Estimating the Abundance of Stone-dwelling Organisms: A New Method

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Abstract
A system of stratified sampling has been developed which allows the estimation of population densities of organisms which adhere to, or are associated with, stones in both lentic and lotic habitats, using the stones themselves as the primary sampling units. Knowing the stone profile of the habitat (i.e. the mean count, relative frequency, or mean surface area of the stones occurring within a size class), and using the appropriate statistical equations, an estimate of the mean number of individuals occurring within a predefined stone size class can be obtained along with its associated error estimates. Densities of the organisms can be expressed in terms of either a standardized unit area of substratum, average stone size in the habitat, or unit area of stone surface. The statistical theory and computational equations for each sampling method are presented, together with worked examples of the application of the methods in the field. The new sampling protocols are superior to conventional quadrat techniques in population size estimation because (1) the derived density and error estimates account for sources of variation arising from both the heterogeneity of the habitat and the spatial dispersion of the organisms; (2) physical disturbance to the habitat during the sampling program is minimal, and (3) the procedures allow simultaneous quantitative measures of both the density and spatial dispersion of a population.