Recruitment and Summer Diapause in the Dynamics of a Population of Nebria brevicollis (Coleoptera: Carabidae)

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Abstract
We studied the population dynamics of the carabid Nebria brevicollis (F.) by conducting a capture-recapture experiment between 1968 and 1978. We used 30 pitfalls, emptied (almost) daily and marked individually 11521 beetles. A part of the experimental area was fenced in 1976 and 1977 (about 360 m2). We used key-factor analysis to analyze the life cycle of this beetle. Density fluctuated heavily in the population investigated. The species is almost completely semelparous; on average only 1.3% of the old beetles reproduced again in following years. The most important factors influencing variation in population size are the number of eggs laid and the survival of the preadult stages. It is not yet possible to decide which of these varies most in the field. There is an indication that a negative correlation exists between recruitment and number of parent females, but this could also be related to differential survival of larvae. Larval mortality is highly affected by the amount and quality of food as well as by predation and parasitism. Previous research suggests that in our area conditions during larval development are suboptimal. Larval mortality appears to vary within a much wider range than egg production. Therefore, we consider variation in survival of the preadult stages as more influential. Young beetles need to increase their hatching weight by 50-60% before entering summer diapause. Although on average one third of these beetles died before or during summer diapause, this could not be considered to be a key-factor. Young adults were more mobile than reproducing beetles, often leaving the hatching site. The pattern of catchability of marked beetles deviated from the expected Poisson distribution. Too many beetles were either caught only once or four or more times. This might explain why estimations of population sizes according to Jolly were lower than the number of beetles actually marked. Calculations based on models of Craig yielded a better fit. His second model rightly predicted population size when the area was fenced off. In our area N. brevicollis is widespread and often abundantly present. In our opinion the success of N. brevicollis is presumably based on high mobility of (young) adults, which favours reaching new suitable sites, together with plasticity in developmental mode of the larvae, which enables them to make the best of an ever changing food situation, and which influences both the quality and quantity of adults.