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OCR
56 | II. Biocoenosis and zoocoenosis These cross-directional interactions will generate the incessant fluctuations in the biocoenosis that creates a perception of a form of balance, because our macroscopic impression is that the association is stable, without noticeable changes. Therefore, a complete defoliation, due to its highly visible consequences, creates an impression that something out of the ordinary happened and, to explain this, it seems logical to assume a disturbance of a balance. The gradating population, though, does not drop out of the biocoenosis, and continues to be under the influence of its factors (Glen, 1954). The effect of biotic factors (e.g. parasites and predators) is greatest at high densities (“density dependent mortality factors’, Smith 1935). The high activity of obstant elements during gradations of Lymantria or Aporia is very well known; several authors see this as a self-regulating ability of the biocoenosis, to maintain some sort of balance (Friederichs, 1930; Schwenke, 1953). To this we respond as follows: the richer the biocoenosis, the more complicated are the interactions among its members, and more mortality factors are likely to decimate every member population. This can explain the inverse relationship between density fluctuations and the complexity of the biocoenosis (Solomon, 1949). This is also supported by the observation that there are more frequent gradations in agrobiocoenoses than in coenoses under lower human influence (Schimitschek, 1942). Is it correct to conclude from this that there is a lack of biological equilibrium in the former? At first sight, it is attractive to explain the gradations of corrumpents in agrobiocoenoses through disturbances of the equilibrium (Friederichs, 1930). The writer himself interpreted insect pest damage this way (Szeényi, 1944), and he may still retain this opinion if: 1) he could define, precisely, the equilibrium, and; 2) his later studies, carried out in biocoenoses less influenced by humans, had not convinced him that gradations of corrumpent elements are no less frequent in such communities. Point 1 cannot be satisfied, because the studies mentioned under point 2 do not support the steady state of the biocoenosis, or any kind of balance. We illustrate this with two examples. One of them are the results for a gall fly, Janetia (Arnoldia) cerris, studied over 10 years in the same area that also exhibited gradations in this system, and where fly abundance can reach a level whereby their galls cause growth disorders on the host plant. Two years after the latest gradation (1955), the abundance became so low that only one Janetia gall was found per several hundred leaves. The second example is the Rubus-Crataegus-Rosa bushland on the southern slope of the Nagyszénäs, above Nagykovacsi, that was censused over several years, and where, in 1953, without any previous signs, the abundance of Cydia tenebrosana (Laspeyresia roseticolana) reached extremely high levels, but hardly any were found in the subsequent year. These biocoenoses are not studied by anyone, while the agrobiocoenoses are always under observation, and as their corrumpents are mostly economically damaging animals and their activity creates attention. We may
