ASSESSMENT OF THE EFFICIENCY OF A HAND-HELD SUCTION DEVICE FOR SAMPLING SPIDERS - IMPROVED DENSITY-ESTIMATION OR OVERSAMPLING

In the study a comparison is made between the results of two sampling procedures, both based on the application of the same hand-held suctio n apparatus. Sampling was aimed at spiders, and was carried out on two alfalfa fields. In the first method suction sampling was applied to a n enclosure of 0.48 m(2). The enclosed area was sampled intensively, w hich was facilitated by the removal of the vegetation. The second meth od was a transect sampling procedure during which the suction apparatu s with a 0.01 m(2) nozzle was applied to single unenclosed sampling po ints Im apart. A linear series of 48 such subsamples comprised a trans ect, thus the total area covered in a transect equalled the area of th e enclosure. In the transect samples three times more spiders were cau ght than in the enclosures. This result was consistent on different oc casions and at both fields. This basic trend was found in all spider f amilies that were present in significant numbers in the samples. Speci es composition in the samples collected by the two methods was similar , and species abundance ranks were highly correlated across dates. We propose that an ''edge effect'' can explain higher catches in transect samples. This edge effect is caused by lateral suction at the edges, which inflated the number of animals caught in the transect applicatio n.