Human Population Ecology

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Human Population Ecology
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Background Thomas Malthus

• The first significant contribution to the theory
  of population ecology was that of Thomas Malthus,
  an English clergyman, who in 1798 published his
  Essay on the Principle of Population.
• Malthus introduced the concept that at some point
  in time an expanding population must exceed
  supply of prerequisite natural resources, i.e.,
  population increases exponentially resulting in
  increasing competition for means of subsistence,
  food, shelter, etc.
• This concept has been termed the "Struggle for
  Existence".

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Background Charles Darwin

• Malthus's theories profoundly influenced Charles
  Darwin 1859, On the Origin of Species, e.g., the
  concept of "Survival of the Fittest".
• Mortality of this type can be termed "facultative
  mortality" (as opposed to catastrophic mortality,
  e.g., weather, insecticides)

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Population Density

• Harry Smith, pioneering biological control worker
  with the University of California (1935),
  proposed the equivalent and now accepted terms
  density-dependent and density-independent.
• Density-dependent mortality factors are those
  that are facultative in effect,
  density-independent mortality factors are those
  that are catastrophic in effect.

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Density Dependent Factors

• A density-dependent mortality factor is one that
  causes a varying degree of mortality in subject
  population, and that the degree of mortality
  caused in a function (i.e., related) to the
  density of the subject (affected) population
  (density-geared, feedback regulation,
  self-regulating or self-limiting) may and
  typically involves a lag effect., e.g., most
  biological control agents.

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Density Dependent Factors

• A density-dependent mortality factor is one that
  causes a varying degree of mortality in subject
  population, and that the degree of mortality
  caused in a function (i.e., related) to the
  density of the subject (affected) population
  (density-geared, feedback regulation,
  self-regulating or self-limiting) may and
  typically involves a lag effect., e.g., most
  biological control agents.

Cycles in the population dynamics of the snowshoe
hare and its predator the Canadian lynx (redrawn
from MacLulich 1937). Note that percent
mortality is an elusive measure, it may, or may
not, be useful since mortality varies with
environment and time.
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Biotic Potential vs. Environmental Resistance

• Royal N. Chapman, Univ. Minnesota, in the 1930s
  proposed the concept of a balance between biotic
  potential and environmental resistance. 
• Chapmans model was a mathematical representation
  of the Malthusian concept, illustrated here by
  the logistic growth of a laboratory population of
  yeast cells.
• Population growth trajectory (N1 N0 (Rm
  -sN0)N0 ) Rm maximum rate of increase, here
  1, s interaction coefficient, here 0,0001,
  and carrying capacity of environment 1000.

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• Population growth (N1 N0 RN0), where N1 10
  and
• R0.5 (blue), R0 (black), and R-0.5 (red).

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Steady-state population model (N1 N0 Rm(1
-sN0 /K)N0, where Rm 2, K 1000, and intitial
displacement from equilibrium x -10.
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Steady-state population model(N1 N0 Rm(1
-sN0 /K)N0), when K 1000, Rm 3  and initial
displacement from equilibrium x -10.
Steady-state population model(N1 N0 Rm(1
-sN0 /K)N0), when K 1000, Rm 1.5 and initial
displacement from equilibrium x -10.
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Balance of Nature

• A. J. Nicholson (Australian entomologist) was a
  leading proponent of concept of density-dependent
  mortality factors.
• He maintained that density-dependent mortality
  played the key role in regulating prey
  populations.
• This is the essence of the so-called "Balance of
  Nature" theory.
• This theory implied a static balance about a mean
  (characteristic) equilibrium density with
  reciprocal (feedback) oscillations in density
  about these means.

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Balance of Nature

• Nicholson and V. A. Bailey (1935) proposed a
  population model that incorporated a "lag
  effect".
• This is particularly appropriate to parasitoids
  were population effects of attack (oviposition)
  may not be evident until the parasitoid has
  completed its immature development and emerges as
  a adult (killing the host).
• Leading proponents of this view of population
  dynamics included the early California biological
  control workers. The theory and practice of
  biological control can be said to revolve about
  this assumption.

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Dynamic Equilibrium

• However, a contrary view of the nature of
  population regulating mortality factors was
  argued by others, especially W. R. Thompson
  (Dominion Parasite Laboratory).
• The Canadian workers held that assumptions of
  Nicholson and like thinkers were unrealistic, and
  did not occur in nature, i.e., that the
  regulating role of a so- called density-dependent
  mortality factors was largely myth.

