Ecology 2: Populations

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Ecology 2 Populations
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POPULATION DYNAMICS

• Four key features of populations
• Population size
• Population density
• Population dispersion
• Population growth

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POPULATION SIZE

• Defined as the number of individuals
• Has an important effect on population survival
• Very small populations - more likely become
  extinct
• random events may threaten the survival
• inbreeding may become common as possible mates
  become scarce
• inbreeding leads to genetic uniformity
  ?population less likely to survive changes in
  environment

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POPULATION DENSITY

• Refers to number of individuals per unit area
• determined by
• social or population structure
• mating relationships
• time of year
• examples high density - fleas on a dog
• low density - hawks in the woodland

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POPULATION DISTRIBUTION

• The way individuals of a population are arranged
  in the environment
• Three main classifications -
• clumped
• uniform
• random

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CLUMPED Distribution

• Individuals are lumped into groups
• ex. flocking birds or herbivore herds
• due to
• resources that are clumped
• positive social interactions
• most common type of dispersion pattern

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UNIFORM Distribution

• Individuals are regularly spaced in the
  environment
• ex. creosote bush
• due to
• negative (antagonistic) relationship between
  individuals
• regular spacing of resources
• rare because resources are rarely evenly spaced

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RANDOM Distribution

• Individuals are randomly dispersed in the
  environment
• ex. dandelions (wind dispersed)
• due to
• random distribution of resources in the
  environment
• neither positive nor negative interaction between
  individuals
• rare because these conditions are rarely met

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POPULATION GROWTH

• Population growth rate
• The amount that a population changes in a given
  time is determined by the birth rate and the
  death rate.
• birth rate the number of births in a period of
  time.
• death rate the number of deaths in a period of
  time.
• If growth rate is a positive number, the
  population is increasing if it is a negative
  number the population is shrinking.

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Exponential Growth

• The exponential model of population growth
  describes a population that begins growing
  slowly, but increases rapidly after only a few
  generations.
• All populations have this ability, although not
  all populations realize this type of growth
• J-shaped curve

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Exponential Growth

• The larger the population gets, the faster it
  grows.
• Examples
• a single pair of house flies could produce gt6T
  offspring in one summer IF all eggs hatched and
  survived
• One penny on one square of a chessboard, doubles
  for each of 64 squares1st doubling adds 1 cent.
  Last doubling adds 4,600,000,000,000,000.
• Exponential growth is growth that is independent
  of population size or density

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Logistic Growth

• Most populations grow exponentially until
  resources (food, space, etc.) become limiting.
• The growth rate of the population begins to slow
  down and eventually stops a pattern of logistic
  growth.
• The population stabilizes at the carrying
  capacity the population size that the
  environment can support.

S-shaped growth curve
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What limits population growth?

• Density-independent factors
• affect populations randomly (without respect to
  density)
• usually abiotic factors
• ex. hurricanes, tornadoes, fire, drought, floods
• poor regulators of populations (dont maintain
  populations at a stable level)
• Density-dependent factors
• affect populations when densities are high
• ex. disease, competition, predation, parasitism
• good regulators of populations
• cause logistic growth
• maintain populations at carrying capacity

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How can you tell if a population is going to grow
or shrink?

• Age Structure Diagrams
• Population is divided into pre-reproductive,
  reproductive and post-reproductive
• A broad-based pyramid indicates growth
• A straight edged diagram indicates zero
  population growth
• An inverted pyramid indicates negative growth

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Life History Strategies

• Individuals have limited energy to put toward
  maintaining their own bodies and reproducing.
• they can devote it to long life and low
  reproductive rate OR
• short life and high reproductive rate

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R strategists

• Populations of species that are r-strategists are
  characterized by exponential growth.
• short life spans, mature and reproduce early
• many, small offspring little parental care
• changing habitats - exploit, grow, move on
• generalists

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K strategists

• Populations that are K-strategists are
  characterized by logistic growth.
• maintain their population levels at K (carrying
  capacity)
• longer life spans, mature reproduce later
• few, large offspring high parental care
• stable environments maximize carrying capacity
• specialists
• many are endangered

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Human Population Growth

• humans are K strategists (but are growing
  exponentially)
• historically the population has remained stable
• agricultural and industrial revolution have
  increased carrying capacity of the earth
• at this rate
• human population is doubling every 35 years
• 6 B currently 94 million every year 230,000/day
• damage to planet should eventually reduce
  carrying capacity and slow human growth rate

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Ways to stabilize population growth

• Delay birth of first child (? age of mother)
• Increase education and jobs for women
• Decrease necessity of children in labor force
• Increase pension plans
• Reduce infant mortality
• Encourage/practice birth control