The life histories and population dynamics of two carabid species on a Dutch heathland : 1. Fecundity and the mortality of immature stages.

We deal with the causes of the synchronously fluctuating numbers of subpopulations of the carabid species Calathus melanocephalus as compared with the asynchronously fluctuating numbers of subpopulations of the carabid Pterostichus versicolor. Both species continuously occupy a large heath area, Dwingelder Veld (1600 ha), in The Netherlands, and are studied there in the same localities with the same methods. Of the adults of C. melanocephalus, 90% do not cover more than 2 ha during the entire reproductive season, while 90% of adults of P. versicolor cover no more than 12 ha. In C. melanocephalus egg production in the field is usually similar to that under optimal feeding conditions in the laboratory, but in P. versicolor egg production seems to be much lower in the field. In the field 70-80% of the eggs most probably are killed by eelworms, followed by more than 90% mortality among the remaining larvae. Comparing mortality of developmental stages in laboratory experiments with that in field experiments in enclosures, it appears that mortality of larvae is not density-dependent, even when density in the experiments is much higher than it ever is in the field. Larval mortality mainly results from the poor ability of the larvae to find prey, even when in field experiments prey density is increased far above natural densities. We discuss why these poor prey-finding abilities are not improved by natural selection. In the spring breeder P. versicolor differences between localities both in abiotic factors, soil moisture and surface temperature, and biotic factors, reactions of prey species to abiotic factors, in spring and summer when the larvae are maturing contribute to the asynchronous fluctuations of numbers between subpopulations. In the autumn breeder C. melanocephalus possible differences in biotic factors between sites are outnumbered by the effects of winters with a higher or lower than normal amount of precipitation respectively. During a wet winter mortality among the larvae is much higher than during a dry winter. As these winter conditions are similar over large areas (many km2) the fluctuations of numbers between subpopulations are synchronous.