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130 IVII. Zoocoenological characteristics The guestion of fidelity has another side to it. It can occur that a certain zoocoenosis contains, albeit at a restricted space and for a limited time, populations that are not normally part of that coenosis. These transitional variants, always limited in space and time, show different facets of the same zoocoenosis; the species creating these components are the differential species. Differential species include populations that characterise various facets of the same zoocoenosis that are limited in space and time. The differential species are distinguished from character species in that the former are not constant, but exceptional, members of the coenosis, limited to a small area and for a short time interval. Such a differential species in a Hyphantriaetena cuneae catenarium are the obstant populations of Tetramopria aurocincta and Diapria nigricornis that were only found in 1948 at Pakozd (Jermy, 1952), or the Theronia atalantae obstant population in the same catenarium, detected in 1954 in the Nyirseg (Szalay-Marzsö, 1957). § QUANTITATIVE CHARACTERISTICS 7. Abundance Abundance (density) indicates the number of individuals of a population in each zoocoenosis. Abundance is an absolute characteristic, using an unequivocal number to indicate actual size of the studied population in a zoocoenosis. Given that, here, we should express relations between numbers, even if this is tangential, we have to relate these numbers to a unit of area. Even if not ideal, we cannot dispute the analytical methods when using area or volume. However, we ought not to lose sight of the fact that a unit of area, or volume, does not represent a zoocoenosis but only a small fragment, that happens to be present in the space being studied. The extrapolation of data obtained in this way is always risky (see Kuehnelt, 1950), and we have to keep in mind that, even if we see absolute numbers, their value is only relative. Abundance itself is of relative constancy. Even in a locality-bound populace (scale insects, leaf miners), the numbers always change; some animals perish, fall prey to obstant elements, or are subject to the incursion of endoparasitoids, and that, then, they do not really represent their own population, but that of the given obstant element. In the case of a population of high vagility, abundance is even more relative because, here, not only space, but time should also be considered (for example, when counting bees, see Móczár, 1954). However, animals do not associate with a unit of space or volume but with the energy sources present in the given space. Consequently, the causes of abundance is not the unit of space, or volume, but the plants occurring, or the animals living, there. If this is so, then the abundance cannot always be related to a unit of area or space, because only the plants that occur there can
