03-Life_Tables - 3-1 Population ecology Lab 3 Population...

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3-1 Population ecology Lab 3: Population life tables I. Introduction to ecological populations, life tables, and population growth models This week we begin a new unit on population ecology. A population is a set of individuals of the same species living in a given region or habitat. Populations are examples of ecological systems. As such, they exhibit both structural and functional properties. The total number of individuals in a population, the age distribution of those individuals, the sex ratio of adults, probabilities of survival (or mortality), and rates of fecundity are key traits of population structure . Ecologists employ a variety of methods to study the structure of populations. Life history tables, or life tables , are a method of quantifying population structure that addresses all of the above population traits. Life tables provide age-specific information on survival and fecundity rates for a particular population. An ecologist can collect two very different types of life history data for individuals in a population, which can lead to two kinds of life tables. Horizontal (dynamic or cohort) life tables require ecologists to follow all the individuals of a single cohort in a population from birth to death. Construction of horizontal life tables frequently depends on the recapture of marked individuals for mobile species or repeated, representative samples of sessile species. Since individuals must be followed from birth to death, the horizontal life table technique is not well suited for the study of long-lived individuals. Vertical (static or time-specific) life tables consist of data on individuals of all ages in a population from a single point in time. In vertical life table studies, it is important to work with a large, random sample of individuals to ensure that the data is representative of the entire population. For example, the age distribution of your sample of individuals should reflect the age distribution of the whole population. Non-destructive sampling methods are particularly useful for the construction of vertical life tables as they minimize the impact of large sampling efforts on population dynamics. Both dynamic and static observations of population structure can be used 1) to quantify the age structure of a population; 2) to estimate an optimal age of sexual maturity; and 3) to predict population growth rates. Population growth projections are based on mathematical models such as the exponential growth model: N t = N 0 (e rt ). In the exponential growth model, the number of individuals at any time (t) can be predicted using the number of individuals at the starting time (N 0 ), the base of the natural logarithm (e = 2.7182818), the intrinsic rate of population growth (r), and the time (t) since N 0 . The intrinsic rate of population growth (r) is a direct consequence of the age structure of a population.
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