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Dynamic Equilibrium

• These workers argued that it was unnecessary to
  postulate such a mechanism of population
  regulation.
• They observed that the environment never remains
  continually favorable or unfavorable for any
  species. If it did so that population would
  inevitably become either infinite or decline to
  extinction.
• They maintained it was more accurate to say that
  populations were (in reality) always in a state
  of "dynamic equilibrium" with their environment.

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Life Table Predictive Model

• One of the most useful starting points for a
  population ecologist is the development of a life
  table.
• A life table is a schedule of mortality for each
  cohort (age group) of individuals in the
  population.
• The methods were developed originally for
  actuarial or demographic studies.
• Multifactor studies which incorporated the life
  table technique were once largely the province of
  forest entomologists, but are now widely used in
  agriculture.

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Life Table Predictive Model
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Predictive Value

• The final test of any population model is its
  usefulness to predict (generation to generation)
  changes in abundance or to explain why changes
  occur at particular population densities.
• Consequences of recent advances in understanding
  of population dynamics has lead to almost
  universal (among ecologists) acceptance of the
  proposition that population growth is geared to
  population density.
• Differences in the relative importance of
  density-dependent and density-independent
  mortality factors varies in different
  environments, e.g., the role of biotic components
  tends to be greater in more stable (benign)
  environments.

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Symbiotic Relationships

• Competitive processes/cooperative processes we
  often think of interactions between individuals
  even within species as only negative, but
  interactions can be positive (even between
  species), e.g., defense against predators,
  genetic diversity (concept of minimum density),
  mate finding (sustainable population), early
  mortality may favor subsequent survival.
• Interactions between species can be very complex
  even when only 2 species are considered (through
  impact on environment of other species).
• Each species can affect environment of other
  positively (), negatively (-), or have no effect
  (0). Major categories include mutualism (),
  commensualism (0), predator/prey (-),
  competition (- -), and amensalism (rare) (-0).

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Symbiosis in Action

• Insects and plants of two types
• 1) good colonizers, e.g., weed species, with high
  reproductive potential (capacity), adaptable,
  invaders, readily dispersed, "r-strategists"
• 2) good competitors, high survival, tend to
  stable population (K equilibrium), exploit
  stable environments, win out in competition,
  "K-strategists" (R. H. MacArthur and E. O. Wilson
  1967).

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Symbiosis in Action

• Most crop pests are r-strategists, e.g., aphids
  and phytophagous insects in general.
• Natural enemies, i.e., parasites and predators,
  are mostly K strategists.
• This is said to be one reason for the high
  failure rate associated with introductions of
  exotic natural enemies.
• Most crop plants are early succession plants,
  i.e., they are weedy species and accordingly they
  also are r-strategists. The r-selected species
  are particularly suited to exploiting the
  ecological patchiness and instability of the
  agroecosystem.

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Island Biogeography
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Island Biogeography

• Equilibrium models for near and distant islands.
• Equilibrium (in number of species present) occurs
  where curves of rates of immigration and rates of
  extinction intersect.
• I is the initial rate of immigration and P is the
  total number in the species pool on the mainland.

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Human Population Dynamics

• Human populations represent another example of
  exponential growth.
• Magnitude of the problems posed by human
  population growth can be seen from the fact that
  it took more than 1 million years for the human
  population to first reach 200,000 (the current
  daily rate of increase
• US Census, Historical Estimates of World
  Population

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Human Population Dynamics

• The human population is estimated to have first
  reached 1 billion persons in 1830, and 2 billion
  in 1930, a doubling time of 100 years.
• In 1960, thirty years later, the population edged
  past 3 billion, and a mere 15 years later, 4
  billion.
• In 1986, we exceeded 5 billion for the first
  time. Despite a slowing of the growth rate, 6
  billion in early 1999.

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Human Population Dynamics

• Barring catastrophic change, it is expected the
  human population will top 7 billion in 2010.
• To continue feeding our growing population, only
  as well as we presently do, it will be necessary
  to increase food production 6 every two years.
• Univ. North Carolina, Chapel Hill's World
  Population Counter

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Human Population Dynamics
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