POPULATION ECOLOGY
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. 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.
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
re-set. 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. 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) 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.
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. 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
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 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.
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)
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.
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.
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
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
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DEER DENSITY MAPS
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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.
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.
Habitat management is critical for game management!
AGE DISPERSAL PATTERNS
Species can have spatial dispersion across a habitat (clumped, uniform, or random)
A species can also have age-dispersal patterns 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.
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 Regulating a species’ population is incredibly complex
because of the intense interaction of factors. A game manager must take into account…
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?
