator-prey relationship
• Ex
• sea otters and urchins
• Sea otters and whales

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Isle Royale

• Well known example of predator-prey relationship
  is the moose/wolf populations on Isle Royale in
  Lake Superior
• When moose numbers are high, wolf s increase
• As wolves feed on moose, moose s decrease,
  causing a decrease in wolves
• With fewer wolves, moose s can increase again

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Parasitism Disease

• Parasites can also limit population growth
• Parasites are similar to predators weaken and
  kill hosts
• Large populations can be under stress making
  them more susceptible to diseases
• More members easier to pass things around

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Density-Independent Factors

• Factors that affect populations regardless of
  their size
• Examples
• Unusual weather
• Season cycles
• Human activities (damming rivers, clear
  cutting)

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• Populations usually respond to such factors with
  a crash in population size
• After crash, numbers may increase quickly, or
  stay low for awhile
• Storms and hurricanes can wipe out populations of
  insects

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• Extreme cold or hot weather can also take a toll
  on populations
• Droughts can affect entire populations of
  vegetation, which can affect populations of
  consumers

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• Environments are always changing
• Most populations can adapt to a certain amount of
  change (grow or shrink)
• Major upsets in ecosystems can lead to long-term
  decline in certain populations
• Human activities have caused some of these upsets

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5-3 Human Population Growth

• Like the populations of many other living
  organisms, the size of the human population tends
  to increase with time.
• For most of human existence, the population grew
  slowly
• Until fairly recently, only half the children in
  the world survived to adulthood.

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• About 500 years ago, the human population began
  growing more rapidly.
• Agriculture and industry made life easier and
  safer.
• The world's food supply became more reliable, and
  essential goods could be shipped around the
  globe.
• Improved sanitation, medicine, and health care
  dramatically reduced the death rate and increased
  longevity.
• With these advances, the human population
  experienced exponential growth, as shown in the
  figure at right.

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Patterns of Population Growth

• English economist Thomas Malthus observed that
  human populations were growing rapidly.
• Malthus predicted that such growth would not
  continue indefinitely.
• Instead, according to Malthus, war, famine, and
  disease would limit human population growth.

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• Scientists have identified a variety of other
  social and economic factors that can affect human
  populations.
• The scientific study of human populations is
  called demography
•   Birthrates, death rates, and the age structure
  of a population help predict why some countries
  have high growth rates while other countries grow
  more slowly.

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Demographic Transition

• Over the past century, population growth in the
  United States, Japan, and much of Europe has
  slowed dramatically.
• Demographers have developed a hypothesis to
  explain this shift.
• According to this hypothesis, these countries
  have completed the demographic transition, a
  dramatic change in birth and death rates.

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Age Structure Diagram

• Demographers can predict future growth using
  models called age-structure diagrams, or
  population profiles.
• Age-structure diagrams show the population of a
  country broken down by gender and age group.
• Each bar in the age-structure diagram represents
  individuals within a 5-year group. Percentages of
  males are to the left of the center line and
  females to the right in each group.

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Future Population Growth

• To predict how the world's human population will
  grow, demographers must consider many factors
  including
• the age structure of each country
• prevalence of life-threatening diseases