OCR
138 IVII. Zoocoenological characteristics This degree of obstancy refers, exclusively, to the relationship between two semaphoront groups; for example, the % infestation by tachinid larvae of the pupae a certain butterfly species. As this interpretation of the degree of obstancy does not indicate the real role of an obstant element in the mortality of a given generation of a species, this empirical degree of obstancy should be distinguished from the gradological degree of obstancy (e.g. apparent vs. real mortality, Thompson, 1929; Chapman, 1931:206). The gradological degree of obstancy is the influence of an obstant population on the host species population size through decimating a given host population. There is no doubt that the empirical degree of obstancy is also a numerical expression of the decrease in density of the host resulting from the activity of a parasite or predator. This role, however, is restricted to one semaphoront group, and the number obtained does not express the importance of this factor among other mortality factors that also contribute to the overall mortality of the generation. If we compare the empirical degree of obstancy to this, it may turn out that the overall effect of this factor among the others is surprisingly small. The real role is expressed by the gradological degree of obstancy. Using an example, we can demonstrate the gradual diminishing of a generation and the gradological degree of obstancy of these factors. The example is the yellow scale insect of the pear, Diaspidiotus pyri (Szelényi, 1936). Mortality factor abstancy% | population, % | _abstaney. 9% Unhatched eggs 80 20 80 Larval mortality, predation 65 7 13 Mortality of established larvae 25 5.25 1.75 Mortality of L2 5 4.99 0.26 Parasitism, L2 5 4.74 0.25 It is evident that the precise quantification of the gradological degree of obstancy requires profound and idiobiological studies that can, realistically, only be madein small catenaria. Given that different populations of the same species may belong to different zoocoenoses, the assessment of the gradological degree of obstancy will, in most cases, go beyond the boundaries of the studied zoocoenosis. For this reason, when studying the relationships among populations of a coenosis, we can only establish the empirical degree of obstancy. The reason for discussing it here is that we can underline the relative value of the numbers expressing the empirical degree of obstancy, and that we appreciate that these are valid only within the studied coenosis, and not related to the species, but its one semaphoront group. The line of argument, through the gradological degree of obstancy, leads from the studied zoocoenosis to other ones.