Attraction of pitfall trap preservation fluids complicates the estimation of Collembola density

Collembola (springtail) communities consist of three eco-morphologically defined life forms: the euedaphics dwell inside the soil matrix, the epedaphics (including atmobiotics) live on the ground and in vegetation, and the hemiedaphics are intermediate. The vast majority of springtail community studies focus on the belowground (eu- and hemiedaphic) forms that are generally collected by taking and extracting soil cores. Few investigations have dealt with epedaphic Collembola that are usually captured with pitfall traps, and only very few studies so far covered all three life forms. When epedaphic and belowground species are sampled using both methods simultaneously, core data (true densities, [individuals m⁻²]) and pitfall data (activity abundances, [individuals trap⁻¹ length of trapping period⁻¹]) may be analyzed independently, but are incompatible in a common statistical framework. As a remedy, two competing numerical approaches to estimate true densities from activity abundances have been described in literature: the nested-cross array and the two-circle method. Attraction or deterrence effects of trap preservation fluids bias the density estimation of the nested-cross array, but not of the two-circle method. To determine whether this bias may be expected for Collembola, and thus which of the two methods should be used in future studies, we experimentally tested potential effects of preservation fluids on trap catch rates. Three preservation fluids (sodium benzoate, propylene glycol, formaldehyde) and a detergent (Tween80) significantly increased the number of captured springtails, thus demonstrating an attraction effect and the deficiency of the nested-cross array. In future studies of collembolan communities, we therefore suggest complementing the traditional focus on the eu- and hemiedaphic life forms by sampling epedaphic species using pitfalls, and subsequently remodelling the trapping data with the two-circle method.