Estimating Species Richness of Arthropods in Azorean Pastures : the Adequacy of Suction Sampling and Pitfall Trapping

The advantages of the complementary use of suction sampling (Vortis) and pitfall trapping in estimating the species richness of arthropods in sown and semi-natural pastures were investigated on three recent isolated oceanic islands in the Azores. In addition, the ability of pitfall trapping and suction sampling to describe species composition and richness were tested in terms of taxonomic and ecological groups. Both suction sampling and pitfall trapping techniques were reliable in terms of sampling efficiency, getting a good estimate of the arthropod species richness as judged by percentage sampling completeness, the ratio of observed to estimated species richness. Using the two methods together values of almost 90% completeness were obtained in some guilds. Pitfall and suction alone were reliable in estimating species richness of predatory arthropods (all species) and spiders. Joint data is needed for getting reliable estimates for herbivores guilds. In terms of presence-absence data, sap-feeding (suckers) and chewing herbivores showed very different patterns, with a higher proportion of the former sampled exclusively by suction sampling, whereas more chewing species were sampled only by pitfall traps. Similar patterns were obtained for two guilds of spiders, where more web-building species were sampled only by suction sampling, whereas most other species were sampled only by pitfall trapping. We conclude that pitfall trapping and suction sampling should be considered as complementary methods, not as alternatives. We therefore strongly recommend that they be used simultaneously in grassland and agroecosystem community studies, in order to derive reliable estimates of arthropod species composition and richness.