The effectiveness of pitfall trapping for the capture of epigaeic fauna has been widely discussed (Luff, 1975 de Oliveira et al., 2019 Privet et al., 2020 Saska et al., 2013 Southwood, 1978 Topping & Sunderland, 1992). This method does not allow reliable estimation of species densities (Curtis, 1980 Lang, 2000). Pitfall traps can be used to generate an estimate of “activity density,” that is, the abundance of each species as a reflection of its activity during the sampling period and the density of the population in the sampled habitat. This method is particularly advantageous for the capture of epigaeic macro- and megafauna with islet-like dispersion and more pronounced differences in circadian activity (Stašiov, 2015). Pitfall traps are simple to use, efficient, and inexpensive and allow for the continuous collection of specimens, including night foragers, thus overcoming interspecific differences in circadian activity rhythms (Koivula et al., 2003 Southwood, 1978 Törmälä, 1982 Ward et al., 2001). It is now a well-established and commonly used quantitative method of zoocoenosis surveys, including harvestman and millipede communities (Štrobl et al., 2019). Pitfall trapping was first used for the research of epigaeic fauna by Dahl ( 1896) and was further developed after the publication of Barber ( 1931) and Stammer ( 1948). Data obtained by pitfall trapping in various studies also have limited comparability (Brown & Matthews, 2016). The ability to make meaningful comparisons of the results of studies is severely hampered by great variation in the design of the sampling equipment and how it is used (Brown & Matthews, 2016 Dornelas et al., 2014 Fischer et al., 2010 Magurran et al., 2010). However, there might be significant constraints in the analysis of long-term and spatially large data, especially when the methodology used differs (Gotelli & Colwell, 2001, 2011). Understanding changes in global biodiversity patterns requires large-scale, long-term monitoring. However, the disadvantage of formaldehyde is its toxicity. Our results revealed that the combination of larger traps ( d = 5 and 12 cm) and formaldehyde was most effective in the capture of both studied groups. By analyzing the differences in the body sizes of the studied arthropods in relation to the trap diameter and fluid, we found that larger traps, as well as traps filled with NaCl solution, captured larger harvestmen more frequently than the other trap types. The same effect was observed for harvestmen species richness, whereas the medium traps ( d = 5 cm) captured the highest mean activity of harvestmen. Large ( d = 12 cm) and medium (5 cm) traps captured significantly more millipede species and individuals than the small-sized traps (3 cm).
We sampled 1,488 individuals representing 11 harvestmen species and 881 individuals representing 11 millipede species. Altogether, 90 traps representing nine combinations of trap diameters and fixing fluid were placed on a mown meadow in spring and autumn intervals for a total of 45 days.
We used pitfall traps with three different diameters: 3 cm, 5 cm, and 12 cm, filled with three types of fixing fluids (saturated fluid of NaCl, 10:1 mixture of 70% ethanol and glycerol and 4% formaldehyde). We evaluated the effects of the trap diameter, the fixing fluid, and their combination on the capture efficacy for harvestmen (Opiliones) and millipedes (Diplopoda). Despite being frequently used, the standardization of this method is problematic due to the large range of combinations of the individual parameters of pitfall traps with varying efficacy under different environmental conditions. Pitfall trapping is one of the standard methods used for the capture of ground-active arthropod groups.
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