Summary The measurement of departure from randomness in spatial distributions has widespread application in ecological work. Several “indices of non‐randomness” are compared with regard to their dependence on sample number, sample size and density. Criteria for the best choice of index for specific situations are discussed. A new coefficient Cx is proposed for use with positively contagious distributions and tests of significance are given. When Cx and another index (S2/m−1) are used for positive and negative contagion respectively, values ranging from −1 through 0 (random) to +1 are obtained, regardless of sample number, sample size or density.

Often a sampling program has the objective of detecting the presence of one or more species. One night 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 Poisson assumption leads to the simple relationship n = -(1/m)log @b, where n is the number of quadrats needed to detect the presence of a species having density m, with a chance @b (the Type 2 error probability) that the species will not be collected in any of the n quadrats. Even if the animals are not randomly distributed the Poisson distribution 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 individuals per sample unit), and the spatial distribution is not highly aggregated. Otherwise a more complicated relationship based on the negative binomial distribution would have to be used. Published sampling distributions of 37 unionid mollusc species over river miles (distance measured along the path of the river; 1 mile = 1.609347 km) in two southern Appalachian rivers were evaluated to determine the appropriateness of the simple Poisson-based formula for estimation of necessary sample size to detect species presence. For each of 273 species x river mile combinations we estimated the mean, the variance, and the negative binomial parameter k, and then estimated "necessary n" from both the Poisson- and the negative-binomial-based formulae. We defined "Poisson adequacy" to be the proportion that the Poisson estimate is of the negative binomial estimate of necessary sample size, and stated the requirement that it be >0.95. Only 8 of the 273 cases represented rare species that failed this requirement. Thus we conclude that a Poisson-based estimate of necessary sample size will generally be adequate and appropriate.

1986. Growth of Lampsi'lis aadiata (~ivalvia: Unionidae) in sand and mud: a reciprocal transplant experiment. Can. j. Fish. Aquat. Sci. 43: 548-552.To examine the effects sf habitat on shell growth and form, freshwater unionid clams (Lampsilis radiata) were reciprocally transplanted between a sandy and a muddy site in inner Long Point Bay, Lake Erie. These were significant differences in the initial shell dimensions sf the two populations, with the sand clams being larger and less obese than the mud clams. Pretransplant growth rate analysis, using annual rings, showed that long-term growth in the sand was greater than that in the mud. After 16 wk the transplanted clams were recovered. Overall growth rate was affected by the source of the clams, while transplant destination affected shape change (height growth per unit length growth). This suggests that shell growth rates may be under direct genetic control, while shell shape can be adaptively modified by environmental cues.Pour examiner les effets de I'habitat sur la croissance et la forme de la coquille, on a transplant6 des coquillages unionides d'eau douce (Lampsilis radiata),dfun site sableux un site boueux et vice versa. Loexperience a eu lieu dans la baie Long Point interieure du lac Erie. Les dimensions initiales des coquilles ktaient tr6s afiferentes d'une population 3 l'autre, les coquillages de sable &ant plus gros et rnsins ob5ses que les coquillages de boue.L'analyse du taux de croissance avant la transplantation, dtapr$s les couches d'accrsissement annuel, a rnontre que la crsissance Zi long terme est plus forte dans le sable que dans la boue. Apres 16 sem, on a recupere les coquillages transplant6.s. h'origine des coquillages influait sur leua taux de croissance global, alors que le milieu dans lequel ils ktaient transplant& eonditionnait leur forme (croissance en hauteur par unit6 de croissance en longueur). Ces r6suItats pot-tent 2 croire que le taux de croissance des coquilles est command6 directement par des facteurs g6n6tiquesf alsrs que la fsrme de la csquille p u t subir des modifications adaptatives dietees par les conditions environnernentales.

No abstract