Association among species of infrequent occurrence: the insect and spider fauna of Polyporus betulinus (Bulliard) Fries.

Abstract In studying how fungus-dwelling species of insects and spiders are distributed among sporophores of the bracket fungus Polyporus betulinus , the question arises as to whether the species are associated, and if so, how. Association may be of two kinds: non-segregative in which the species, although mutually independent, are apparently crowded into fewer brackets than are present; and segregative , in which recurrent groups of species are formed owing to differences among the species in their reactions to the different brackets or to one another. The number of brackets (the sampling units) in each collection was small; the numbers of species were large, but most species occurred in only one or a few brackets of a collection. Therefore new methods of treating the data had to be devised. They involve a comparison between the observed and expected numbers of distinguishably different classes of bracket, regardless of the number in each class, when brackets were classified according to the species they contained. It was found that slight non-segregative association occurred generally. Evidence of segregative association was obtained from only three of the 13 bracket collections examined. Studies of association, and of recurrent groups of species have usually been concerned with abundant species found in a great many sampling units. Similar studies when there are many infrequent species, and when only a few sampling units are obtainable, are also needed. The method here proposed should be generally useful in interpreting data from such populations.