Comparison of the efficacy and ethicality of pitfall and vacuum sampling in the collection of spiders (Araneae) in grassland environments

Invertebrates, including spiders, are ubiquitous components of terrestrial ecosystems and are sensitive to rapid habitat fluctuations. Assessing populations of invertebrates can be a useful bioindicator of environmental change. In the ecological sector, such data is most often gathered with the use of pitfall traps or vacuum sampling; each technique having its own benefits and ethical considerations. In a modern world, where many invertebrate species are shown to be in decline, the ethical scrutiny and refinement of sampling methods is more pertinent than ever. This study evaluated spider populations across grassland locations in Cornwall, UK, using pitfall trapping and vacuum sampling, in order to assess which technique was the most effective in the collection of spiders. Vacuum sampling collected a significantly higher abundance and species richness of spiders than pitfall trapping. In order to assess the ethical implications of each technique, trapped bycatch was evaluated to order level. Vacuum sampling achieved a significantly higher abundance of bycatch, though no significant difference in the bycatch richness at order level could be detected. The study finds that vacuum sampling far outperforms pitfall trapping in measuring spider abundance and species richness and collects a greater number of non-target taxa should this be of interest to the operator. We suggest that the underlying ethical advantage of vacuum sampling is that it gives the operator control of which specimens are to be euthanized. This allows non-target taxa to be released back to the environment, whereas pitfall trapping does not. The results provide a strong argument for the use of vacuum sampling in the place of pitfall trapping in the collection of spiders, however further work is needed to improve repeatability and understand species collection biases.