Relationships between body length, number of species and species abundancein soil mites and beetles

Three relationships of lasting interest to ecologists are those between bod y length and the number of species, between body length and individual spec ies abundances and between body length and the total number of individuals in a size class. We present data on these relationships for soil oribatid a nd mesostigmatid mites and beetles that are among the smallest animals in t errestrial ecosystems. These relationships have rarely been determined for the soil fauna but are of special interest because current models may not a pply to such very small animal size classes. Soil core samples from the Lan sdowne State Forest, NSW, Australia contained 79 oribatid, 34 mesostigmatid and 45 beetle species. The number of species did not in crease with decrea ses in body size as rapidly as theory predicts and the number of species de clined in the smallest size classes. The relative abundance and range of bo dy sizes were similar to those reported elsewhere, hence this trend is expe cted to be repeated across regions. Regression analysis of total abundance and body length showed a strong linear relationship with a slope of -2.31. Linear relationships between individual species abundance and body length w ere significant with slopes ranging from -0.67 to -2.25. The similarity in slopes be tween the two abundance relationships suggests that total abundan ce in a size class is independent of the number of species in that class.