popeco

Mr. Kousen is...Water Man
POPULATION ECOLOGY
Population ecology is a science that deals with measuring changes in population size and composition and identifying the factors that cause these changes. A population is define as a group of one species of organisms occupying the same general area, using the same resources, and acted upon by the same environmental factors. Populations cannot grow indefinitely, many populations will become stable over a period of time while others will show sharp increases followed by similar decreases. Population characteristics that afford study are its density and the spacing of its individuals. Population density is the number of individuals per unit area or volume. Population dispersion is the pattern of spacing among the inin the parameters of the geographical boundaries of the population.
Measuring Density:
Due to the impracticality of capturing or counting each individual in a given area, ecologists use a variety of methods to determine the density of various populations of organisms. Some of the sampling techniques used are as follows: Counting the number of nests or burrows in a given area, examining the number of tracks, examination of solid waste products left behind, and an actual capture method called mark - recapture. In the mark-recapture method, animals are trapped within their boundaries, marked with a long lasting sign, and released. At a later time some of the animals will be recaptured along with other that are not marked. The data is then placed into the following formula to calculate the population's density
This method assumes individuals have the same probability of being captured as unmarked individuals. This assumption is not always valid.
Patterns of Dispersion:
Local densities, within a population's range, may vary substantially due to differences in the limiting factors present. There are three general patterns of dispersion in relationship to other individuals: clumped, uniform, and random.

Clumped. is a pattern when individuals are aggregated in patches. This style is caused by a heterogeneous environment with resources concentrated in patches. Mating or social behavior of the individuals may also contribute to this type of dispersion.
Uniform. is a pattern of equally spaced individuals. Competition between individuals may set up zones or territories for feeding, nesting or breeding.
Random. is a spacing pattern based on total unpredictability. This form of dispersal is highly uncommon in nature. If it does it usually results from the absence of a strong competition among individuals.

While the above patterns apply to individuals within a population, keep in mind that populations within a species show dispersal patterns. Biogeography is the study of factors that influence the distribution of a species over its range.
Demography:
Demography is the study of the vital statistics affecting population size. This branch of science deals with the affects that immigration and emigration have on a given population. Other than new organisms leaving or entering the population birth and death rates are also studied.

Age Structure and Sex Ratio. Many populations have overlapping generations where individuals of more than one generation coexist. This situation produces an age structure in most populations. Every age group has a characteristic birth and death rate. A standard rule of thumb sets a high mortality for the lower and upper age groups and a low mortality rate for the intermediate age group. Generation time refers to the average time span between the birth of an individual and the birth of their offspring. Body size has a major effect on generation time. Mice,which are small,can produce a litter of individuals ever 21 days. Their offspring usually are ready to reproduce in 3 to 4 weeks after birth. On the other hand, the Elephant takes 2 years to produce an offspring and several more years before the offspring can successfully reproduce. The sex ratio plays an important role in the development of the population. Female number plays a dominant role in a population since they are the ones to produce the offspring. Males may mate with several females, diminishing them to a subordinate reproductive role.
Life Tables and Survivorship Curves. Life tables describe how birth rates and death rates vary with age over time corresponding to maximum life span. A variety of information about change in population size can be obtained from life tables. In a particular species of bird fewer than 20% of the original females survive to an age of three years, and a three year old female has a 44% chance of surviving to age 4. Mortality rates for females are constant through the middle years but higher in young and older birds. The birth rate is highest for 4 year olds and lower in very young and very old females. Survivorship curves plot the numbers in a cohort still alive at each age. There are three types of survivorship curves: Type I curves flat during early and middle life and drops suddenly as death rates increase among the older individuals. Type II curves are intermediate with mortality being more constant over the life span. Type III curves show very high death rates for the young followed by lower death rates after the individuals have survived to a certain critical age.

Density-Dependent and Density-Independent Factors:

Density Dependent Factors. Increasing population size reduces available resources and this limit ultimately limits population growth. In restricting population growth, a density-dependent factor intensifies as the population size increases, affecting each individual more strongly. population growth declines because the death rate increases, birth rate decreases or both. There is a reduction in the food supply which restricts reproduction resulting in less offspring. The competition for space to establish territories is a behavioral mechanism that may restrict population growth. Predators concentrate in areas where there is a high concentration of organisms. As long as the natural resources are available in sufficient quantity the population will remain constant. As the population decreases so do the predators. The accumulation of toxic wastes may also limit the size of a population. Intrinsic factors may play a role in limiting a populations size. High densities may cause stress syndromes resulting in hormonal changes that may delay the onset of reproduction. Immune disorders are also reported to be related to stress in high densely populated areas.
Density-Independent Factors. Weather, climate, and natural disasters such as freezes, seasonal changes, hurricanes, and fires are examples. These factors are unrelated to population size and affect everyone in the population regardless of population size.

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