POPULATION ECOLOGY
By C. Kohn, Waterford WI
WILDLIFE MANAGEMENT

Population Ecology is the study of the factors
that affect the population levels, survival, and
reproduction of individual species in a specific
area.
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A population is the number of individuals of a species
in one area at one time.
Wildlife management is the application of
scientific knowledge and technical skills to
protect, preserve, conserve, limit, enhance, or
extend the value of wildlife and its habitat

Wildlife are any non-domesticated vertebrate
animals, including birds, mammals, reptiles, and
amphibians
DETERMINING THE SIZE OF A POPULATION

Most population sizes are estimates
It is impossible for ecologists and managers to count
every single species of wildlife.
 Most biologists use mathematical formulas to
estimate the size of a population rather than count
each individual.

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The Mark-Recapture Method is the most widely
used approach.
Mark-Recapture involves trapping and marking
individuals of a species.
 These individuals are then released and traps are reset.
 The proportion of the newly caught individuals is
used to determine the overall size of a population.

EXAMPLE
For example, let’s imagine we are counting
pheasant populations in the Waterford area.
 We set traps and catch 12 birds, which we then
tag.
 These birds are released, and several weeks later
we re-set the same traps.
 On the second try we catch 12 birds. Of the 12
birds, 4 have been previously tagged.


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This means that for this area, 4 out of 12, or 1/3 are
tagged.
If 1/3 are tagged, and we tagged 12 total, that would
mean that 12 is 1/3 of the total population for this
area.
If we multiply 12 times 3, we’d get the total
estimated population: 36 pheasants for the Waterford
area.
MARK RECAPTURE EQUATION

The Mark-Recapture Equation:

If N = the total population of individuals of a species in a
given area, then
N = [1st catch] x [2nd catch] /[number caught twice]

For example, in our pheasant example –
We caught 12 the first time.
 We caught 12 the second time.
 We re-caught 4 the second time.
 N = (12 x 12) / 4
 N = 144/4 = 36
 N = 36

FECUNDITY & FERTILITY


In Population Ecology, two terms serve as a basis
for the ability to maintain a population of a
species.
Fecundity – the maximum reproductive ability of
a breeding female of a species
E.g. whitetail deer can have 2-3 fawns per year max
 Human females have had over 40 children


Fertility – the actual reproductive performance of
a breeding female of a species
E.g. most whitetail does have 1 fawn per year
 Most human females have 1-2 children if they have
any

FACTORS THAT NATURALLY LIMIT
POPULATION GROWTH
 In
nature, no species ever reaches its full
reproductive potential (fecundity)

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This is because of natural population limits such as
predation, competition, and disease.
Genes do not code for natural population limits
A species cannot genetically limit its own population
levels
 With unrestricted access to resources, populations
increase without limit.
 Factors outside of a species’ genes must limit the
growth and reproduction of a species’ population.

CARRYING CAPACITIES

A game manager must also consider what is too many
of an animal for a particular habitat.

Every habitat has a maximum carrying capacity for
each species.

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The Carrying Capacity, or K-value, represents the
maximum number of individuals of a species that a
habitat can sustainably maintain.
Note: a Carrying Capacity is not a fixed number – it
will change each year based on weather, competition
from other species, and availability of resources.

Most K-values naturally fluctuate from year and from
place to place depending on the availability of resources.
FECUNDITY & FERTILITY

As species increase their population, their
performance in their habitat can change their
physical attributes.

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This can have a significant impact on game
management practices.
High game densities may at first seem ideal,
especially to hunters. However, dense game
populations can…
Reduce birthrates
 Reduce individual size of game
 Increase the spread of disease

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Source: http://www.cals.ncsu.edu/course/fw353/Estimate.htm
K-VALUES AND SATURATION POINTS

A species can temporarily surpass its carrying
capacity, but not for a long period of time

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If it does surpass its carrying capacity, its population
will crash if not reduced due to a shortage of
resources.
If a species reaches the K-value for its habitat
(the carrying capacity), this is known as the
Saturation Point.
The habitat is “saturated” with individuals of that
species and has as many as it can sustain.
 Exceeding the saturation point of a habitat can
quickly drive other species in that habitat to
extinction locally.

WISCONSIN DEER DISPERSION (DNR.GOV)

Game population estimates may be expressed in
terms of abundance or density.
Abundance estimates are the total number of
individuals estimated for an entire unit.
 Density can be calculated by dividing the abundance
estimate by the area (square miles) within the unit.

