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# What models are useful in describing the growth of a population?

// How is population size regulated by abiotic and biotic factors?

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Organisms and Populations -- Ecology

Topic: Population Dynamics

Relevance

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 individuals within the parameters of the geographical boundaries of the population.

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

(number marked) x ( total catch the second time)

N= ________________________________________

number of marked recaptures

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.

Density-Dependent and Density-Independent Factors:

Density Dependent Factors. Increasing population size reduces available resources limiting 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 of death rate increase, birth rate decrease 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 prey. 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.

• 4 years ago

The butterfly is the elected symbol for chaotic math. The story goes that scientists were using a sophisticated computer program in an attempt to make long range weather forecasts. The computer was storing all values to far more decimal places than were being printed out. A subsequent computer run used only the printed out data values (neglecting the omitted decimal places) and the projection was vastly different. When they 'discovered' the cause, they concluded that even if they could produce a computer model with almost unlimited input, the fluttering of a butterfly's wings would upset the data and the projection would still be fatally flawed at some point into the future. Any program would be highly sensitive to the assumed initial conditions that is generally true for all chaotic math.