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

Three relationships of lasting interest to ecologists are those between body length and the number of species, between body length and individual species abundances and between body length and the total number of individuals in a size class. We present data on these relationships for soil oribatid and mesostigmatid mites and beetles that are among the smallest animals in terrestrial ecosystems. These relationships have rarely been determined for the soil fauna but are of special interest because current models may not apply to such very small animal size classes. Soil core samples from the Lansdowne State Forest, NSW, Australia contained 79 oribatid, 34 mesostigmatid and 45 beetle species. The number of species did not increase with decreases in body size as rapidly as theory predicts and the number of species declined in the smallest size classes. The relative abundance and range of body sizes were similar to those reported elsewhere, hence this trend is expected 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 were significant with slopes ranging from -0.67 to -2.25. The similarity in slopes between the two abundance relationships suggests that total abundance in a size class is independent of the number of species in that class.