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Density estimates are useful for comparing
population estimates among deer management
units.
The amount of game per unit area matters more than
the total amount of game.
 For example 40 deer is a lot for one small forest but
not a lot for an entire county!

DISPERSAL PATTERNS OF WILDLIFE

Deer and other game never disperse themselves evenly.

This means that dividing total population of an animal by the
total land area will give you a density that is too low.
3 dispersal patterns include…
 Clumped: when individuals of a
species are more likely to be
together in groups (most common)

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Uniform: when individuals of a
species are more likely to equally
distanced from each other (rare).
Random: when the arrangement
of a species follows no pattern and
is not predictable.
DENSITIES AND CARRY CAPACITIES

Density is more than just deer per area.

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The habitat quality and amount in that area matters as much
or more as the total number of deer in that area.
Deer and other game do not disperse themselves evenly
across a county or deer management unit.
The level patchiness of their habitat affects the actual density
of deer and other game.
 For example, if only 20% of a county is suitable habitat for
deer, their density is 5x greater than the calculated density for
an entire county or deer management area.

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This is because 80% of the area would be unusable to them.
This means that even with a relatively low density for the
county, if there is a “perceived” high population density for
the deer it will result in smaller bucks, lower reproduction,
and increased spread of disease.
Deer densities will always be high if suitable habitat is
low.
 Why? TPS What impact would this have on hunting?

DISPERSAL PATTERNS

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Wildlife rarely have uniform dispersal
Their type of dispersal can create unequal
pressures on the resources of a particular
habitat.
For example, one part of a habitat may be over
its K-value while another part of the same
habitat may be under.
Deer are managed in state units rather than as
an entire state herd for this reason.
DEER ABUNDANCE MAPS
DEER DENSITY MAPS
Which area
will have the
bigger bucks?
DEER ABUNDANCE AND DENSITIES IN
WISCONSIN DEER MANAGEMENT UNITS
DNR.GOV

Density estimates for deer management units are
based largely on the number of antlered bucks
harvested in the unit.
The resulting density estimates are averages for the entire
unit and may not accurately reflect local deer density.
 Density within a unit can vary greatly from habitat to
habitat.

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There can be considerable local variation in density
within deer management units due to differences in
deer habitat quality and local hunting pressure.
i.e. a well managed habitat will have a higher density of
deer but can also allow for more reproduction and bigger
bucks.
 i.e. a habitat with low hunting pressure will have a higher
density but may also have smaller bucks and lower
reproduction.

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Habitat management is critical for game
AGE DISPERSAL PATTERNS

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Species can have spatial dispersion
across a habitat (clumped, uniform,
or random)
A species can also have age-dispersal patterns

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The investigation of changes in a species population
due to age is also major a part of population ecology.
This information can then be graphed to create a
survivorship curve.

A survivorship curve represents the numbers of a
species that are alive at each stage of life.
SURVIVORSHIP CURVES
A survivorship can fall into one of three
categories.
 Type I on the survivorship curve starts off
relatively flat and then drops off steeply at
an older age.
Death rates are relatively low until later in life when old age claims
most individuals.
 The death rate for Type I species is highest at old age. These species
tend to produce few young, as they are less likely to die due to good care.

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Type II is the intermediate category, with a steady even death
rate over the course of a species expected lifespan.


The risk of death is fairly consistent over the individual’s lifespan
Type III curves drop off steeply immediately, representing high
infant mortality, but then levels off for adults.
This type of curve is affiliated with species that produce large numbers
of young with the expectation that few of them will make it to maturity.
 Fish and frogs lay large numbers of eggs with only a small percentage
making it to adulthood. Plants often tend to be good examples,
producing many seeds, few of which become adults.

SURVIVORSHIP CURVES
REGULATING POPULATIONS

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Regulating a species’ population is incredibly complex
because of the intense interaction of factors.
A game manager must take into account…
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Resource Consumption (food, water) & predation
Breeding/nesting (cover)
Habitat suitability (lack of pollution, invasive species, and
fragmentation)
Availability of Mates (e.g. Earn of Buck vs. Earn a Doe)
Emigration and Immigration (individuals leaving, individuals
coming)
Carrying Capacity of a Habitat & Patchiness of Habitat
Average age of a species and its survivorship curve
Dispersion of a species and their resources
Bottom line – a population is not just a number, but
a collection of highly varying factors and inputs.
TPS – how could each of these factors increase and
decrease the population of a species in a particular habitat?