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140 IVII. Zoocoenological characteristics
detour from the path of guantitative analyses, which is the only correct route;
we need to acknowledge the fact that the dominance of obstant populations
can only be assessed correctly in the context of the whole (meaning whole¬
year) life of the studied zoocoenosis, otherwise we risk overemphasising the
importance of the role of populations that dominate in certain parts of the
season. If we register, for example, that on tortricid eggs, there is hardly any
obstant activity, but the larvae are exposed to Colpoclypeus florus, and the
pupae to some Brachymeria sp., even if both have similar densities, due to
the higher degree of obstancy, the formergualifies as the dominant population
of the coenosis, because its role in host mortality is greater. For the same
reason, the larval population of Trichogramma evanescens will be classified
as the dominant one of the Cacoeciatena, if its degree of obstancy is high,
even if it is only linked to the catenarium for a short period. Its dominance
effects on the fate of the population was more decisive than any of the obstants
that appear later in the season and, due to a density-dependent effect, also
influenced their abundance.
Therefore, we did not deviate from the quantitative route when assessing
dominance but, by introducing the term “degree of obstancy’, and using it
to rank the obstant elements, we created an additional, new platform for
assessment. This allowed us to evaluate co-dominance correctly, and we could
also put the dominance limited to a single aspect into perspective.
However, we also found an additional complication, caused by obstant “A”
living with 25-30 of its conspecifics in a single caterpillar host, while “B” and
“C” are solitary. In the Hyphantriatena cuneae catenarium, an example of the
former is Psychophagus omnivorus and, of the latter Theronia atalantae and
Pimpla rufipes, both as larval populations. This is a typical matter of degree
of obstancy because, by abundance, the Psychophagous is clearly dominant,
but its semaphoront represents a smaller degree of obstancy than a Theronia
or a Pimpla. The Psychophagous degree of obstancy can be calculated by
dividing the number of pupae parasitised with the number of emerging
adults. The number obtained by these means is only an approximation,
because the number of parasitoid larvae per host is very variable.
In this case, the Psychophagus is host to Pedobius pyrgo, and the Pimpla to
Dibrachys microgastri, that are obstant hyperparasites; these cannot be
considered together when calculating degrees of obstancy, because
Psychophagus and Pimpla are obstants on corrumpent elements, but the
hyperparasites act on the primary parasitoids. The dominance values
calculated for the populations of the catena must, therefore, be supplemented
by the degrees of obstancy: to the abundance of the primary obstants, the
individuals hosting the secondary hyperparasites will have to be added. The
degree of obstancy of the primary parasitoid must be calculated this way.
This is fully justified, because the decrease in abundance in the host population
can be linked to the primary parasites, and its absolute value is not changed
by the fact that part of the primary parasitoid population fell victim to the