9
Population Distribution and
Abundance
Chapter 9 Population Distribution and Abundance
CONCEPT 9.1 Populations are dynamic
entities that vary in size over time and
space.
CONCEPT 9.2 The distributions and
abundances of organisms are limited by
habitat suitability, historical factors, and
dispersal.
CONCEPT 9.3 Many species have a
patchy distribution of populations across
their geographic range.
Chapter 9 Population Distribution and Abundance
CONCEPT 9.4 The dispersion of
individuals within a populations depends
on the location of essential resources,
competition, dispersal, and behavioral
interactions.
CONCEPT 9.5 Population abundances and
distributions can be estimated with areabased counts, distance methods, mark–
recapture studies, and niche models.
From Kelp Forest to Urchin Barren: A Case Study
Waters around the Aleutian Islands have
abundant marine life, including the sea
otter and kelp forests.
From Kelp Forest to Urchin Barren: A Case Study
Kelp are brown algae that form dense
forest-like stands and harbor a diverse
community of organisms.
Some islands are surrounded by “urchin
barrens”—many sea urchins, very little
kelp, low species diversity.
Grazing of kelp by sea urchins could
explain the differences.
From Kelp Forest to Urchin Barren: A Case Study
Field studies show a negative correlation
between urchin abundance and kelp
cover.
Experimental studies: Urchins were
removed from plots with no kelp. Kelp
abundance increased dramatically.
But what determines urchin abundance?
Figure 9.2 Do Sea Urchins Limit the Distribution of Kelp Forests?
Introduction
Distribution: Geographic area where
individuals of a species occur.
Abundance: Number of individuals in a
given area.
Ecologists try to understand what factors
determine the distribution and
abundance of species.
Introduction
Populations are dynamic—distribution
and abundance can change over time
and space.
Understanding the factors that influence
these dynamics helps us manage
populations for harvest or conservation.
CONCEPT 9.1
Populations are dynamic entities that vary
in size over time and space.
Concept 9.1
Populations
Population: Group of interacting
individuals of the same species living in
a particular area.
Interactions include sexual reproduction
and competition.
Concept 9.1
Populations
Abundance can be reported as
population size (# of individuals), or
density (# of individuals per unit area).
Example: On a 20-hectare island there
are 2,500 lizards.
Population size = 2,500
Population density = 125/hectare
Concept 9.1
Populations
Sometimes the total area occupied by a
population is not known.
It is often difficult to know how far
organisms or their gametes can travel.
When the area isn’t fully known, an area
is delimited based on best available
knowledge of the species.
Concept 9.1
Populations
Individuals can be defined as products of
a single fertilization: The aspen grove
would then be a single genetic
individual, or genet.
If members of a genet are independent
physiologically, each member is called
a ramet, e.g. strawberries.
CONCEPT 9.2
The distributions and abundances of
organisms are limited by habitat suitability,
historical factors, and dispersal.
Concept 9.2
Distribution and Abundance
Habitat Suitability
Abiotic features: Moisture, temperature,
pH, sunlight, nutrients, etc.
Some species can tolerate broad ranges
of physical conditions, others have
narrow ranges.
Concept 9.2
Distribution and Abundance
Creosote bush is very tolerant of dry
conditions, as well as cold, and occurs
widely in North American deserts.
Saguaro cactus can tolerate dry
conditions, but not cold temperatures
and has a much more limited
distribution.
Figure 9.7 The Distributions of Two Drought-Tolerant Plants (Part 1)
Figure 9.7 The Distributions of Two Drought-Tolerant Plants (Part 2)
Concept 9.2
Distribution and Abundance
Some species distributions depend on
disturbance:
Events that kill or damage some
individuals, creating opportunities for
other individuals to grow and
reproduce.
Example: Some species persist only
where there are periodic fires.
Concept 9.2
Distribution and Abundance
Historical Factors
Evolutionary history and geologic events
affect modern distribution of species.
Example: Polar bears evolved from
brown bears in the Arctic. They are not
found in Antarctica because of an
inability to disperse through tropical
regions.
Concept 9.2
Distribution and Abundance
Dispersal
Dispersal limitation can prevent
species from reaching areas of suitable
habitat.
