Download Population Ecology

POPULATION ECOLOGY
By C. Kohn, Waterford WI
WALKER & WHITETAILS
Milwaukee Journal Sentinel – PolitiFact Analysis
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“Walker made increasing the number of deer -- and the number of
hunters -- a central part of a tourism plan he unveiled Oct. 14,
2010.”
“In the three-page document, Walker says Gov. Jim Doyle and the
state Department of Natural Resources have engaged in "political
games" and "put bureaucrats in Madison ahead of hunters of the
state."
“The result, he argues, is a smaller herd, fewer deer taken and fewer
hunters. In a news release, Walker claimed that the "deer
population has dwindled" as a result of "mismanagement" by Doyle
and the DNR.”
“Is it true the deer population has dwindled? And, if so, is the
frustration of hunter a result of political games and
mismanagement in Madison?”
CRITICAL QUESTIONS
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To determine if Walker’s stance is correct, we
have to address a few key questions first:
How do we actually determine how many deer are in
Wisconsin?
 How do we determine the impact of hunting?
 What other factors besides hunting affect deer
management?
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Farming damage
 Traffic accidents
 Disease transmission
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How do the needs of the deer and the wishes of
hunters affect the management of Wisconsin’s deer
herd?
 Should this be a political debate? Or should this be
handled by scientists and ecologists instead?
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WILDLIFE MANAGEMENT
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This debate falls under the umbrella of wildlife
management and population ecology.
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
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Wildlife are any non-domesticated vertebrate animals,
including birds, mammals, reptiles, and amphibians
DETERMINING THE SIZE OF A POPULATION
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Most population sizes are estimates
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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.
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.
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EXAMPLE
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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.
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.
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MARK RECAPTURE EQUATION
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The Mark-Recapture Equation:
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If N = the total population of individuals of a species in a
given area, then
N = [1st catch] x [2nd catch] /[number caught twice]
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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
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FECUNDITY & FERTILITY
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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
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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
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FACTORS THAT NATURALLY LIMIT
POPULATION GROWTH
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nature, no species ever reaches its full
reproductive potential (fecundity)
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Direct killing and limits to reproduction inhibit
population growth
Genes do not code for natural population limits –
a species cannot genetically self-regulate its
population levels
With unrestricted access to resources, populations
increase indefinitely
 Factors outside of a species’ genes must limit the
growth and reproduction of a species’ population.
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FECUNDITY & FERTILITY
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With unlimited access to resources and no
population limits, a species’ population will
increase without limit.
NATURAL LIMITING FACTORS
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If a game manager’s goal is to increase the size of Wisconsin’s
deer herd, simply reducing hunting of a species is not enough.
A population ecologist or game manager must take into
consideration the impact of natural limits to population
growth as well as fertility and fecundity
These factors include…
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Resource Consumption (food, water) & predation
Breeding/nesting (cover)
Habitat suitability (lack of pollution, invasive species, fragmentation)
Availability of Mates
Emigration and Immigration (individuals leaving, individuals coming)
If game managers need to change a species’ population, they
must use one or more of these factors. All must be taken into
consideration in any game management decision.
CARRYING CAPACITIES
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A game manager must also consider what is too many
of an animal for a particular habitat.
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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.
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Most K-values naturally fluctuate from year depending on
the availability of resources.
FECUNDITY & FERTILITY
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With unlimited access to resources and no
population limits, a species’ population will
increase without limit.
K-VALUES AND SATURATION POINTS
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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.
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The habitat is “saturated” with individuals of that
species and has as many as it can sustain.
DISPERSAL PATTERNS
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Carrying Capacities, or K-values, are more like
abstract ideas rather than concrete numbers.
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You won’t find a specific maximum number for a habitat,
only a general idea of what would be an unsustainable
population.
K-values can also be affected by the dispersal
patterns of a species.
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.
 For example, deer are managed in state units rather than
as an entire state herd for this reason.
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DISPERSAL PATTERNS OF WILDLIFE
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Density: the concentration of the individuals of a species
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Dispersion: the pattern of spacing of a population’s
individuals.
3 dispersal patterns include…
 Clumped: when individuals of a
species are more likely to be
together in groups
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Uniform: when individuals of a
species are more likely to equally
distanced from each other.
Random: when the arrangement
of a species follows no pattern and
is not predictable.
WISCONSIN DEER DISPERSION (DNR.GOV)
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Deer population estimates may be expressed in
terms of abundance or density.
Abundance estimates are the total number of deer
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 because they standardize abundance
estimates by taking into account the difference in
size of deer management units.
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This takes into account how dispersed deer are in a
unit.
DEER ABUNDANCE MAPS
DEER DENSITY MAPS
DEER ABUNDANCE AND DENSITIES IN
WISCONSIN DEER MANAGEMENT UNITS
DNR.GOV
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It is important to keep in mind that 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.
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Density within a unit can vary greatly from habitat to
habitat
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
 i.e. a habitat with low hunting pressure will have a higher
density
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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.
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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.
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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.
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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
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?
NEXT TIME
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Next Week – Game Management Techniques