THE IMPORTANCE OF SMALL-SCALE HETEROGENEITY IN BOREAL FORESTS - VARIATION IN DIVERSITY IN FOREST-FLOOR INVERTEBRATES ACROSS THE SUCCESSION GRADIENT

Heterogeneity in species assemblages of forest-floor arthropods carabi d beetles, ants and spiders - within and between different forest age classes was studied in the southern Finnish talga. The importance of p rocesses operating on the local scale (within the movement radius of t he species) vs on the regional scale (among forest stands) in determin ing the observed variation was assessed. Four data sets with different spatial resolutions in mesic forests in the same general study area w ere used. The material consists of 18 283 carabids of 51 species. 48 7 69 spiders of 212 species, and 126 718 worker ants of 23 species. Anal yses of abundance variation and species complementarity among successi onal stages revealed that in all the three taxa species occurring in t he mature forest were prevalent in the younger successional stages as well, constituting more than half of the catch in any age class. A gre at majority of carabid and spider species were widely distributed acro ss the forest age classes, whereas ants include a higher proportion of species with a narrower amplitude across the succession gradient. Com parisons of similarity between samples at increasing distance from one another on the local scale within forest stands (a few tens of meters to a few hundreds of meters) showed a quite consistent pattern in car abids and spiders: there was more variation between sampling sites in young successional forests than in the mature forest. Furthermore, onl y in the mature forest a slight, albeit statistically not significant, negative relationship between similarity of samples and distance betw een sampling sites was detected. In carabids and spiders, comparisons between samples located at a distance of 10-15 m from each other showe d considerable heterogeneity, the mean percentage similarity being c. 0.6 (in ants c. 0.8). On the regional scale, systematic variation betw een young and mature forest stands is a major element increasing the t otal diversity (species turnover c. 50% in carabids and spiders: compo sitional similarity c. 0.3 0.4 in carabids, 0.2 0.3 in spiders), but v ariation within forest stands on a spatial scale of 10-15 meters is an other important component in the total heterogeneity. The results sugg est that regional abundance variation is a primary factor influencing the composition of local assemblages: a set of hypotheses elaborating this conclusion is formulated. The results implies that maintenance of habitat heterogeneity on a small scale (10-15 m) is needed to preserv e biodiversity in managed forests.