Population Ecology
Population – a group of organisms of the same
species occupying a particular space at a
particular time.
Populations are units of study.
Population Attributes
• Density – size of a population in relation to
a definite unit of space
• Affected by:
– Natality – the reproductive output (birth rate) of
a population
– Mortality – the death rate of organisms in a
population
– Immigration – number of organisms moving
into the area occupied by the population
– Emigration – number of organisms moving out
of the area occupied by the population
Population Density
Four primary population parameters:
Density Examples
Two fundamental attributes that affect our choice of
techniques for population estimation are the size and
mobility of the organism with respect to humans.
Range of Population Density
Two Types of Density Estimates
• Absolute Density – a known density such
as #/m2
• Relative Density – we know when one area
has more individuals than another
Measuring Absolute Density
• Total Count – count the number of
organisms living in an area
– Human census, number of oak trees in a
wooded lot, number of singing birds in an area
– Total counts generally are not used very often
• Sampling Methods – use a sample to
estimate population size
– Either use the quadrat or capture-recapture
method
Quadrat Method
• A Quadrat is a sampling area of any shape
randomly deployed. Each individual
within the quadrat is counted and those
numbers are used to extrapolate
population size.
– Example: a 100 square centimeter metal
rectangle is randomly thrown four times and
all of the beetles of a particular species within
the square are counted each time: 19, 21, 17,
and 19. This translates to 19 beetles per 100
cm2 or 1900 per m2.
Transects as Quadrats
• Each transect was 110 meters long and 2m
wide (220 m2 or 0.022/ha). All trees taller
than 25 cm counted.
Capture-recapture Method
• Important tool for estimating density, birth
rate, and death rate for mobile animals.
• Method:
– Collect a sample of individuals, mark them,
and then release them
– After a period, collect more individuals from
the wild and count the number that have marks
– We assume that a sample, if random, will
contain the same proportion of marked
individuals as the population does
– Estimate population density
Assumptions For All CaptureRecapture Studies
• Marking technique does not increase
mortality of marked animals
• Marked individuals are allowed to mix with
population
• Marking technique does not affect catch
probability
• Marks are not lost or overlooked
• No significant immigration or emigration
• No significant mortality or natality
Peterson Method or Lincoln Index
Marked animals in
second sample
Total caught in
second sample
5 = 16
20 N
=
Marked animals in
first sample
Total population
size
N = (20)(16)
5
N = 64
Indices of Relative Density
• Assume that samples represent some
relatively constant but unknown
relationship to total population size.
– # cars in the Piggly Wiggly parking lot
• Provides an index of abundance
– Is population increasing, decreasing, or
staying the same
– Are there more animals in one location than
another?
– Can not quantify differences between sites
 Twice the number of tracks does not = twice
as many animals
Some Indices Used
• Traps
• Number of Fecal
Pellets
• Vocalization
Frequency
• Pelt Records
• Catch per Unit Fishing
Effort
•
•
•
•
•
Number of Artifacts
Questionnaires
Cover
Feeding Capacity
Roadside Counts
Natality (birth rate)
• Fecundity – physiological notion that
refers to an organism’s potential
reproductive potential
– Usually inversely proportional to the amount of
parental care given to young
• Fertility – Ecological concept that is based
on the number of viable offspring
produced during a specific period
– Realized fertility – actual fertility rate
 One birth per 15 years per human female in
the child-bearing ages
– Potential fertility – potential fertility rate
 One birth per 10 to 11 months per human
female in the child-bearing ages
Natality Continued
• Absolute or crude natality – number of
new individuals per unit time
– 50 protozoa increase to 150 by division in one
hour, then crude natality = 100
• Specific natality – number of new
members divided by the population size
– 100 new protozoa in one hour from original 50
– 100/50 = 2 protozoa per original per hour
• 400 births in one year in a town of 10,000
– Absolute natality = 400
– Specific natality = 400/10,000 = 0.04 = 4%
Mortality
• Opposite of mortality is survival
• Longevity focuses on the age of death of
individuals in a population
– Potential longevity – maximum lifespan by an
individual of a particular species
 Set by the organisms physiology (dies of old age)
 Sometimes described as the average longevity of
individuals living in optimal conditions
– Realized longevity – actual life span of an organism
 Measured as an average for all animals living
under real environmental conditions
Determining Mortality
• Mark several individuals and measure how many survive
from time t to t+1.
• If abundance of successive age groups is known, then
you can estimate mortality between successive age
groups.
• Can use catch curves for fish:
Survival between
age 2 and 3=
147/292=0.50
Or develop regression
equation
292
147
Immigration and Emigration
• Seldom measured
• Assumed to be equal or insignificant
(island pop’s)
• However, dispersal may be a critical
parameter in population changes
Life Tables - Mortality
• Mortality is one of the four key parameters that
drive population changes.
• We can use a life table to answer particular
questions about population mortality rates.
– What life stage has the highest mortality?
– Do older organisms die more frequently than
young organisms
• A cohort life table is an age-specific summary of
the mortality rates operating on a cohort of
individuals.
