Populations

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Populations

• Lecture 13

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In order the manage populations or understand
populations

• For the purposes of
• extracting resources
• restoring habitats to natural conditions
• satisfying scientific curiosity
• We need to be able to measure populations
• We need understand the processes that
• regulate populations
• control biological diversity

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Populations

• Vary in
• density
• Abundance
• distribution
• Examples
• Maple trees common in a beach-maple forest
• Rare in oak-pine forests
• Lupins on an old dune ridge occur in dense
  patches
• Lady slipper orchids widely dispersed in a swamp
  or damp lowland forest

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Population

• Defined as a group of individuals of the same
  species in a particular place and time
• Example
• All humans living in Cornwall
• All the moose in Algonquin Park
• All the yellow perch in Lake St. Francis

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Deme

• Sub-population which is composed of
  inter-breeding organisms
• There may be some gene flow between demes but
  genetic exchange is greater within demes
  (sub-population) than among them

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Population size/density

• Affected by four primary population parameters

Immigration ()
Death or Mortality (-)
Density
Birth or Natality ()
Emmigration (-)
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Population density varies widely

• Diatoms
• 5 000 000/cu. m.
• Soil Arthropods
• 500 000 /sq. m
• Trees
• 200/hectar
• Elephants lt 1-10 per sq. km

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Much of what ecologists do

• Requires measuring population sizes
• Identifying rare species
• Increasing populations of endangered species
• Controlling pests

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Measuring populations

• Population density measurements
• Rare to be able measure or enumerate all the
  individuals of a population
• Exceptions extremely rare species
• Density must be measured in sub-populations
• Extrapolate to entire population

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Defining an individual

• For many species like birds, mammals, fish it is
  easy
• Unitary (determinate growth)
• Plants are more difficult
• Modular (indeterminate growth)

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Defining an individual

• Genet results from the union of a single egg
  sperm
• Ramet result from vegetative (asexual
  reproduction)

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When counting individuals

• Unitary organisms are straightforward
• Modular organisms are more difficult
• Do we count all trees?

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When counting individuals

• For modular organisms what we count may depend on
  the type of question we ask

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Defining individuals
Unitary organisms 6 little brown bats
Modular organisms 1 sumac genet
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Density measurements

• Can be
• Absolute estimates
• Relative estimates
• Relative density of population A is greater
  than the density of population B
• Absolute density of population A is 100
  plants/Hectare

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Density measurements

• Absolute estimates
• May provide more information but are more
  difficult to obtain
• Cost more (time and energy)
• Fewer sites or species can be monitored
• Must decide whether the extra information is
  worth the added cost

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Absolute measurement techniques

• Two basic types
• Quadrat sampling
• Mark recapture sampling
• Most sampling techniques are variations of these
  two types

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Quadrat sampling

• All the individuals within a measured area are
  counted
• Extrapolated to the entire area of study
• Assumes that the quadrat is representative of the
  entire area

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Quadrats

• Can be any shape
• Size and number of quadrats will depend on the
  species and habitat being studied

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Mark recapture estimates

• Individual animals are captured and marked for
  future identification
• Marked animals are then released back into the
  population
• Population is then re-sampled and the proportion
  of marked individuals is used to calculate the
  population size

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Example

• 200 fish captured, marked and released
• Population is re-sampled 300 fish captured
• 30 marked and 270 unmarked
• 10 of the population is marked
• 200 fish are marked
• Therefore there are 2000 fish in the population

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Relative estimates

• Detecting vocalizations
• Frogs
• Bats (echolocation call surveys with bat
  detectors)
• Hunter or angler surveys
• Signs of activity
• Tracks, Scats, Slides

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Population size

• Is not the only way in which populations or
  species differ

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Life stages and reproductive cycles

• If we wish to understand population regulation
• We need to know which life stages are the
  critical ones and what factors influence these
  stages
• e.g. mortality may not be evenly distributed
  throughout the organisms life cycle

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Age distributions

• Many ecological factors are age related
• Death
• Reproductive rate
• Summarizing age specific information can be a
  useful way of comparing populations
• Three common ways
• Life tables
• Cohort analysis
• Survivorship curves

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Life table and fecundity schedule
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Life tables and cohort analysis
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Sometimes following a cohort is not practical and
we must use age frequency distributions
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Classic survivorship curves
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Real survivorship curves are sometimes more
complex
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Population characteristics

• Carrying capacity
• K
• Ability of the environment to support a
  population
• Intrinsic rate of increase
• r
• Instantaneous rate of increase per individual

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Two types of species

• K selected species
• Slow growth rates
• Low reproductive rate
• Large investment in raising offspring
• R selected species
• High growth rate
• High reproductive rate ( r )
• Little parental investment

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Two types of populations

• K selected species
• Climax species
• Good competitors
• R selected species
• Good colonizers
• Poor competitors

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Types of life cycles

• We can classify animal life cycles in different
  ways
• e.g., whether generations overlap
• Overlapping generationshumans and other long
  lived species
• Non-overlapping generationstemperate
  grasshoppers

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Types of life cycles

• e.g., time to compete life cycle
• Annualscompete life cycle in one yearannual
  plants (most weeds) and many temperate insects
• Biennialstake two years
• Perennialstake several yearsMaple trees

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Types of life cycles

• e.g., timing of reproduction
• Iteroparous
• Reproduce repeatedly throughout their lives
• Devote some energy to reproduction and some to
  survival (and future reproductive opportunities)
  
• Semelparous
• Reproduce once and then die