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OCR
§ Quantitative characteristics | 139 The degree of obstancy can be of minimal importance and, indeed, it frequently is; gradology expresses this by saying that the studied animals have no effective natural enemies. We have a substantial body of evidence proving that the population dynamics of insects, in most cases, are largely determined by abiotic factors, dwarfing the influences from the direction of the zoocoenosis. Opposite cases are also known (Szelényi, 1954), and we ought not forget that, in the absence of systematic zoocoenological analyses, we know very little about the processes operating within zoocoenoses. Especially little is known about the degree of obstancy of temporal elements, yet their study can lead to surprising results, as shown in studies on graininfesting Heteroptera by Trem] and Batkina (1951). Several studies indicate that the degree of obstancy is influenced by the abundance of the host population. Smith (1935), among the mortality factors acting on populations, distinguished both density-dependent and densityindependent ones. Solomon (1949) modified these terms by referring to processes not factors (density-dependent or density-independent processes or actions), correctly arguing that these factors do not act in isolation but within the framework of the ecosystem dynamics. The biotic factors usually act in a density-dependent manner, while the physical factors act independent of density. If this is so, then the degree of obstancy will clearly be greater with an increase of density. This does not hold for all obstants. Parasitoids are most effective when the density of the host population triggers a noticeable transformative effect, while at lower densities, the degree of obstancy by predators is at the forefront (MacPhee and Sanford, 1954). The degree of obstancy of a phytophagous predator, thrips (Haplothrips kurdjumovi), on phytophagous mites, depends on density, but the effect of the same obstant on the codling moth and the apple tortricid is independent of density (MacPhee, 1953). The obstants on the scale insect Lepidosaphes ulmi are exposed to the effect of low winter temperatures, and this will determine whether the parasitoids, or the predators, have a higher degree of obstancy (Lord, 1947). The density-independent factors are often combined with density-dependent ones, and the intensity of a given factor is also variable in time, but this is not influenced by the density of the population that is affected (Solomon, 1949). The degree of obstancy can reach high levels and, the earlier in the life cycle it is manifested, the bigger the impact. The importance of obstants usually declines along the egg-larva-pupa-adult continuum. A long diapause of an ontostadium can increase the degree of obstancy of relevant obstant elements, as in the case of the pine sawfly (Ruevkin, 1953). Therefore, we must distinguish between obstant elements, not only from a gradological but, also a zoocoenological viewpoint. This also involves dominance, because it seems appropriate if, besides the statistically based evaluation, we also consider the zoocoenological role of the population in question. This does not mean that, when evaluating dominance, we would
