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Chapter 9
Population Distribution
and Abundance
Terminology
Population (def.): A collection of all
organisms of a single species living in a
specific geographic area
Niches

Niche: Summarizes environmental
factors that influence growth, survival,
and reproduction of a species.
– Grinnell’s definition focused on the effects
of the physical environment
– Elton’s definition included biotic and
abiotic factors
Niches

Hutchinson defined niche as:
– n-dimensional hyper-volume

n equates the number of environmental
factors important to survival and reproduction
of a species.
– Fundamental niche - hypervolume
– Realized niche includes interactions such
as competition that may restrict
environments where a species may live.
Population Ecology
Population
Characteristics
Birth Rate
Immigration
Dispersion
Density
Sex Ratio
Age Distribution
Growth Rate
(#organisms / time)
Death Rate
Emigration


“Large-scale” patterns of distribution:
Refer to variation in species abundance w/in
range
– due to variation in habitat quality
Describing Where a Species Lives

Range
– Geographic description
– Where on a map ?
– “Large-scale” distribution
Kangaroo Distributions and Climate

Caughley - relationship between climate +
distribution of three largest kangaroos in
Australia
Macropus giganteus – eastern grey
Eastern 1/3 of continent
temperate forest, tropical forest
Macropus fuliginosus – western grey
southern and western regions
temperate woodlands and shrubs
Macropus rufus – red
arid / semiarid interior
Distributions
largely based
on climate

Kangaroo Distributions and Climate
Limited distributions may not be directly
determined by climate.
– Climate often influences species distributions
via:
food production
 water supply
 habitat
 incidence of parasites, pathogens and competitors

Describing Where a Species Lives
 Habitat
– Ecological description
– What environmental conditions ?
– Determined by:

Terrestrial Environments: Climate /
Elevation / Soil conditions / Topography

Aquatic Environments: Water depth /
Temperature / Nutrient concentration
Environmental Conditions and Habitat
Poor
Habitat
Good
Habitat
Stress
No
Stress
Fatal Sub-Optimal
Poor
Habitat
Stress
Optimal Sub-Optimal
Environmental Gradient
Fatal
What Defines Habitat Quality ?
Habitat
Quality
=
Resource
Utilization
Capacity of the
environment to
support a
population
Habitat
Space
Area
Volume
x
Time
Resource x
to Acquire
Concentration
Resources
Resources
Hours / day
Energy
Days / year
per unit space
What Defines Habitat Quality ?
Habitat
Quality
Resource
Utilization
=
Habitat
Space
Area
Volume
x
Time
Resource x
to Acquire
Concentration
Resources
Resources
Hours / day
Energy
Days / year
per unit space
Limited by:
Physical barriers
Environmental
requirements
What Defines Habitat Quality ?
Habitat
Quality
Resource
Utilization
=
Habitat
Space
Area
Volume
x
Time
Resource x
to Acquire
Concentration
Resources
Resources
Hours / day
Energy
Days / year
per unit space
Controlled by:
Light availability, Precipitation,
Soil characteristics, Water depth,
Prey abundance
What Defines Habitat Quality ?
Habitat
Quality
Resource
Utilization
=
Habitat
Space
Area
Volume
x
Time
Resource x
to Acquire
Concentration
Resources
Resources
Hours / day
Energy
Days / year
per unit space
Controlled by:
Daily / Seasonal variation
in temperature, precipitation,
and nutrient availability
Habitat
Specialist
Figure 9.3
Habitat
Generalist
This temperature
occurs everywhere
This temperature
occurs only in some
high elevation
habitats
Figure 9.4
Distributions of Barnacles
- Intertidal Gradient

Organisms in intertidal zone have
evolved different degrees of resistance
to drying
– Barnacles - distinctive patterns of
zonation within intertidal zone
Connell found pattern in
barnacles:

Chthamalus stellatus restricted to
upper levels; Balanus balanoides
limited to middle and lower levels
Distributions of Barnacles
Along an Intertidal Gradient

