POPULATION DISTRIBUTION
AND ABUNDANCE
Chapter 9
Molles: Ecology 2nd Ed.
Chapter Concepts
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Physical environment limits geographic
distribution of species
On small scales, individuals within pops. are
distributed in random, regular, or clumped
patterns; on larger scales, individuals within
pop. are clumped
Population density declines with increasing
organism size
Rarity influenced by geographic range, habitat
tolerance, pop. size; rare species vulnerable
to extinction
Molles: Ecology 2nd Ed.
Populations
•
Ecologists define a population as group of
individuals of single species inhabiting
specific area.
Molles: Ecology 2nd Ed.
Habitat
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Physical environmental conditions that allow
individuals of species to survive AND
reproduce
Molles: Ecology 2nd Ed.
Habitat quality
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Ability of environmental conditions to support
repro and survival
 Habitat area/volume
 Resource concentration
 Time
High habitat quality = organisms acquire
many resources; high survival + repro =
large pop.
Molles: Ecology 2nd Ed.
Population size
Population numbers vary with habitat
quality
Poo r
habitat
Low
Good
habitat
Optimal
Environmental gradient
Molles: Ecology 2nd Ed.
Poo r
habitat
High
Distribution Limits
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Physical environment limits geographic
distribution of species
 Organisms can only compensate so much
for environmental variation
Molles: Ecology 2nd Ed.
Geographical range
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Geographic area where species is found
(based on macroclimate, salinity, nutrients,
oxygen, light, etc.)
Molles: Ecology 2nd Ed.
•
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“Large-scale” patterns of distribution:
Refer to variation in species abundance w/in
range
 due to variation in habitat quality
Molles: Ecology 2nd Ed.
Kangaroo Distributions and Climate
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Caughley - relationship between climate +
distribution of three largest kangaroos in
Australia
Molles: Ecology 2nd Ed.
Macropus giganteus – eastern grey
Eastern 1/3 of continent
temperate forest, tropical forest
Molles: Ecology 2nd Ed.
Macropus fuliginosus – western grey
southern and western regions
temperate woodlands and shrubs
Molles: Ecology 2nd Ed.
Macropus rufus – red
arid / semiarid interior
Molles: Ecology 2nd Ed.
Distributions
largely based
on climate
Fig 9.2
Molles: Ecology 2nd Ed.
Kangaroo Distributions and Climate
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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
Molles: Ecology 2nd Ed.
Tiger Beetle of Cold Climates
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Tiger beetle (Cicindela longilabris) - higher
latitudes + elevations than other NA species
 Schultz found metabolic rates of C.
longilabris are higher and preferred temps.
lower than other species
 Physical env. limits species distributions
Molles: Ecology 2nd Ed.
Fig 9.3
Metabolic rates of C.
longilabris higher;
preferred temps
lower than other
beetle species
Adapted to cool
climates
Molles: Ecology 2nd Ed.
•
Distributions of Plants Along a MoistureTemperature Gradient
Encelia spp. distributions + variations in
temp and precipitation
Fig 9.7
Molles: Ecology 2nd Ed.
Fig 9.5
Molles: Ecology 2nd Ed.
•
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
Molles: Ecology 2nd Ed.
Connell found pattern in barnacles:
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Chthamalus stellatus restricted to upper
levels; Balanus balanoides limited to middle
and lower levels
Molles: Ecology 2nd Ed.
•
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
Molles: Ecology 2nd Ed.
Competition?
How do we know that Balanus
outcompetes Chthamalus?
Molles: Ecology 2nd Ed.
Fig 9.8
Fig 9.9
Molles: Ecology 2nd Ed.
Distribution of Individuals on Small Scales
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Three basic patterns:
 Random: equal chance of being anywhere
 Regular: uniformly spaced
 Exclusive use of areas
 Individuals avoid one another
 Clumped: unequal chance of being anywhere
 Mutual attraction between individuals
 Patchy resource distribution
Molles: Ecology 2nd Ed.
Fig 9.10
Molles: Ecology 2nd Ed.
Importance of scale in determining
distribution patterns:
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At one scale pattern may be random, at
another scale, might be uniform:
Molles: Ecology 2nd Ed.
Distribution of Tropical Bee Colonies
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Hubbell and Johnson predicted aggressive
bee colonies have regular distributions;
Predicted non-aggressive species have
random or clumped distributions
Molles: Ecology 2nd Ed.
Hubbell and Johnson results:
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4 species with regular distributions were
highly aggressive
 Fifth non-aggressive and randomly
distributed
Molles: Ecology 2nd Ed.
Fig 9.11
Molles: Ecology 2nd Ed.
What causes overall pattern?
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Behavior!
Aggressive bees were uniformly spaced due
largely to their interactions.
Non-aggressive species were random - did
not interact.
Molles: Ecology 2nd Ed.
Fig 9.10
Molles: Ecology 2nd Ed.
Distributions of Desert Shrubs
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Traditional theory suggests desert shrubs
are regularly spaced due to competition
 Phillips and MacMahon - distribution of
desert shrubs changes from clumped to
regular patterns as they grow
Molles: Ecology 2nd Ed.
Hypothesis:

Young shrubs clumped for (3) reasons:
 Seeds germinate at safe sites
 Seeds not dispersed from parent areas
 Asexual reproduction
Molles: Ecology 2nd Ed.
Distributions of Desert Shrubs
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Phillips and MacMahon proposed as plants
grow, some individuals in clumps die =
reducing clumping
 Competition among remaining plants
produces higher mortality
 Eventually creates regular distributions
Molles: Ecology 2nd Ed.
Fig 9.13 - their hypothesis
Molles: Ecology 2nd Ed.
