Ecology 2
Population Ecology
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A population is a
group of individuals
of a single species
living in the same
general area.
They rely on the same
resources and are
influenced by the
same environmental
factors.
Soay Sheep from the
Scottish island Hirta, an
ancient and rare breed.
Qu. Why do populations of
some species fluctuate
greatly while others do
not?
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Hector’s Dolphins
If organisms move too
quickly to count, biologists
may use the Mark-Recapture
sampling method to estimate
population size.
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Density is the number
of individuals per unit
area.
In rare cases, biologists
can physically count
the number of
individuals.
However, most often
biologists use a variety
of sampling
techniques.
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Additions to
populations occur
through births and
immigration (the influx
of new individuals
from other areas.)
Removing individuals
occurs by death and
emigration (the
movement of
individuals out of the
population)
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Demography is the
study of the vital
statistics of
populations and how
they change over time
Life Tables are agespecific summaries of
the survival patterns
of a population
Life Table for Belding’s Ground
Squirrels
Age
No.
No.
Death Avg.
(yrs)
alive
deaths rate
life
at start
exp.
0-1
337
207
.61
1.33
1-2
252
125
.50
1.56
2-3
127
60
.47
1.60
3-4
67
32
.48
1.59
4-5
35
l6
.46
1.59
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A survivorship curve
is a plot of the
proportion or
numbers in a
population that are
still at each age.
Assume 1,000
squirrels were present
at the beginning, and
plot the numbers vs.
age.
This survivorship curve
indicates relatively constant
rates of death; males have a
lower survival rate overall
than females. (Type II curve)
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Survivorship curves can
be classified into three
general types:
Type I—flat at the start,
reflecting low death
rates, then drops steeply
as population ages
Type III—drops sharply
at start, reflecting very
high death rates for
young, but flattens out
as death rates decline
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The exponential
model describes
population growth in
an idealized,
unlimited
environment.
Under these
conditions, there are
no restrictions on the
abilities of individuals
to harvest energy,
grow and reproduce.
Change in population size = (B + I) – (D + E)
 (Births + Immigrants )- (Deaths + Emigrants)
 N = (population size) ∆N = change in
population size
 t = time ∆t = time interval
Simplified:
 Thus: ∆ N
∆t = Births - Deaths

Population ecologists are most interested in the
difference between the per capita birth rate and
the per capita death rate. This difference is the
per capita rate of increase, or r:
 r = b-d (Rate of increase = birth rate – death rate)
 Zero Population Growth occurs when the per
capita birth rate and death rate are equal. Births
and deaths still occur,
but they balance
each other exactly.
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Birth rates exceed death rates. The population
accumulates more new individuals per unit of
time when it is large than when it is small, thus
the curve gets increasingly steep. Can this be
sustained?
The population of elephants in
Kruger National Park, South Africa
grew exponentially for 60 years after
they were first protected from
hunting.
The increasingly large number of
elephants eventually caused enough
damage to vegetation that a collapse
in their food supply was likely,
resulting in action by park managers.
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The logistic model
describes how a
population grows
more slowly as it
nears its carrying
capacity.
Carrying capacity (K)
is the maximum
population size that a
particular
environment can
sustain.
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As the population nears
the carrying capacity,
the per capita rate of
increase approaches
zero
The logistic model
assumes that
populations adjust
instantly to growth. In
reality, there is often a
lag time before the
negative effects are
realized.
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K-selected organisms=
Populations of a
roughly constant size
whose members have
low reproductive rates
The offspring produced
require extensive
postnatal care until they
have sufficiently
matured.
Humans are examples
of a K-selected species.
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r-selected organisms=
They are populations that
experience rapid growth
of the J-curve variety.
The offspring produced
are numerous, mature
quite rapidly, and require
very little postnatal care.
This population grows
fast, reproduces quickly,
and dies quickly. Bacteria
and mice are examples of
r- selected species
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Many factors that
regulate population
growth are densitydependent.
A death rate that rises
as population density
rises is density
dependent.
In other words, there is
a negative effect with a
large population
density.
In a crowded population,
increasing population density
intensifies competition for
declining nutrients and other
resources, resulting in a lower
birth rate.
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Without some type of
negative feedback
between population and
the vital rates of birth
and death, a population
would never stop
growing.
Density-dependent
regulation provides that
feedback, helping to
reduce birth rates and
increase death rates.
Territoriality--Gannets nest
virtually a peck apart. Up to
a certain population
density, birds can find a
nesting site—beyond that,
few additional birds can
reproduce successfully.
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Competition for Resources
Territoriality
Disease—can be worse in
crowded conditions
Predation—predators can
encounter and kill more prey
when crowded
Toxic Wastes—build up in
crowded conditions
Aggressive interactions that
increase with population size
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These factors occur
when the birth rate or
death rate does not
change with
population density.
A drought, for
example will affect
the death rate of
grasses regardless of
how crowded the
plants are.

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Human population
growth is no longer
growing exponentially,
but is still increasing
rapidly
The rate of growth
began to slow in the
1960s. Current models
project an annual
growth rate of .4%
By 2050, population
could be 9 billion.
This shows the movement of Sweden
to Zero Population Growth where
there is a low birth rate and a low
death rate. This took about 150 years.
Mexico should achieve this sometime
after 2050.