• Cohort – a collective group of individuals
– Fish year class, all mice born in March, tadpoles from a
single frog, freshman year class
Cohort Life Table:
X = age
nx = number alive at time t
lx = proportion of organisms surviving from the start of the life
table to age x (ex: l1 = n1/n0, 0.217 = 25/115; l2=n2/n0,
0.165=19/115)
dx = number dying during the age interval x to x + 1 (ex:d0=n0-n1,
90 = 115-25; d1=n1-n2, 6=25-19)
qx = per capita rate of mortality during the age interval x to x + 1
(ex: q0=d0/n0, 0.78 =90/115; q1=d1/n1)
Per Capita Rates
• Per capita is a presentation of data as a
proportion of the population.
• Suppose a disease kills 400 ducks:
– If total duck population = 250,000 then the per
capita mortality = 400/250,000 = 0.16%.
– If total duck population = 2,500 then the per
capita mortality = 400/2,500 = 16%.
• Per capita gives us an idea of how the
entire population is affected.
• Allows us to standardize a population
Suvivorship Curve
Types of Survivorship Curves
• Type 1 – Mortality
rates high late in life
span
• Type 2 – Mortality
rates fairly constant
with age
• Type 3 – Mortality
rates highest early in
life span
Survivorship Curves
• Can be a reflection of
the amount of parental
care
• Can be affected by
population density
Unmanaged
area
Managed for
hunting
Adding Reproduction
Fertility Schedule
Population net reproductive rate
0.6% increase each generation
bx = natality; (lx)(bx) = reproductive output for that age class
R0 =  (lx)(bx) = net reproductive rate
Net Reproductive Rate
Under stable conditions R0 is usually around 1.
R0 < 1 population is declining, R0 = 1 population is
stable, R0 > population is increasing.
Can greatly affect the population growth rate, and
natural selection works towards an adaptive
reproductive schedule
Population Age Structure
R0 > 1
R0 = 1
R0 < 1
Population Age Pyramids
Dominant Year Class
• Fish reproduction
strongly affected by
year-to-year
fluctuations
• Population Age
Pyramids will be
different
Relationships
Natality Rates
Environmental
factors
Age
Composition
Mortality Rates
Rate of increase
or decrease of
the population
Growth With Discrete Generations
• Species with a single annual breeding season
and a life span of one year (ex. annual plants).
• Population growth can then be described by the
following equation:
Nt+1 = R0Nt
• Where
– Nt = population size of females at generation t
– Nt+1 = population size of females at generation t + 1
– R0 = net reproductive rate, or number of female offspring
produced per female generation
• Population growth is very dependent on R0
Multiplication Rate (R0) Constant
• If R0 > 1, the population increases geometrically
without limit. If R0 < 1 then the population
decreases to extinction.
• The greater R0 is the faster the population
Geometric Growth
increases:
Multiplication Rate (R0) Dependent
on Population Size
• Carrying Capacity – the maximum population size
that a particular environment is able to maintain
for a given period.
– At population sizes greater than the carrying capacity,
the population decreases
– At population sizes less than the carrying capacity, the
population increases
– At population sizes = the carrying capacity, the
population is stable
• Equilibrium Point – the population density that =
the carrying capacity.
Net Reproductive rate (R0) as a
function of population density:
Y = mX + b
Y = b – m(X)
N = 100, then R0 = 1.0
population stable
N > 100, then R0 < 1.0
population decreases
Intercept
N < 100, then R0 > 1.0
population increases
Remember, at R0 = 1.0
birth rates = death
rates
Growth With Overlapping Generations
• Previous examples were for species that
live for a year, reproduce then die.
• For populations that have a continuous
breeding season, or prolonged
reproductive period, we can describe
population growth more easily with
differential equations.
Multiplication Rate Constant
• In a given population,
suppose the probability of
reproducing (b) is equal to
the probability of dying (d).
– Instantaneous rate of
population growth = r = b – d
– Then dN/dt = (b – d)N = rN
– Population grows
geometrically
Multiplication Rate Dependent on
Population Size
dN
K-N
= rN
dt
K
Where:
N = population size
t = time
r = intrinsic capacity for increase
K = maximal value of N (‘carrying capacity’)
r
dN
K-N
= rN
dt
K
K
Pop. Size (K-N)/K
Growth Rate
1.0
1
99/100
0.99
1.0
50
50/100
25.00
1.0
75
25/100
18.75
1.0
95
5/100
4.75
1.0
99
1/100
0.99
1.0
100
0/100
0.00
Logistic Growth
Theoretical Growth Forms
Population Fluctuations
• Irruption – a boom in numbers followed by a bust
• Unpredictable, but usually happen when weather,
food, and shelter are all ideal
Population Cycles
• Population increase and decrease follow a
multiple year cycle
• Usually predator prey cycles, but not
always
Modeling Population Cycles
Population Level Pulsing
Density Independent / Dependent
Factors
• Physical factors such as unpredictable
weather, water currents, chemical limiting
conditions, or pollution can affect the
population no matter the size
• Biotic factors such as competition,
parasites, and predation often work as a
density dependent factor (more important
at higher densities)
Metapopulations
• Subpopulations
occupying discrete
patches of suitable
habitat separated by
unsuitable habitat
(except dispersal
corridors)
• Source vs. sink
r vs K selection
• r – intrinsic rate of increase
– r-selected species have evolved to put a lot of
energy into reproduction and growth
• K – carrying capacity
– K –selected species have evolved to put a lot
of energy into maintenance
r vs K selection