Pitfall trapping bias and arthropod body mass

.  1. Pitfall trapping is one of the most widely used arthropod sampling techniques. However, relative species abundance in pitfall trap catches differs from that found using quadrat samples. This difference, here termed bias, reflects the fact that pitfall trap catch is influenced not only by abundance but also by other factors, including activity, which may be linked to body size. Here, we investigate whether the bias affecting pitfall trap catches of a particular species is related to the typical body mass of individuals of that species. 2. Data were extracted from five studies where pitfall trapping bias was quantified and covered 32 species of spiders and carabid beetles. Bias was expressed as the ratio of biomass captured by pitfall traps to that measured by quadrat counts (four field studies), or as the product of mean speed of movement and probability of capture per trap encounter, at standard density (one laboratory study). 3. Pitfall trapping bias and body mass were strongly related on a log–log scale, with log(body mass) explaining 78% of the variation in log(bias) ( P  < 0.0001). There was no significant effect of arthropod group (spiders, carabids) or study location (field, laboratory). 4. A method is proposed to correct pitfall catches of each of a group of species, based on the typical body mass of an individual of each species. This approach may remove much of the bias in pitfall trap data and so improve the value of this type of data in studies of arthropod communities.