PATTERNS OF ABUNDANCE AND DEMOGRAPHY - COLLEMBOLA IN A HABITAT PATCH GRADIENT

The demography and abundance of Collembola in relation to a adient of increasingly isolated tussocks of Carex ursina were investigated near Ny-Alesund, Svalbard. The study area was divided into three zones acco rding to tussock density. Ten tussocks were sampled in each zone. In a ddition, samples were taken between tussocks, which consisted of groun d covered with a layer of cyanobacteria. A total of ten Collembola spe cies were found, five of which were chosen for further studies. The Ca rex tussocks were the preferred habitat for the majority of these spec ies. Only one species, Hypogastrura viatica, was found regularly betwe en tussocks, although at low density. The gradient in tussock distribu tion was probably an important factor in determining the distribution, abundance and the underlying demographic processes of most species. T his was indicated by an increase in demographic heterogeneity with pat ch isolation. The different species were affected differently, however . Whereas one species appeared to be unaffected by the gradient (H. vi atica), two species (H. longispina) and (Folsomia sexoculata) were som ewhat surprisingly found to have their highest density where tussocks were furthest apart. Factors other than the spatial configuration of t he habitat are probably important in determining the distribution of t hese two species, indicated by a positive correlation at tussock level between them. In accordance with general hypotheses on the effect of patchiness on population dynamics the remaining two species, F. quadri oculata and Onychiurus groenlandicus, occurred in very low numbers or not at all, respectively, in the zone where tussocks were furthest apa rt. Their response is probably dependent on their ability to successfu lly colonise isolated tussocks. We predict that different species spec ific demographic strategies, and in particular dispersal rates, may ac count for the observed patterns.