Demography and Life History

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Demography and Life History  

• Describe a static life table and the concept of
  life history
•  Contrast different life history strategies.
• Present a hypothesis for how different life
  history strategies evolved.
• Concepts and terms r selected, K selected
• Assigned reading Chapter 52

• By the end of this lecture you should be able to
• Discuss the major factors that determine the rate
  of change in population size.
• Concepts and terms birth rate, death rate, age
  structure
• Discuss the relationship between the age
  structure of a population and its growth rate.
•  Discuss the significance of different
  survivorship curves.

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Demography is the study of factors that affect
the growth and decline of populations

• Additions occur through birth, and subtractions
  occur through death.
• Demography studies the vital statistics that
  affect population size.
• Life tables and survivorship curves.
• A life table is an age-specific summary of the
  survival pattern of a population.

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Factors that control population size
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Population size and age structure
http//www.xist.org/charts/pop_graph2.htm
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http//www.census.gov/cgi-bin/ipc/
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http//www.census.gov/cgi-bin/ipc/
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From an evolutionary perspective, if the game
is about leaving the maximum number of viable
offspring, the ultimate life history strategy
would be begin reproducing at an early age,
produce many well provisioned offspring each time
you reproduce, and reproduce often during a
lifetime. Can you think of an organism that
behaves this way?
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Life histories are very diverse, but they exhibit
patterns in their variability

• Life histories are a result ofnatural selection,
  and oftenparallel environmental factors.
• Some organisms, such as theagave plant,exhibit
  what isknown as big-bangreproduction, where
  largenumbers of offspring areproduced in each
  reproduction,after which the individualoften
  dies.

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r-selected
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K-selected
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part 1
r-selected
K-selected
Population size
high rate of increase (r) variable not in
equilibrium
at or near carrying capacity (K) fairly constant
Examples
desert ephemerals (opportunistic)
redwood (equilibrium)
Climate
variable unpredictable
fairly constant predictable
Survivorship
Usually Type II or III
Usually Type I
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part 2
r-selected
K-selected
Mortality
often catastrophic (high) depending upon
external factors density dependent independent
(abiotic)
usually low density dependent highest
mortality when population size is high (biotic)
Competition
variable
keen
Life span (maturation time)
short (annuals) (short)
long (perennials) (long)
Age at first reproduction
lt 1 year
gtgt 1 year
many
Numbers/repro- ductive episode
few
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part 3
r-selected
K-selected
Selection favors
Rapid development early reproduction small
size single reproduction period in life span
slow development greater competitive ability
delayed reproduction large size repeated
reproduction
Seeds
small dispersed over large area
large poor dispersal
Allocation
mostly to reproduction
mostly to growth
Overall results
productivity of seeds is high short life span
efficiency of plant is high long life span