Understanding Population Ecology: Density, Dispersion, and Limiting Factors

1. Chapter 4 Notes, Population Ecology

2. 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.

3. 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.

4. 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)

5. Dispersion (Spatial Distribution)

6. 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.

7. 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.

8. 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.

9. 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.

10. 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 steepj-shaped curve. • The j-shape represents the rapid increase in growth, which is called exponential growth.

11. Exponential Growth Model

12. 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.

13. Population Growth Models

14. 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

15. 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