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.