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§ The question of balance in the biocoenosis | 55
over-utilised by corrumpent elements. Consequently, one can assume that
other corrumpents (e.g. gall wasps, leaf miners) were removed, or, at least,
their abundance drastically decreased. These phenomena can be easily
explained by the workings of the interaction network. Why would we
complicate the explanation by forcibly drawing up this mysterious equilibrium,
when the process affected only a part of the biocoenosis and, although the
consequences are obvious, it could not have affected those populations that
do not use oak leaves as their energy source?
The view that the biological equilibrium is manifested by the sustenance
of a population around its mean abundance, and this balanced “iron minimum”
is maintained by intraspecific competition for food (Nicholson, 1933), is
contradicted by the fact that there are large between-generation changes in
population densities (Solomon, 1949; Thalenhorst, 1950; Schwedtfeger, 1951).
The mean is only a theoretical value, obtained by considering several factors,
and it does not reflect the real conditions at all; the competition for food is
not a universal phenomenon, either, because the biotic factors that influence
populations operate in concert with abiotic ones, and in turn will be prominent
as the main regulator of density (Glen, 1954).
What several authors call biological equilibrium, based on current
knowledge, cannot be more than an interplay among the components of the
biocoenosis, i.e. interactions. These interactions occur because no animal is
a completely self-dependent organism; it consumes energy which it must
acquire externally, thus any animal can satisfy its trophic needs only in the
presence of other living things.
This need will generate synphysiological relationships and, does it follow
from this, that satisfying these needs involves a fixed relationship in densities?
The arguments mentioned above indicate that we cannot speak of a balanced,
stable density. The view that, due to “environmental resistance’, nearly 100%
of every generation must perish (Thomson, 1929) is theoretically plausible,
and occasionally occurs (Diaspidiotus pyri [Aspidiotus piri] - Szelényi, 1935),
but projected onto the biocoenosis, it does not follow that the abundance of
the populations and their ratio would be constant.
Widely fluctuating numbers entail that sequential generations always have
different starting conditions, and it matters whether, at times with favourable
conditions, their numbers are low or high. This may determine whether the
starting generation will reach gradational densities, or only a high density
that does not threaten the sustenance of the energy source. Density fluctuations
can be caused by many factors and, even if in some cases one can establish
a causal relationship between fluctuations in food plant densities and those
of corrumpent populations relying on them (Melanoplus mexicanus, Scharff,
1954), in most cases, we do not see clear causes of fluctuations in abundance.
The importance of a given regulating factor is not equal in space and time,
and it is not indifferent which ontopopulation it will affect; therefore, it will
never act in isolation but in combination with other factors (Glen 1954).