Example: The Hawaiian Islands have
only one native mammal, the hoary bat,
which was able to fly there.
Concept 9.2
Distribution and Abundance
Dispersal limitation has also been shown
in plant species:
When seeds of the annual plant
Impatiens capensis were dispersed by
hand, the distribution of the plants
expanded.
Figure 9.10 Populations Can Expand after Experimental Dispersal
Concept 9.2
Distribution and Abundance
27 populations of English bluebells were
established experimentally in suitable
habitat in 1960.
After 45 years, 11 populations persisted,
each with many individuals.
This suggests that dispersal limitation
prevented the bluebells from reaching
habitat where they could thrive.
Concept 9.2
Distribution and Abundance
Dispersal can also affect population
density, and vice versa.
Many species of aphids produce winged
forms (capable of dispersing) in
response to crowding.
The percentage of offspring that
develop wings increases as the density
of aphids increases.
CONCEPT 9.3
Many species have a patchy distribution of
populations across their geographic range.
Concept 9.3
Geographic Range
Geographic range—the entire
geographic region over which a species
is found.
Concept 9.3
Geographic Range
There is great variation in species
geographic ranges:
Devil’s Hole pupfish live in a single
small desert pool.
Many tropical plants have small ranges.
In 1978, 90 new species were
discovered, restricted to a single
mountain ridge in Ecuador.
Concept 9.3
Geographic Range
Geographic range includes areas
occupied during all life stages.
Some species, such as monarch
butterflies, migrate long distances
between summer and winter habitats.
For some species, it is difficult to find all
the life stages and the ranges they
inhabit.
Figure 9.13 Monarch Migrations
Concept 9.3
Geographic Range
Not all habitat within a range is suitable,
resulting in patchy distributions.
This can operate at different spatial
scales.
At large scales, climate may dictate
locations of populations. At small scales,
soils, topography, other species, etc.,
can determine patchiness.
Concept 9.3
Geographic Range
Patchiness at different scales is
illustrated by the shrub Clematis
fremontii.
It is restricted to areas of dry, rocky soil
with few trees, called barrens or
glades.
The glades occur on outcrops of
limestone on south- or west-facing
slopes.
Figure 9.14 Many Populations Have a Patchy Distribution
CONCEPT 9.4
The dispersion of individuals within a
population depends on the location of
essential resources, competition, dispersal,
and behavioral interactions.
Concept 9.4
Dispersion within Populations
Dispersion: Spatial arrangement of
individuals within a population:
Regular—individuals are evenly spaced
Random—individuals scattered randomly
Clumped—the most common pattern
Figure 9.16 Dispersion of Individuals within Populations
Concept 9.4
Dispersion within Populations
Dispersion patterns often result from the
distribution of resources.
Random or clumped dispersion can also
result from short dispersal distances.
Competition appears to result in the
regular dispersion of some species.
CONCEPT 9.5
Population abundances and distributions
can be estimated with area-based counts,
distance methods, mark–recapture studies,
and niche modeling.
Concept 9.5
Estimating Abundances and Distributions
Complete counts of individual organisms
in a population are often difficult or
impossible.
Several methods are used to estimate the
actual abundance or absolute
population size.
Concept 9.5
Estimating Abundances and Distributions
Relative population size: Number of
individuals in one time period or place
relative to the number in another.
Estimates are based on data presumed to
be related to absolute population size.
Examples: Number of cougar tracks in a
given area, or number of fish caught per
unit of effort.
Concept 9.5
Estimating Abundances and Distributions
Interpretation of relative population size
can be tricky.
Example: Number of cougar tracks is
related to population density, but also
activity levels of individuals.
Concept 9.5
Estimating Abundances and Distributions
Area-based counts: individuals in a
given area or volume are counted.
Used most often to estimate abundance
of immobile organisms.
Concept 9.5
Estimating Abundances and Distributions
Quadrats: Sampling areas of specific
size, such as 1 m2.
Individuals are counted in several
quadrats; the counts are averaged to
estimate population size.