Balanus - more vulnerable to desiccation,
excluded from upper intertidal zone
– Chthamalus adults excluded from lower areas by
competition with Balanus
Competition?
How do we know that Balanus
outcompetes Chthamalus?
Distribution of Individuals
on Small Scales
All Species Have Clumped
Dispersion At Large Spatial Scales
A species occurs only in locations where
environmental conditions are suitable for metabolic
activity, resource acquisition, and reproduction
All Species Have Clumped
Dispersion At Large Spatial Scales
Geographic Scale:
Latitudinal / Longitudinal gradients of climate.
Barriers to migration (Oceans, Mountain ranges)
All Species Have Clumped
Dispersion At Large Spatial Scales
Topographic Scale:
Variation in temperature, precipitation, soil
conditions at different elevations and slope
positions (e.g., upper, lower, N-facing, S-facing)
Muncie
Figure 9.15
Importance of scale in
determining distribution
patterns:

At one scale pattern may be random, at
another scale, might be uniform:
Distribution of Tropical Bee
Colonies


Hubbell and Johnson predicted aggressive bee
colonies have regular distributions;
Predicted non-aggressive species have random or
clumped distributions
Hubbell and Johnson
results:

4 species with regular distributions
were highly aggressive

Fifth non-aggressive and randomly distributed
Figure 9.11
What causes overall
pattern?



Behavior!
Aggressive bees were uniformly
spaced due largely to their
interactions.
Non-aggressive species were random did not interact.
Figure 9.13
Characteristics of Populations:
Density

Density: The average number of
individuals in a unit of space (area or
volume).
Characteristics of Populations:
Density
Density: The average number of individuals
in a unit of space (area or volume).
“Unit Area”
Large for big organisms
Small for small organisms
Examples:
Number of trees per acre
Number of hawks per km2
Number of clams per m2
Number of algae cells per liter
Unit Volume
For Aquatic
Species
Determining Density:
Sessile Organisms (Ex. plants)

Randomly locate “representative”
sample points (randomization &
replication)

Establish “plots” of known area

Count number of individuals in each plot

Average number of individuals per plot
Determining Density:
Sessile Organisms (Ex. plants)

Randomly locate “representative” sample
points

Establish “plots” of known area

Count number of individuals in each plot

Average number of individuals per plot

Average
number
per
plot
_____________________
Plot Area
Density =
Example: Computing Density
1. Randomly locate 20 points in Christy Woods.
2. Establish a 10m x 10m square plot at each point.
(Plot area = 100 m2 = 0.01 hectare (1 ha = 10,000 m2)
3. Count the number of sugar maples in each plot.
4. Suppose average number of maples per plot = 4.5
5. What is the density of sugar maple per hectare?
Sugar
Maple
= 4.5 trees/plot = 450 maples / ha
Density
0.01 ha/plot
Determining Density:
Mobile Organisms

Mark-Recapture Method
– Obtain random sample of organisms, “mark” them
and release them back to the population. M
– After a period of time, obtain another random
sample of organisms. n
– Count the number of marked organisms in the
second sample. m
Population = M (n + 1)
Size (N)
(m + 1)
Example: Estimating Population
Size from Mark-Recapture

Number of animals marked in 1st sample M = 100

Total number of animals in 2nd sample

Number of marked animals in 2nd sample m = 11
n = 150
Population = M (n + 1) = 100 (151) = 1258
Size (N)
(m + 1)
12
Note: N is not “density”, as there is no “unit space”.
Larger Species Tend to Have Lower
Density Than Smaller Species.
Why?
Big organisms take-up more space.
Example: More ants can fit into an acre than trees.
Big organisms need more resources to live, requiring
larger areas in which to obtain those resources.
Big organisms tend to produce fewer, larger offspring.
Small organisms tend to produce many small offspring.
Figure 9.21
Figure 9.20
Species Abundance
and Risk of Extinction

Local density within a habitat area: Small
populations at higher risk.
Species Abundance
and Risk of Extinction

Local density within a habitat area: Small
populations at higher risk

Habitat Tolerance: Number of habitat-types
within a landscape where present (habitat
specialists at higher risk)
Species Abundance
and Risk of Extinction

Local density within a habitat area: Small
populations at higher risk.

Habitat Tolerance: Number of habitattypes within a landscape where present
(habitat specialists at higher risk).

Large-scale geographic distribution:
Species found in only one location at
higher risk (“All the eggs in one basket”).
Least
vulnerable to
extinction
Increasing
Rarity
Increasing
vulnerability to
extinction
Moderate
vulnerability to
extinction
High
vulnerability to
extinction
Other Example ?
Highest
vulnerability to
extinction
The End