Page 83 [83]
§ The smallest category of a zoocoenosis: the catena | 81
high abundance in arvideserta of corrumpent populations, can be partially
attributed to this lack of intermediary hosts.
The endoparasite Bracon sp. is a parasite of the weevil Neoglocianus
(Ceutorrhynchus) maculaalba (macula-alba) living in poppy heads. The
species Baryscapus (Tetrastichus) diaphantus (terminalis) is a parasite of
Bracon. All three are, therefore, members of a catena around the weevil, but
the last one also appears in the catena linked to Eurybia cardui that lives in
the flowerhead of Centaurea sadleriana and, moreover, also in the catena
connected to the oak gall wasp Biorrhiza pallida. We know from our own
studies that the adult B. diaphantus that is linked to the poppy head weevil
appears in the middle of summer, while those active in the catenae around
Eurybia cardui and Biorrhiza pallida emerge in spring. It is likely that this
population could not insert itself into the catena around the poppy head
weevil if the overwintering generation did not have the two other catenae at
their disposal nearby.
Examining the role of constituent populations in a catena, we can distinguish
two sharply different groups. It is obvious that we need to distinguish between
the larval populations of Cydia pomonella, its parasite, Perilampus laevifrons,
and its hyperparasite Dibrachys microgastri (cavus) and the great tit, for
example. One or two semaphoronts of the latter may visit the orchard, and
among other prey, may consume the pupae of the codling moth (with its
parasite) and fly away, perhaps even the same day, not to be seen for several
days. A different one would be a goshawk catching the tit, or a cat preying
on its distant nest. Likewise, the larval populations of Syrphus, Aphidius,
Pachyneuron spp. active in a colony of aphids must be considered differently
from the Coccinella septempunctata adult that stays there for 1-2 hours,
destroying a few aphids, and then moves to feed on scale insect nymphs on
the next tree. This is not a matter of mono- or polyphagy, which would be
an idiobiological and coenological view. The above mentioned larval
populations of Perilampus and Dibrachys, or the Syrphus, Aphidius and
Pachyneuron are permanent residents around the codling moth- or aphid¬
based catenae, while the other animals are not.
Therefore, all catenae have a core, that stays together for longer time scales,
and cannot remove itself, and there are elements that are transients, temporarily
contacting the core catena, but soon cutting their links to it. This casts a sharp
light onto the dynamics of the zoocoenosis: the composition (and, also its
species composition) can change from day to day, and even from hour to hour.
An overabundant Agrotis segetum infestation will have a rich larval
population, and numerous obstant elements, parasitic flies and wasps, and
episitic beetles will also be present. This catena can remain unchanged for a
week, when a murmuration of starlings arrives, and for a few hours, will feed
on the larvae of the moth. There is no doubt that, during these hours, the
starlings will belong to the catena; yet they were not until they physically
appeared, and will not belong there on the next day when they have flown