Population Ecology

Population Ecology By C. Kohn, Waterford WI

Walker & Whitetails Milwaukee Journal Sentinel – PolitiFact Analysis • “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 • 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? • Farming damage • Traffic accidents • Disease transmission • 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?

Wildlife Management • 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. • 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, thenN = [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) • 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.

Fecundity & Fertility • With unlimited access to resources and no population limits, a species’ population will increase without limit.

Natural Limiting Factors • 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… • 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 • 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 depending on the availability of resources.

Fecundity & Fertility • With unlimited access to resources and no population limits, a species’ population will increase without limit.

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 SaturationPoint. • The habitat is “saturated” with individuals of that species and has as many as it can sustain.

Dispersal Patterns • Carrying Capacities, or K-values, are more like abstract ideas rather than concrete numbers. • 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.

Dispersal Patterns of Wildlife • Density: the concentration of the individuals of a species • 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 • 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) • 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. • 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. • 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 • 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. • 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

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 • 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 • Next Week – Game Management Techniques

Population Ecology