Oldal 58 [58]
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