Overcoming sampling bias in studies of terrestrial gastropods

Permanent plots, in the form of square masonite boards (30 × 30 cm), were placed in a variety of habitats as a means of sampling the fauna of living gastropods on a bighorn sheep range in southwestern Alberta. Fourteen species of gastropods, ranging in age from newly hatched young to overwintered adults, were found adhering to the undersides of these boards. The number of individuals found on plots was increased in some habitats by wetting them the day before inspection. In comparing this method with other sampling procedures, I concluded that the problem of differential visibility and fragility was resolved when using these boards. Furthermore, since all gastropods recorded were living individuals, there was no need to rely on the remains of dead animals (shells) as in sifted samples, with the inherent problem of deciding which shells were from animals recently dead. The use of permanent plots enables one to sample the exact location repeatedly without destroying it and thereby to compare numbers of individuals of each species in both space and time.