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On the question of how many visits are needed, it might be worth looking at Chao's Estimator. It must be on the web somewhere. Once you have made a few collections, it predicts how many species are present in total. It is a simple calculation based on the number of species you have found only once and the number you have found more than once, though there are a lot of ifs and buts to take into account. The principle is: if there are lots of species you have found only once, there are probably still many present that you have not yet found at all.

John