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138 IVII. Zoocoenological characteristics
This degree of obstancy refers, exclusively, to the relationship between two
semaphoront groups; for example, the % infestation by tachinid larvae of the
pupae a certain butterfly species. As this interpretation of the degree of
obstancy does not indicate the real role of an obstant element in the mortality
of a given generation of a species, this empirical degree of obstancy should
be distinguished from the gradological degree of obstancy (e.g. apparent vs.
real mortality, Thompson, 1929; Chapman, 1931:206).
The gradological degree of obstancy is the influence of an obstant population
on the host species population size through decimating a given host population.
There is no doubt that the empirical degree of obstancy is also a numerical
expression of the decrease in density of the host resulting from the activity
of a parasite or predator. This role, however, is restricted to one semaphoront
group, and the number obtained does not express the importance of this
factor among other mortality factors that also contribute to the overall mortality
of the generation. If we compare the empirical degree of obstancy to this, it
may turn out that the overall effect of this factor among the others is surprisingly
small. The real role is expressed by the gradological degree of obstancy. Using
an example, we can demonstrate the gradual diminishing of a generation and
the gradological degree of obstancy of these factors. The example is the yellow
scale insect of the pear, Diaspidiotus pyri (Szelényi, 1936).
Mortality factor abstancy% | population, % | _abstaney. 9%
Unhatched eggs 80 20 80
Larval mortality, predation 65 7 13
Mortality of established larvae 25 5.25 1.75
Mortality of L2 5 4.99 0.26
Parasitism, L2 5 4.74 0.25
It is evident that the precise quantification of the gradological degree of
obstancy requires profound and idiobiological studies that can, realistically,
only be madein small catenaria. Given that different populations of the same
species may belong to different zoocoenoses, the assessment of the gradological
degree of obstancy will, in most cases, go beyond the boundaries of the
studied zoocoenosis. For this reason, when studying the relationships among
populations of a coenosis, we can only establish the empirical degree of
obstancy. The reason for discussing it here is that we can underline the relative
value of the numbers expressing the empirical degree of obstancy, and that
we appreciate that these are valid only within the studied coenosis, and not
related to the species, but its one semaphoront group. The line of argument,
through the gradological degree of obstancy, leads from the studied
zoocoenosis to other ones.