Introduction to Agriculture AAEC – Paradise Valley Population Ecology Introduction to Agriculture AAEC – Paradise Valley

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.

5th Period Starts on Slide #5 – “Factors that Naturally Limit Population Growth”

Factors that Naturally Limit Population Growth In nature, no species ever reaches its full reproductive potential 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.

Population Growth 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 various species, 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 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 carrying capacities naturally fluctuate from year depending on the availability of resources.

Population Growth With unlimited access to resources and no population limits, a species’ population will increase without limit.

Carrying Capacity 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 the carrying capacity for its habitat, this is known as the Saturation Point. The habitat is “saturated” with individuals of that species and has as many as it can sustain.

Dispersal Patterns Carrying Capacities 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. Carry Capacities 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 carrying capacity 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.

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.