Lecture 6: Abundance and populations

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Lecture 6 Abundance and populations

• EEES 3050

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Relationship between distribution and abundance.

• What do we mean by distribution?
• Where a species lives?
• Geographic range?
• Habitat?
• What do we mean by abundance?
• The number of animals?
• At what scale?

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A hierarchy of scales. Distribution of Tetraphis
A species range can be measured at many different
scales.
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Do range sizes differ between similar species?

• Do most species have small or large ranges?

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Do range sizes change with geography?

• Rapoports Rule
• Geographic ranges decrease from the poles to the
  equator.
• Example Canadian mammals have large ranges,
  while Mexican mammals have smaller ranges.

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Do range sizes change with geography?

• Rapoports Rule
• Geographic ranges decrease from the poles to the
  equator.
• Pattern holds for many trees, fishes, reptiles,
  some birds, and mammals on most continents.
• What explains this pattern?
• Climatic variability
• Product of glaciations
• Not a good explanation for southern hemisphere.
• Competition

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Example Climatic variability hypothesis

• Where is climate variability the highest in
  oceans?
• Mid-latitudes
• Example Marine fishes

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Geographic range and abundance

• What do you expect to be the relationship between
  range and abundance?
• Many species have their highest abundance in the
  center of their range.

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Geographic range and abundance
Western grey kangaroo
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Geographic range and abundance

• If a species has its highest abundance in the
  center of its range
• What would be expected regarding the relationship
  between the range sizes and abundance?
• Are organisms with larger ranges more abundant?

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Example British moth species.

• Question
• Is range size positively related with abundance?
• Test
• Document average abundance and distribution.
• Results
• Moth species in UK with broader distributions
  have higher abundance

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Why are distribution and abundance correlated?

• Sampling Model
• Purely an artifact of sampling
• Rare species are hard to find.
• Ecological specialization model or Browns model
• Generalists are more broadly distributed.
• Local population model
• Effect of good dispersers.

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What happens when species decline?

• If a species is in decline, what happens to its
  range?
• Some bird species appear to be declining based on
  breeding bird survey routes.

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What happens when species decline?
X

• Why would eastern meadowlarks decline in
  abundance and range?
• Why would grackles increase their range while
  their abundance is declining?

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The problem of Abundance Populations

• Population Ecology
• Population ecology takes into account (amongst
  other things) the relationship of animals to
  their food, and to other sorts of animals that
  eat the same sort of food, or prey on them or are
  related to them in some other way
• Andrewertha 1961 Introduction to the study of
  animal populations.
• What is a population?
• A group of organisms of the same species
  occupying a particular space at a particular
  time.

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Problems with studying populations

• A group of organisms of the same species
  occupying a particular space at a particular
  time.
• Organisms are distributed in a continuum.
• Populations exist within a community.

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Population Ecology

• What is the main issue of interest?
• Density
• What determines it?
• Natality i.e. egg, seed, births, etc.
• Death
• Immigration
• Emigration

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Factors that determine density natality

• Includes the production of new individuals.
• Other terms
• Fecundity potential reproductive capacity
• Fertility Based on of offspring
• Realized vs. potential fertility
• Natality usually reported as organisms produced
  per female per unit time.

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Factors that determine density Mortality

• Converse of survival
• Potential vs. realized longevity.
• How to measure mortality
• Example catch curves

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Catch curve
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Factors that determine density Mortality

• Converse of survival
• Potential vs. realized longevity.
• How to measure mortality
• Example catch curves

Relative abundance of age III fish
Survival rate between years 2 and 3

Relative abundance of age II fish
147

0.50
292
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Factors that determine density Immigration and
emigration

• Common assumptions in population studies.
• Immigration emigration
• Assume closed system, i.e. islands, where
  dispersal is of little importance.
• We know better.
• Dispersal can be measured
• Advent of radio-telemetry
• Obviously, primarily limited to larger organism.

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Modular vs. unitary organisms

• Modular
• Some organisms come in simple units of
  individuals.
• Example aspen clone
• Unitary
• Individual units
• You, me, deer, mice, etc.

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How can density be determined?

• Absolute vs. relative density
• Measuring absolute density
• Census, counting trees,
• Can you do this for insects?
• Sampling and statistics
• Sample areas and extrapolate

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Class exercise

• How to calculate the number of students in class?
• Census
• Sample a proportion
• Mark and recapture.

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Sampling quadrates

• Divide area into quadrats
• Count density in subset of squares

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Sampling quadrates

• Divide area into quadrats.
• Count density in subset of squares
• Average of survey 4.8 individuals
• Estimate of whole area 4.8 x 25 120

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Other sampling methods

• Transects
• Time counts
• Mark and recapture
• Marked animals in 2nd sample Marked animals in
  1st
• Total caught in 2nd
  Total Popn size

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Example of mark and recapture

• Marked 100 individuals in first catch
• 33 Marked in 2nd catch
• 75 total caught in 2nd
• Marked animals in 2nd sample Marked animals in
  1st
• Total caught in 2nd
  Total Popn size
• ( 33 ) (
  100 )
• ( 75 )
  ( Total Popn size )
• 33/75 100/N
• N (10075)/33
• N 227 total individuals.

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Three assumptions

• Marked and unmarked animals are captured randomly
• Marked animals are subject to same mortality rate
  as unmarked.
• Some methods assume no mortality during sampling
  interval
• Marked animals are neither lost nor over-looked.

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Measures of relative density

• Many methods
• Traps
• Vocalization frequency
• Catch per unit effort
• Percent cover
• Roadside counts
• Etc

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Summary

• Distribution is a function of scale.
• Often range and abundance are positively
  correlated.
• Population ecology often concerned about density.
• Natality, Mortality, Immigration and emigration
  affect density.
• Measuring density can be done on a relative or
  absolute scales.