The effects of spatial habitat configuration on recruitment, growth and population structure in arctic Collembola

The population density and demography of five species of arctic Collembola were studied in a naturally patchy habitat, consisting of Carex ursinae tus socks with varying degrees of isolation. Focal predictor variables were tho se describing the spatial configuration of tussocks, including tussock size and isolation and the amount of habitat (cover) at a 1-m(2) scale surround ing each tussock population. The Collembola populations were heavily influe nced by environmental stochasticity in the form of winter mortality and sum mer drought, and the influence of patchiness on population characteristics was evaluated in this context. The five species showed very different respo nses to the structuring effect of the habitat. depending on life history ch aracteristics, mobility and habitat requirements. Population density was hi ghly variable in both time and space. Spring densities indicated larger win ter mortality compared to observations from a previous study, and the snow- and ice-free season from June to August only resulted in population growth for Folsomia sexoculata. In the other species, adult mortality must have b een high as there was no net population growth despite observed reproductio n. The exception was Hypogastrura viatica, whose population decline was mor e likely to have been the result of migration out of the study area. Cover was the most important variable explaining density. No pure area or isolati on effects at the tussock level were detected, even in areas with very low habitat cover. Drought was probably an important mortality factor, as July was particularly warm and dry. Due to qualitative differences in the tussoc ks and the matrix substrate, desiccation risk would be higher during disper sal between tussocks. We suggest that increased dispersal mortality gave th e observed pattern of increased density in relation to cover, both in gener al and in F. quadrioculata, an opportunistic species otherwise known for ra pid population growth. Onychiurus green landicus, which had a similar densi ty response to cover, may also be influenced by a rescue effect sustaining densities in areas with high cover. The cover effect can be viewed as a lar ge-scale factor which encompasses the general spatial neighbourhood of each tussock, where inter-population processes are important, as opposed to int ernal patch dynamics.