Chapter 4 Notes, Population Ecology

Population Characteristics Three main characteristics of a population are. Population density is the number of organisms per unit of area. Dispersion is the spatial distribution or the pattern of spacing or how the population is arranged within an area. Population growth rate explains how fast a population grows over time.

Population Density Population density is a number that can be determined by dividing the number of organisms in an area, by the size of the area. For example, let's say the Alaska Department of Fish and Game conducted a moose population survey and they counted 500 moose in a 100 square mile area. The population density would be 500 / 100 = 5 moose per square mile. 100 square miles would be 10 miles wide by 10 miles long. That's roughly about the size of the Wasilla and Palmer area.

Dispersion (Spatial Distribution) Uniform dispersion – organisms that are solitary and/or territorial often have a uniform dispersion pattern. (bear or wolverine) Clumped dispersion – organisms that live in colonies, herds, flocks, or packs have a clumped dispersion pattern. (like a pack of wolves or a herd of caribou) Random dispersion – organisms that are not very territorial or do not live in big groups tend to have a more random dispersion due to the availability of good habitat. (snowshoe hare)

Dispersion (Spatial Distribution)

Population-Limiting Factors Limiting factors that affect population size are called population-limiting factors. Density-independent factors are limiting factors that are not dependent upon the density of the population. These factors are usually abiotic. Examples of density-independent factors could be droughts, floods, extreme heat or cold, tornadoes, hurricanes, snowfall etc. Many moose and deer die in years of heavy snowfall regardless of their density.

Deep Snowfall's Effect on Moose Moose are extremely vulnerable to predation in the deep snow. Especially when the snow is hard enough to support the weight of a wolf.

Population-Limiting Factors Density-dependant factors are limiting factors that are affected by the population density. Density-dependant factors are often biotic factors like predation, disease, parasites, and competition for limited resources. Predation is high when the density of prey species is high. Prey are much easier to catch. Diseases and parasites spread more easily when densities are high because there is more contact between individuals within a population.

Population Characteristics Another important characteristic of populations is the population growth rate. The population growth rate explains how fast a population grows. Two primary factors you need to know in order to determine a population's growth rate are the natality (birth rate) and mortality (death rate). Two other factors that are also important are emigration and immigration. Emigration is moving away from the population. Immigration is moving into a population.

Population Growth Rate Models One way to understand population growth rates is to graph them to produce a model. An exponential growth model shows that as a population grows, its growth rate increases. When this happens the population experiences a period of slow growth called the lag phase. Then, after several generations, the curve begins to change into a steep j-shaped curve. The j-shape represents the rapid increase in growth, which is called exponential growth.

Exponential Growth Model

Population Growth Rate Models Another type of model that shows population growth rates is called the logistic growth model. The logistic growth model is similar to the exponential growth model, except the growth rate slows down or stops once the population reaches the carrying capacity. The carrying capacity is the maximum number of individuals in a species that a habitat can support. Logistic growth occurs when deaths exceeds births, or emigration exceeds immigration.

Population Growth Models

Reproductive Patterns or Strategies The rate strategy is used by r-strategists High natality and mortality Low parental care Short life spans Begin breeding early Fish, mice, rabbits

Reproductive Patterns or Strategies The carrying-capacity strategy is used by k- strategists Low natality and mortality High parental care Long life spans Begin breeding late Elephants, bears, humans