Distribution Patterns, Life Cycle and Phenology of Pterostichus Oblongopunctatus F. (Col., Carabidae) and Philonthus Decorus Grav. (Col., Staphylinidae)

Abstract

This study concerns the distribution, both of populations in a landscape and of individuals in a population, and the annual cycle of two species of beetles of the forest floor, Pterostichus oblongopunctatus and Philonthus decorus. The distribution pattern ofpopulations of Pt. oblongopunctatus in a given landscape changes very little from year to year, and less than Ph. decorus. This comment holds also within the population. Pt. oblongopunc- tatus has a more stable pattern of favourable places within its habitat than has Ph. decorus. So individuals of the latter species have to cope with a more unpredictable environment than the former. A description of the preferred habitats is given. The habitat of Pt. oblongopunctatus is determined more strictly by a number of structural and physical properties of the environment than is the case for Ph. decorus. Although density fluctuations differ considerably in each population in the landscape, they run parallel in the various places within the habitat of one population. Absolute densities have been established by means of mark recapture techniques. Mortality is lower for Pt. oblongopunctatus and its life span may be more than 4 years as opposed to 2 years for Ph. decorus. The relative locomotor has been measured throughout the reproduction period. It is found to be positively correlated with temperature. However, the quantitative relation changes with time: Pt. oblongopunctatus becomes progressively less active at all temperatures from April till July, culminating in aestivation, and Ph. decorus may become increasingly active at all temperatures. The form of the activity curve, based on pitfall catches, can be explained by the mortality rate and changes in locomotor activity. The reproductive cycle and the dispersal capacity of the species have been studied by dissecting specimens. For Staphylinids flight muscle dimorphism may be a partial alternative for the wellknown wing dimorphism in Carabids. Results of the various studies are discussed in terms of life strategies. As the distribution pattern within a population is rather fixed, conditions for triggering the mechanism of the spreading of risk, i.e. making use of the heterogeneity of the environment and thus levelling off fluctuations in density, are not fulfilled. The climatic stability of the forest floor and self-regulation of the density of the species must account for the relatively stable populations. Development of the dispersal capacity is set against high reproduction capacity. Both may make conflicting demands on the resources of the animals. It is suggested that for many insects the high dispersal capacity is the more decisive factor in determining whether the species will survive in unpredictable environments.