Ecological Toolkit 9.1, Figure A An Underwater Quadrat
Concept 9.5
Estimating Abundances and Distributions
Example:
40, 10, 70, 80, and 50 chinch bugs are
counted in five 10 cm x 10 cm (0.01
m2) quadrats.
(40  10  70  80  50) / 5
2
 5000/m
0.01
Concept 9.5
Estimating Abundances and Distributions
Distance methods
Distances of individuals from a line or
point are converted into estimates of
abundance.
Concept 9.5
Estimating Abundances and Distributions
Line transects:
Observer travels along line and counts
individuals and their distance from the
line.
Ecological Toolkit 9.1, Figure B Counting Trees from a Line Transect
Concept 9.5
Estimating Abundances and Distributions
Mark–recapture studies:
Used for mobile organisms.
A subset of individuals is captured and
marked or tagged, then released.
At a later date, individuals are captured
again, and the ratio of marked to
unmarked individuals is used to
estimate population size.
Ecological Toolkit 9.1, Figure C Release of Marked Salmon
Concept 9.5
Estimating Abundances and Distributions
Niche Modeling
Ecological niche: Abiotic and biotic
conditions that a species needs to
grow, survive, and reproduce.
A niche model predicts a species’
distribution based on conditions at
locations the species is known to
occupy.
Concept 9.5
Estimating Abundances and Distributions
Niche model for chameleons in
Madagascar:
Environmental data were recorded for 1 x
1 km2 “grid cells.”
“Habitat rules” for each species described
environmental conditions where it was
most likely to be found.
Concept 9.5
Estimating Abundances and Distributions
A computer program (GARP) compared
grid cell data with habitat rules for each
species.
Predictions of species occurrences were
correct 75–85% of the time.
In areas where predictions were not
correct, researchers found 7 previously
unknown chameleon species.
Figure 9.19 Predicted Distributions of Madagascar Chameleons
A Case Study Revisited: From Kelp Forest to Urchin Barren
Sea urchin barrens:
Can persist for years after kelp is gone.
Urchins can survive on other foods—
other algae, benthic diatoms, detritus.
When food is very scarce, they can
reduce their metabolic rate, reabsorb
sex organs, and absorb dissolved
nutrients directly from seawater.
A Case Study Revisited: From Kelp Forest to Urchin Barren
But urchins are vulnerable to predation
by sea otters.
Aleutian Islands that have sea otters
have few sea urchins, leading to the
hypothesis that sea otters control
population size of urchins, and thus the
distribution of kelp forests.
A Case Study Revisited: From Kelp Forest to Urchin Barren
Estes and Duggins (1995) compared
sites with and without otters to confirm
the hypothesis.
In southern Alaska sites newly colonized
by sea otters, urchins disappeared
within two years, and kelp densities
increased dramatically.
Figure 9.20 The Effect of Otters on Urchins and Kelp (Part 1)
A Case Study Revisited: From Kelp Forest to Urchin Barren
In Aleutian Islands sites, otters reduced
large urchin biomass by about 50%, but
new urchin larvae arrived via ocean
currents to continuously recolonize.
Kelp forest recovery was slower at these
sites.
Figure 9.20 The Effect of Otters on Urchins and Kelp (Part 2)
A Case Study Revisited: From Kelp Forest to Urchin Barren
Historically, sea otters were abundant
throughout the North Pacific, but were
hunted to near extinction for furs.
By 1911, only scattered colonies
survived.
By the 1970s, otter populations had
begun to recover, but declined again in
the 1990s, and urchins made a
comeback.
Figure 9.21 Killer Whale Predation on Otters May Have Led to Kelp Declines (Part 1)
A Case Study Revisited: From Kelp Forest to Urchin Barren
Recent sea otter declines may be due to
predation by killer whales, but it is
unclear why killer whales began to eat
more sea otters.
Whaling may have reduced killer whale
preferred prey, causing them to switch to
seals, sea lions, and then otters.
A Case Study Revisited: From Kelp Forest to Urchin Barren
Or, reduced fish populations and lack of
food reduced seal and sea lion
populations and killer whales turned to
sea otters.
Figure 9.21 Killer Whale Predation on Otters May Have Led to Kelp Declines (Part 2)