POPULATION ECOLOGY
CH 53
Study of the growth, abundance and
distribution of populations
I. POPULATION ABUNDANCE AND
DISTRIBUTION
A. Population size and density
• Population size = N = Total number of
individuals in the population
• Population Density = Number of
individuals per unit area
Calculating population size: Mark recapture
method
• Capture and tag a population then release it
• Recapture a population a count number
tagged
N-= population size
m = #captured and tagged
n = #second capture
x = #tagged in 2nd capture
x/n = m/N
B. Population dispersion: how is the
population dispersed in an area
Clumped is often
due to availability of
resources or social
behavior
Uniform is often due
to territoriality
Random dispersion is
due to no real
interaction between
members
Age structure
• The abundance of individuals of each
age
• Pyramid shaped curves indicate a
rapidly growing population
• Rectangular shaped curves indicate a
stable population
Survivorship curves
• Describes the mortality of individuals
during their lifetime
Type I species: most survive
to middle age or older
tend to be large mammals
that produce few offspring
but provide a lot of care
Type II species: length of
survival is random. Equal
likelihood of death at any
age
example rodents
Type III species: most of the individuals
die young
Tend to be organisms that produce a lot
of offspring with very little care
example oysters
II. POPULATION GROWTH
Population growth rate
1. exponential growth:
Maximum growth rate
of a species under
IDEAL conditions
dN/dt = rmaxN
Results in J shaped
curve
Rmax = biotic potential
Factors that affect the rmax of a species
• Age at which they reproduce
• Average # offspring produced in each
breed
• How frequently they reproduce
• How long they can reproduce
• How many offspring survive to
reproduce
• Greater rmax fly or elephant?
2. Logistic Growth
• Exponential growth is often seen in
very small, newly established
populations but can’t be sustained for
long
• As the population reaches the
carrying capacity of the environment,
growth decreases and approaches
zero
• Represented by the following
formula:
dN/dT = rmaxN(K-N/K)
• K = carrying capacity (maximum #
individuals environment can support)
• N= #individuals
• K-N = #of new individuals the
environment can support
• K-N/K = fraction of K that is still
available for population growth
• The smaller N is the closer KN/K is to 1 and the growth rate
approaches the maximum rate
(exponential)
• The larger N is the closer K-N/K
is to zero and the closer the
growth rate approaches zero
• What happens if N>K?
Carrying capacity:
• the maximum number of
individuals of a population
that the environment can
support
• Some populations overshoot
carrying capacity before
stabilizing at a certain density
• Some populations fluctuate
greatly
Factors that limit the carrying capacity and
biotic potential of a population
• Parasites and disease
• Resources
• Toxic effects of waste
• Stress of too many individuals inhibits
reproduction
• predators
• These are density dependent. What does
that mean?
III. REPRODUCTIVE PATTERNS ARE
THE RESULT OF NATURAL SELECTION
• Any trait that would enhance an
organism’s chances to survive and
reproduce is favorable
• However, there are tradeoffs
between survival and reproduction
• Reproduction requires an
investment of energy
• The greater the chance of survival,
the more energy can be invested in
reproduction
o Species whose young have a high
rate of survival like primates
tend to produce few larger
young and invest energy in their
care
The lower the chance of survival the less
energy will be invested in reproduction
o Species whose young have a lower
chance of survival, like insects,
produce a lot of smaller young
and don’t invest energy in their
care
reproductive strategies can be kselected or r-selected
K-selected strategies
• Tend to operate in a stable environment and is
under the influence of density dependent factors
• Selects for traits that are sensitive to population
density
• Organisms that are extreme kstrategists tend to have the following
reproductive characteristics:
o Fewer larger offspring
o Lots of parental care
o Slower maturity
o Often reproduce more than once
o Most offspring survive to
reproduce
• Because most offspring survive
they can invest a lot of energy in
their young
R-selected strategies
• Tend to operate in an unstable
environment and subject to density
independent factors
• Select for traits that maximize
reproduction
• Characteristics of r-strategists:
o
o
o
o
o
many small offspring
little or no parental care
early reproductive age
often reproduce only once
most offspring die before
they reproduce
• because survival of offspring is low
they tend to invest little energy in
their young