Population Dynamics of Two Carabid Beetles at a Dutch Heathland: II. Egg Production and Survival in Relation to Density

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Abstract
Egg production in 2 abundant carabid species at Kralo Heath was estimated by summing the weekly averages of the number of eggs in the ovaries per female during the reproductive period. For both Pterostichus versicolor Sturm and Calathus melanocephalus L., this variable correlates with the number of eggs laid. Laboratory experiments show egg production to be determined largely by food supply, but with shortage of food, total numbers of eggs in the ovaries, though low, overestimate the numbers laid. In 2 subpopulations, egg production correlated inversely with total catch, representing mean density, suggesting that competition for food results in density-dependent fecundity. Density-dependent fecundity seems a general feature in other subpopulations of C. melanocephalus, but in a subpopulation of P. versicolor at a rich site, no inverse relationship was found; catches were higher than those observed at the heathland. In P. versicolor, the number of recruits appearing in any year correlated significantly with egg production of the previous year; at low numbers of eggs in the ovaries, relatively few recruits were found. Though similar in C. melanocephalus, the relative number of recruits was more variable: egg production correlated positively with survival of adults to the next season, and low recruitment was generally coupled with low survival, since scarcity of food probably influences egg production and physical condition. The negative regressions of number of eggs per female with density resulted in parabolic recruitment curves for both species. Absolute recruitment declines at densities comparable to the highest in the field. Analysis of log Nt+1/log Nt regressions for 8 subpopulations failed to detect density dependency, a large variance of the net reproductive rate at moderate densities obscuring the non-linear relationship between Nt+1 and Nt at high densities. Enclosure experiments supported the density-dependent/fecundity hypothesis and indicated that food is generally limited in the field. Both P. versicolor and C. melanocephalus densities at Kralo Heath are restricted by scrambling for food. Below the carrying capacity, abiotic factors may determine fluctuations in numbers.