SAMPLING TO DETECT RARE SPECIES

Often a sampling program has the objective of detecting the presence o f one or more species. One might wish to obtain a species list for the habitat, or to detect the presence of a rare and possibly endangered species. How can the sampling effort necessary for the detection of a rare species can be determined? The Poisson and the negative binomial are two possible spatial distributions that could be assumed. The Pois son assumption leads to the simple relationship n = -(1/m)log beta, wh ere n is the number of quadrats needed to detect the presence of a spe cies having density m, with a chance beta (the Type 2 error probabilit y) that the species will not be collected in any of the n quadrats. Ev en if the animals are not randomly distributed the Poisson distributio n will be adequate if the mean density is very low (i.e., the species is rare, which we arbitrarily define as a true mean density of <0.1 in dividuals per sample unit), and the spatial distribution is not highly aggregated. Otherwise a more complicated relationship based on the ne gative binomial distribution would have to be used. Published sampling distributions of 37 unionid mollusc species over river miles (distanc e measured along the path of the river; 1 mile = 1.609347 km) in two s outhern Appalachian rivers were evaluated to determine the appropriate ness of the simple Poisson- based formula for estimation of necessary sample size to detect species presence. For each of 273 species x rive r mile combinations we estimated the mean, the variance, and the negat ive binomial parameter k, and then estimated ''necessary n'' from both the Poisson- and the negative-binomial-based formulae. We defined ''P oisson adequacy'' to be the proportion that the Poisson estimate is of the negative binomial estimate of necessary sample size, and stated t he requirement that it be >0.95. Only 8 of the 273 cases represented r are species that failed this requirement. Thus we conclude that a Pois son-based estimate of necessary sample size will generally be adequate and appropriate.