Environmental Science: Population Principles

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Environmental Science Population Principles

• Chapter 7

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Population Characteristics

• Population - Group of individuals of the same
  species inhabiting the same area simultaneously.

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Historical Human Population Growth
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Population Growth in Different countries
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Natality and Mortality

• Natality - Number of individuals added through
  reproduction.
• Birth Rate (Humans Born / 1,000)

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Natality and Mortality

• Mortality - Number of individuals removed via
  death.
• Death Rate (Humans Died / 1,000)

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Population Density and Spatial Distribution

• Population Density - Number of individuals per
  unit area.
• High population may lead to increased competition
  for resources.
• Dispersal - Movement of individuals from densely
  populated locations to new areas.
• Emigration - Movement from an area.
• Immigration - Movement into an area.

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Sex Ratio

• Relative number of males and females in a
  population.
• Females determine the number of offspring
  produced in sexually reproducing populations.

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Population Characteristics

• Population Growth Rate - Birthrate minus the
  death rate. Often expressed as a percentage of
  the total population.

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Survivorship Curve

• - Shows proportion of individuals likely to
  survive to each age.
• High mortality in young.
• Mortality equitable among age classes.
• Mortality high only in old age.

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Age Distribution

• Age Distribution - Number of individuals of each
  age in the population.
• Greatly influences population growth rate.

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Age Distribution in Human Populations
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Population Growth Curve

• Biotic Potential - Inherent reproductive
  capacity. (Biological ability to produce
  offspring)
• Generally, biotic potential is much above
  replacement level.
• Natural tendency for increase.
• All living populations follow an exponential
  growth curve.

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Exponential Growth Curve

• Lag Phase - First portion of the curve slow
  population growth.
• Exponential Growth Phase (Log Phase)- More
  organisms reproducing causing accelerated growth
  continues as long as birth rate exceeds death
  rate.
• Stable Equilibrium Phase - Death rate and birth
  rate equilibrate population stops growing.

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Typical Population Growth Curve
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Carrying Capacity

• Carrying Capacity - Number of individuals of a
  species that can be indefinitely sustained in a
  given area without harming the habitat.
• Dependent and varies with type of habitat

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Carrying Capacity
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Environmental Resistance

• Environmental Resistance - Any factor (limiting
  factor) in the environment limiting carrying
  capacity.
• Four main factors
• Raw Material Availability
• Energy Availability
• Waste Accumulation and Disposal
• Organism Interactions

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Reproductive Strategies and Population
Fluctuations

• Not all species reach a stable carrying capacity.
• Species can be broadly lumped into two
  categories
• K-strategists
• r-strategists

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Population growth death phase

• If growth exceeds carrying capacity
• Environmental resistance lower pop size
• Population enter the Death phase
• Threatened
• Endangered
• Extinct

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K strategist

• Elephants, humans, cougar
• Low biotic potential
• Rely on learning
• Long gestation
• Lower population growth

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K - Strategists

• Large organisms
• Long-lived
• Produce few offspring
• Provide substantial parental care
• Populations typically stabilize at a carrying
  capacity.
• Usually occupy relatively stable environments.
• Reproductive strategy is to invest in a few,
  quality offspring.

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K - Strategists

• Controlled by density-dependent limiting factors.
• Factors that become more severe as the size of
  the population increases.
• Diseases
• Deer - Lions - Swans

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R strategist

• Insects, fish, frogs
• Short lives
• High bio potential
• High reproductive rate
• Rely on instinct

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r - Strategists

• Small organisms
• Short-lived
• Produce many offspring
• Little if any parental care
• Usually do not reach carrying capacity (boom-bust
  cycles).
• Exploit unstable environments.
• Reproductive strategy is to produce large numbers
  of offspring to overcome high mortality.

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r - Strategists

• Controlled by density-independent limiting
  factors.
• Population size is irrelevant to the limiting
  factor.
• Weather Conditions
• Grasshoppers - Gypsy Moths - Mice

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Human Population Growth

• Major reason for increasing human population
  growth rate is an increase in medical care, and a
  consequential decrease in death rates.

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Doubling Time

• Doubling Time of a Population (years)
• 70 / Population Growth Rate ()
• ( 70 / 2.0 35 years )

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Doubling Time for the Human Population
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Human Population Growth

• Interactions Affecting Carrying Capacity
• Available Raw Materials
• Available Energy
• Waste Disposal
• Interaction With Other Organisms

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Social Factors Influence Human Population

• Humans are social animals who have freedom of
  choice.
• People make decisions based on history, social
  situations, ethical and religious beliefs, and
  personal desires.
• Biggest obstacles to controlling human population
  are not biological, but are the province of
  philosophers, theologians, politicians, and
  sociologists.

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Ultimate Size Limitation

• If the world continues to grow at current rate,
  population will surpass 12 billion by 2060.
• Human population subject to same biological
  constraints as other species.
• Human population will ultimately reach a carrying
  capacity and stabilize.
• Disagreement about exact size and primary
  limiting factors.

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What do you think the carrying capacity is?

• Have we reached it?
• What warning signs will tell us we may be
  overreaching our carrying capacity?

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