Population Ecology: population regulation

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Population Ecology population regulation

• BS 111
• Leanne Hepburn

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Re-cap on last lecture

• Population groups of organisms of same species
  which occupy a given area
• Ways to measure pops. mark-recapture, estimating
  abundance, long-term studies, life tables, pop.
  Pyramids
• Survivorship curves
• Evolutionary strategies (r K)
• Human population dynamics

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Learning Objectives

• describe the exponential growth of populations,
  the logistic equation and the concept of carrying
  capacity
• compare density dependent and independent factors
  controlling populations and discuss how these may
  lead to population regulation

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Population regulation

• Population regulation control of size of pop.
• Regulation implies a tendency of the pop. to
  achieve or return to size at equilibrium or in
  harmony with surrounding environment.
• same size stable

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Population growth without regulation
exponential growth

• Populations are dynamic always changing
• Organisms die, are born, immigrate, emigrate
• But what would happen if a pop. Grew in size
  without stopping???

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Darwinian references

• The elephant is reckoned to be the slowest
  breeder of all known animals .it breeds when 30
  yrs old and goes on breeding until 90 yrs old,
  bringing forth 3 pairs of young in this interval
  if this be so, at the end of the 5th century
  there would be alive 50 mil elephants, descended
  from the 1st pair

Darwin 1859, Chapter 3
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Exponential growth

• Unlimited, rapid growth
• dN/dt rN
• Where
• dN/dt rate of growth of pop.
• r per capita rate of increase
• N no. of organisms in pop. at time t.

Pop accumulates more inds. Per unit of time when
large so curve gets progressively steeper What's
missing?
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Exponential Population Growth

• Examples of this?
• Elephant pop of Kruger national park, SA
• Grew exponentially for 60 years after first
  protected from hunting
• Often an unnatural occurrence
• Eventually increase in pop. Caused enough damage
  to vegetation in park that collapse in food
  supply likely

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• Get them to bring calculators and work through a
  real example equation, e.g. elephants
• Use lecturer resources from campbell or reproduce

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In reality.

• Populations do not normally grow exponentially
• Why not?
• Many factors can slow exponential growth
• Some of these are selective forces
• Can you think of some examples?
• These are density-dependent factors
• What are some forces that might slow growth
  without exerting selection?
• These are density-independent factors

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Why Does Population Size Change?

• Density Independent Forces
• Forces that are at work irrespective of the
  population density
• Density Dependent Forces
• Forces that oscillate depending on the population
  density

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Density Independent Forces

• effect of factor on the size of the pop. is
  independent of and does NOT depend upon the
  original density or size of the population
• Climate
• Topography
• Latitude
• Altitude
• Rainfall
• Sunlight
• In Sum Abiotic factors
• Exceptions do exist!

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Exponential Growth and Density Independent
Regulation
If a pop. growing exp. becomes subject to
density-ind. factors that regulate growth, what
will our graph look like?
N
Time ?
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Characteristics of populations controlled
primarily by density-independent factors

• "resource-limited" type
• less biological control control is a more
  haphazard, physical control.
• population size often goes over the carrying
  capacity before some other physical factor
  decreases the population size.
• Unlike the case for density-dependent factors,
  growth rates do not seem to show any trend at all
  relative to population density
• This type of regulation will usually occur in
• i) ecosystems where the communities have FEW
  species, i.e., where fewer biological
  interactions are taking place,
• ii) ecosystems ARE usually stressed periodically
  by physical factors (such as periodic flooding
  through a flood plain).

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Density Dependent Forces

• effect of the factor on the size of the pop.
  depends upon the original density or size of the
  population
• Within species
• Breeding spaces
• Food
• Mates
• Foraging spots
• Between species
• Predation
• Parasitism
• Pollinators
• Competition
• In Sum Biotic factors
• Exceptions do exist!

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Density-dependent Forces

• Density-dependent growthBirth and death rates
• As population size (density) increases
• per capita birth rates tend to decline
• per capita death rates tend to increase
• what about competition?

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Logistic growth

• Growth within natural limits
• Carrying Capacity
• Logistic equation
• dN/dt rN(1-N/K)
• where
• - dN/dt rate of pop. growth
• r per capita rate of increase
• N no. of organisms in pop. at time t
• K carrying capacity

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Carrying Capacity (k)Unique to Logistic Growth

• Definition?
• the maximum, equilibrium number of organisms of a
  particular species that can be supported
  indefinitely in a given environment

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Graphing Logistic Growth
K
N
?
inflection point
Time ?
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• Logistic model predicts different per capita
  growth rates for pops. Of low or high density
  relative to carrying capacity of environment
• Per capita rate of increase approaches 0 as
  reaches k
• E.g. high density -
• Each individual few resources available, pop.
  Grows slowly
• Vice versa

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• Get them to work through a real example of
  logistic growth with calculators

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Characteristics of populations controlled
primarily by density-dependent factors

• "self-limiting" growth form
• growth rates are usually inversely proportional
  to pop. density. i.e. if pop. density high,
  growth rate low.
• Because of the nature of these biological
  factors, e.g. disease and comp., this type of
  reg. usually occurs in
• i) ecosystems where the communities have MANY
  species, i.e., where many biol. Inter. taking
  place,
• ii) ecosystems NOT usually stressed periodically
  by physical factors (e.g. periodic flooding),
  i.e., ecosystems that are usually more stable.

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• NOTE
• Pop. Reg. factors, then, can be classified as
  above into two types, but what usually happens in
  nature is that a pop. is actually controlled by a
  COMBINATION of density-dep. and density-indep.
  factors.
• Some pops. will be primarily controlled by 1 type
  and others primarily controlled by the other.
• Note that both types of factors are external
  forces on the population.

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Factors regulating pop. sizes

• Density dependent
• Density independent
• Stochastic

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Stochastic

• Real Populations
• Of course we know that real populations do not
  always behave as smoothly as our graph suggests
• Why not?

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Deterministic vs. Stochastic processes

• Stochasticity
• variability in the system, in this case in the
  factors determining population growth
• Environmental stochasticity
• variability associated with good and bad years
  for population growth.

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Stochasticity

• Environmental Stochasticity (cont.)

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Why are these models important???

• Starting point for constructing more complex
  models
• Conservation biology, e.g. estimate critical size
  below which white rhinoceros may become extinct

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Applications Harvesting pops.

• Pop. Models help us to understand the key
  features of pop. Growth
• Harvesting a pop.
• E.g. fish species
• We need details of age and sex structure
• Life history strategy
• Reproductive strategy

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Summary

• Pop. Reg. control of size of pop.
• Pops. are dynamic always changing
• Exponential growth unlimited, rapid growth
• Logistic growth growth within natural limits
• Density independent forces generally abiotic
  factors, e.g. floods
• Density dependant forces generally biological
  factors, e.g. disease, comp., predation
• Carrying capacity the max., equilibrium number
  of organisms of a particular species that can be
  supported indefinitely in a given environment
• Stochasticity variability in the system

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Recommended reading

• Campbell Reece, Biology, 8th Edition, Pearson
  pp1181 (53.3)-1195
• Detailed section on human pop. growth
• Mackenzie, A, Ball, AS and Virdee, SR (1998)
  Instant Notes in Ecology.Oxford BIOS pp71-92