Brisson and Reynolds
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Dug up roots, map distribution of 32 bushes
found competitive interactions with
neighboring shrubs influences distribution of
creosote roots
Molles: Ecology 2nd Ed.
So what?
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Creosote bush roots do not overlap with
nearby plant roots
Only 4% overlap between bushes
Fig 9.14
Molles: Ecology 2nd Ed.
Distributions of Individuals on Large Scales
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Bird Pops North America
 Root - at continental scale, bird pops have
clumped distributions (Christmas Bird
Counts)
 Clumped patterns in species with
widespread distributions
Molles: Ecology 2
Fig
9.14
nd
Ed.
Similar distribution pattern for species
with small range: few “hot spots”
Fish crow
Fig 9.14
Molles: Ecology 2nd Ed.
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Brown et al. (1995)
Relatively few study sites gave most
records for each bird species in Breeding
Bird Survey (June):
clumped only during breeding season?
Fig 9.16
Molles: Ecology 2nd Ed.
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Density = number individuals per unit
area/volume
Sedentary organisms: plot approach
Moving/secretive organisms: mark/recapture
Relative abundance = percent cover, CPUE
Molles: Ecology 2nd Ed.
Estimating density
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Sedentary animals and plants
Plot methods
 Area of known size
 Randomly located plots
 Count individuals in plots
 Average / plot
 Density = average no. / plot area
Molles: Ecology 2nd Ed.
Estimating density
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Mobile or secretive animals: mark/recapture
1. Sample animals and mark
2. Release (M out of N in pop marked)
3. Wait for mixing
4. Sample (n), count how many marked (m)
5. Compute estimate of pop size:
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N = M (n + 1)
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(m + 1)
Molles: Ecology 2nd Ed.
Example: Estimating Population Size
from Mark-Recapture
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Number of animals marked in 1st sample = 100
Total number of animals in 2nd sample = 150
Number of marked animals in 2nd sample = 11
Population = M (n + 1) = 100 (151) = 1258
Size (N)
(m + 1)
12
Molles: Ecology 2nd Ed.
Another Example
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Sample M = 38 squirrels, marked, released
After 2 weeks, resample, n = 120
m = 12 of 120 marked
Estimate of pop. size:
 N = M (n + 1) / (m + 1)
 = 38 (120 + 1) / (12 + 1) = 353.7
 ~ 354
Molles: Ecology 2nd Ed.
Example: maple trees
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20 randomly located plots, 10 x 10 m
squares (area = 100 m2)
Average sugar maple stems per plot = 4.5
Unit area for trees = hectare (10,000 m2)
Density = 4.5 maples per plot / 0.01 hectare
plots = 450 maples / ha
Molles: Ecology 2nd Ed.
Example: zooplankters
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35 lake water samples, 50 ml each
Average copepods per sample = 78
Unit volume for zooplankton = liters
Sample volume = 0.05 l
Density = 78 copepods per sample / 0.05 l
samples
– = 1560 copepods / l
Molles: Ecology 2nd Ed.
Organism Size and Population Density
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Population density decreases with larger
organism size
 Why?
 Bigger organisms need more space and
resources
 Bigger organisms have lower repro rates
Molles: Ecology 2nd Ed.
Damuth (1981)
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Pop density of 307 spp. of herbivorous
mammals decreased with increased body
size
Molles: Ecology 2
Fig 9.19
nd
Ed.
Peters and Wassenberg (1983)
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Aquatic invertebrates had higher pop
densities than terrestrial invertebrates of
similar size;
 mammals have higher pop densities than
birds of similar size
Fig 9.20
Molles: Ecology 2nd Ed.
Plant Size and Population Density
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Plant population density decreases with
increasing plant size
 Underlying details different from animals
Molles: Ecology 2nd Ed.
White (1985)
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Tree seedlings can live at high densities, but
as trees grow, density declines until mature
trees are at low densities
Molles: Ecology 2nd Ed.
Rarity and Extinction
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Rabinowitz - 7 forms of rarity
commonness classification based on (3)
factors:
 Geographic Range of Species
 Habitat Tolerance
 Local Population Size
Molles: Ecology 2nd Ed.
Rarity
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Non-rare populations have large geographic
ranges, broad habitat tolerances, some large
local populations
All seven other other combinations create
some kind of rarity
= risk of extinction
Molles: Ecology 2nd Ed.
Rarity
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Rarity I
 Large Range: Broad Habitat Tolerance:
Small Local Pops
 Peregrine Falcons
Molles: Ecology 2nd Ed.
Rarity II

Large Range: Narrow Habitat Tolerance:
Small Local Pops
 Passenger Pigeons
Molles: Ecology 2nd Ed.
Rarity
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Rarity III
 Small Range: Narrow Habitat Tolerance:
Small Pops
 Mountain Gorilla
Molles: Ecology 2nd Ed.
Least
vulnerable to
extinction
Increasing
Rarity
Increasing
vulnerability to
extinction
Moderate
vulnerability to
extinction
High
vulnerability to
extinction
Other Example ?
Highest
vulnerability to
extinction
Example: NA suckers
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White sucker large range
Broad habitat
requirements
Large body size
Molles: Ecology 2nd Ed.
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Yacqui sucker small range
Narrow habitat
requirements
Small body size
Molles: Ecology 2nd Ed.
Summary
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Physical environment limits geographic
distribution of species
On small scales, individuals w/in pops. are
distributed in random, regular, or clumped
patterns; on larger scales, individuals w/in
pop. are clumped
Population density declines with increasing
body size
Rarity influenced by geographic range,
habitat tolerance, pop size; rare species
vulnerable to extinction
Molles: Ecology 2nd Ed.
Molles: Ecology 2nd Ed.