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A Population Study of Cis bilamellatus

Kitty Paviour-Smith

Journal of Animal Ecology,Vol. 37, No. 1 (Feb., 1968), pp. 205-228,(article consists of 26 pages),Published by: British Ecological Society


1. An Australian fungus beetle, Cis bilamellatus, is a very important destroyer of dead birch bracket fungi (Polyporus betulinus) and other tree fungi in the lowlands of southern Britain.

2. Fungus fruiting-bodies are small, scattered, fairly temporary habitats which can be destroyed completely by their fauna.

3. In 1957-58 a study was made of the Cis bilamellatus populations of ten 1956-grown, one 1955-grown and one 1954-grown brackets of Polyporus betulinus on two birch trees in Wytham Woods, Berkshire.

4. The adult beetles, with a significantly higher proportion of female female than of male male, first colonized the newly dead brackets in the spring, the sex ratio remaining biased in favour of female female until the first filial generation appeared, and later not differing from 1:1. The dying fungi on the older tree tended to be colonized earlier and by larger numbers of beetles than those on the younger tree.

5. A value of the geometric rate of increase `r’ of 0.0256+-0.0015 per head per day was found for four Cis bilamellatus populations growing in Polyporus betulinus in the laboratory at 24 degrees C and c. 66% R.H. This was found to agree remarkably well with a value calculated for the probable rate of increase of the population of one fungus in the field (r = 0.0207) between May and July when the mean daily temperature was only 15.2 degrees C.

6. The population changes throughout the first year are followed in the two fungi sampled most often over the longest period. After the initial immigration period, there was a fairly sudden increase in numbers by births, followed by a gentle decrease during the rest of the year, but this decline in numbers cannot be accounted for by any of the known causes of mortality, viz: the two known `predators’ of larvae (a Cecidomyiid fly larva and the larva of a Bethylid wasp), interspecific competition with other Ciidae and Dipterous larvae, intraspecific competition or even low winter temperatures (which in really severe winters such as that of 1962-63 can kill the whole population).

7. There is evidence for emigration occurring in the autumn and winter as well as the spring movement which originally gave rise to the populations studied. Such emigration of newly matured adults would account completely for the annual post-breeding decline in the first and later years.

8. In the 1955-grown (and therefore second-year) fungus, the overall population picture for Cis bilamellatus was the same as in the first-year fungi except that the Bethylid wasp had been rather more abundant; in the 1954-grown (and therefore third-year) fungus the picture was again similar to that in the first-year fungi, except that other species of Ciidae were more abundant, and the C. bilamellatus population of this fungus had suffered an extremely heavy mortality, probably from low temperatures in the winter of 1955-56.

9. It was probably because of this heavy mortality that the third-year fungus lasted so long, i.e. up to the beginning of its fourth year, as a habitat and food for Ciidae. Both the second-year and third-year fungi were in 1957-58 becoming eaten out and, by then, competition for their remaining food and space must have been strong among the fungivorous insects.

10. It is considered that the density-independent mortality caused by low temperatures in a severe winter can only delay the effects of competition among the fungivorous insects (mainly C. bilamellatus in the area studied) for a limited and consumable resource such as the dead fruiting-bodies of Polyporus betulinus, and that an additional factor in the population regulation of this beetle species over a wide area is probably an overall scarcity of this habitat compared with the numbers of emigrating beetles.

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