Testing the effect of pitfall-trap installation on ant sampling

Digging-in effect is related to higher epigaeic invertebrate catches immediately after pitfall-trap installation, as first reported for the Australian fauna. However, an installation effect has not been tested for arboreal pitfalls. We tested whether samples taken with pitfalls dug (epigaeic stratum) or tied in a tree (arboreal stratum) at the same time, but were opened for sampling after different time periods, showed some pattern of ant activity density and richness in Brazilian closed-forest habitat. We did not observe any effect for epigaeic activity density and species richness catches caused by pitfall installation. The lack of evidence of a digging-in effect is probably due to differences in habitat complexity and the natural history of the Australian and South American ant faunas. We also observed an increase for arboreal-ant activity density and species richness with increased time after the arboreal pitfall installation. Probably, arboreal ants avoid strange objects (pitfalls) in the tree in the early phase, and then, over time, they might become familiarized with the pitfall and start to fall into the trap. We term this process the familiarization effect, referring to the time that the ants require to become familiar with the trap, which was about 4–7 days after the installation. These results suggest that in closed-forest habitats, precautions to avoid a digging-in effect may be unnecessary for epigaeic samples, but that it is best to wait at least 5 days after an arboreal pitfall is installed to begin sampling ants.