The age structure in a local population of Carabus auronitens under the influence of an abiotic key factor for reproduction
On an area of 0.2 ha in a forest of the Westphalian Lowlands abundance and net reproduction of the carabid beetle Carabus auronitens have been measured since 1982. In summer 1992 this area was surrounded by a beetle-proof fence, after which the interseasonal mortality and age structure of the population could be measured. Beetles were caught by pitfall trapping and were individually marked. C. auronitens is a spring breeder with summer larvae. The young are obligatorily active after hatching in late summer-autumn, during which season surviving old beetles are dormant. Net reproduction is calculated as the quotient “number of new beetles in the late summer-autumn season divided by number of females active in the spring of the same year”. 73.58 % of the variability of net reproduction was found to depend on meteorological mean temperature in the 2nd ten-day period of May. The maximum net reproduction rate measured in 1993 was 3.11; in 1987 and 1996 net reproduction was near zero. The average probability of surviving from the season of hatching until the 1st spring season was around 75 % for females and males of the generations autumn 1992 – autumn 1994; for both females and males of the generation autumn 1995 the survival probability was considerably smaller (30.4 % I 17.5 %). The survival probabilities of the adults that have reached the first spring season were nearly invariable in the years of investigation (about 65 % from the 1st until the 2nd spring season; about 60 % from the 2nd until the 3rd spring; about 55 % from the 3rd until the 4th spring and about 40 % from the 4th until the 5th spring). The extreme dependence of reproduction on temperature in the 2nd ten days of May is evidently tolerable only because of these conspicuously high survival rates of the adults.