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# Several methods are used to estimate the actual

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Several methods are used to estimate the actual abundance or absolute population size . Relative population size : Number of individuals in one time period or place relative to the number in another. Estimates are based on data presumed to be related to absolute population size. Examples: Number of cougar tracks in a given area, or number of fish caught per unit of effort. Area-based counts Used most often to estimate abundance of immobile organisms. Quadrats : Sampling areas of specific size, such as 1 m 2 . Individuals are counted in several quadrats; the counts are averaged to estimate population size. Example: 40, 10, 70, 80, and 50 chinch bugs are counted in five 10 cm ‘ 10 cm (0.01 m 2 ) quadrats. Distance methods Distances of individuals from a line or point are converted into estimates of abundance. Line transects : Observer travels along line and counts individuals and their distance from the line. Mark–recapture studies : Used for mobile organisms. A subset of individuals is captured and marked or tagged, then released. At a later date, individuals are captured again, and the ratio of marked to unmarked individuals is used to estimate population size. A life table is a summary of how survival and reproductive rates vary with age. A cohort life table follows the fate of a group of individuals all born at the same time (a cohort). Mostly used for sessile organisms. Organisms that are highly mobile or have long life spans are difficult to track. Static life table : Survival and reproduction of individuals of different ages during a single time period. Survivorship curve : Plot of the number of individuals from a hypothetical cohort that will survive to reach different ages.(3 types) A population can be characterized by its age structure —the proportion of the population in each age class. Growth rate (λ): Ratio of population size in year t + 1 ( N t +1 ) to population size in year t ( N t ). When age-specific survival and fecundity rates are constant over time, the population ultimately grows at a fixed rate. The age structure does not change—it has a stable age distribution . Geometric growth and exponential growth can lead to rapid increases in population size. Geometric growth : If a population reproduces in synchrony at discrete time periods and growth rate doesn’t change. The population increases by a constant proportion : The number of individuals added is larger with each time period. λ = geometric growth rate or per capita finite rate of increase . 0 N N t t Exponential growth : When individuals reproduce continuously , and generations can overlap. r N d t d N rate of change in population size at each instant in time.

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• Spring '11
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