What Limits Growth Rates and Population Sizes?

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What Limits Growth Rates and Population Sizes?

• Density-independent factors
• usually abiotic (e.g. hurricanes, fire)
• change birth rates and death rates irrespective
  of population size
• Density-dependent factors
• usually biotic (e.g. predation, disease,
  competition for resources and in general, stress)
• fertility and fecundity rates ? and mortality ?,
  as population ?
• cause logistic population growth

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Population Dynamics

• Population dynamics changes in populations
  through time
• some ?, ?, remain unchanged
• some cycle/fluctuate at regular intervals
• population cycles thought to involve
  density-dependent factor

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Population Dynamics

• Some species show regular fluctuations in
  population
• red grouse populations in Britain rise/fall in
  regular 4-8 yr cycles
• population cycle is density dependent
• infection rates of roundworm fluctuate depending
  on population
• untreated populations showed typical dramatic
  four-year cycle, treated populations maintained
  high densities

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Population Structure

• Populations age structure has large influence on
  population growth
• age structure proportion of indiv.
  of each
  possible age
• represented by age pyramids
• Developed nations have more
  uniform age
  distribution
• similar people in most age classes
• result of long periods of
  minimal
  population growth
• populations not expected to
  grow
  quickly
• Sweden

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Population Structure

• Developing nations often have bottom-heavy age
  distribution
• dominated by very young individuals
• results when populations undergo
  period of rapid
  growth
• extremely rapid growth
  expected to continue
  
• India

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Human Population Growth

• Shape of curve is exponential
• ? since 1400
• highest growth rates btwn 1965-70
  averaged 2.04/yr
• since 1970, growth rate ?
• btwn 1990 and 1995, worldwide
  growth rate
  averaged 1.46/yr
• current rate averages 1.2/yr

2005 6.4 Billion
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Human Population Growth

• Fertility rates average of children per ?
• current worldwide ave. fertility rate 2.7
  children per ?
• was 5 in 1950
• replacement rate (ave. fertility rate required
  for each ? and ? to produce exactly 1 offspring
  that survives to breed) 2.1
• r 0 at replacement rate
• zero population growth (ZPG) results

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Human Population Growth

• UN Population Division projections of worlds
  population in 2050 based on 3 fertility rates
  (2.5, 2.1 or 1.7 children per ?)
• at 2.5 rate, world population 10.7 B in 2050,
  no signs of peaking
• at 1.7 rate, world population 7.3 B in 2050,
  already peaked

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Human Population Growth

• Projections drastically altered btwn 1992 and
  1998 to account for impact of AIDS
• 500-year period of rapid growth coming to end
• changes in fertility and AIDS epidemic will
  determine how fast growth rates ? and max.
  population size

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Population Structure

• Geographic structure of populations
• habitat of many sp. restricted despite large
  range map
• individuals occupy isolated patches (niches)
  within broader range
• many sp. exist as metapopulation
• metapopulation population made up of many small
  populations isolated in fragments of habitat
• over time, each population w/i larger
  metapopulation wiped out
• immigration from nearby habitats recolonize empty
  fragments of habitat, thus maintaining overall
  stable of individuals
• small, isolated populations (even on nature
  reserves) unlikely to survive over long term for
  this reason

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Demography and Conservation

• Conservationists draw heavily on concepts and
  techniques from population ecology
• design reserves for threatened sp. sizeable
  enough to maintain
  large populations
• systems of small reserves, connected by
  corridors, can be
  alternative
• crucial to preserve some patches of currently
  unoccupied habitat

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Demography and Conservation

• use life tables to make population projections
  and guide conservation programs
• allows to determine aspects of survivorship and
  fecundity that are sensitive to particular sp.
• endangered sp. tend to have ? juvenile mortality,
  ? adult mortality (type III curve) and ?
  fecundity, populations sensitive to ? in adult
  mortality
• b/c young are likely to die anyway
• thus ensure adults dont die
• population growth of sp. w/ type I curve
  sensitive to changes in age-specific fecundity
• to ? population growth in humans birth control
  and delayed 1st pregnancy are advocated

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Population Viability Analysis

• Population viability analysis (PVA) model to
  estimate likelihood that population will avoid
  extinction for period of time
• combines demographic models w/ data on geographic
  structure and the rate and severity of habitat
  disturbance
• leadbeaters possum (endangered marsupial)
• w/ migration vs. no migration
• extensive timber harvesting
  poses
  threat b/c no migration

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Lessons from Natural and Human-Caused Disasters

• Recovery after Exxon Valdez oil spill of 1989 or
  hurricane Hugo
• recovery rapid if nearby populations are
  available to serve as source of immigration
• habitat must be allowed to recover