Succession of pitfall-trapped insects and arachnids on eight Norwegian glacier forelands along an altitudinal gradient: Patterns and models

Abstract

Invertebrate primary succession based on pitfall traps is described in eight glacier-foreland chronosequences across a major geo-ecological gradient in the Jotunheimen and Jostedalsbreen subregions of southern Norway. Regional (predominantly altitudinal) and local environmental gradients are shown to be effective influences on the nature and rate of colonization and succession at both species- and community-levels of analysis. Inclusion of flying taxa identified only to higher taxonomic units had little effect on the species-level results for epigeal taxa because the flying taxa were found more-or-less evenly throughout each chronosequence. A relatively large number of pioneer taxa colonize within <20 years of deglacierization in the alpine zone (46–61% of taxa, compared with 22–43% in the subalpine and boreal zones) and contribute to a relatively simple, two-stage succession. Later colonizers are proportionately more important in the subalpine and boreal zones where a three-stage succession results in several divergent late-successional trajectories conditioned by local exposure and moisture differences. The Pioneer Persistence Index (PPI), which measures the percentage of pioneer taxa that persist into the mature stage of succession, is consistently high (65–86%), while the Pioneer Importance Index (PII), which measures the percentage of the taxa at the mature stage that comprise the persistent pioneer element, is higher in the Jotunheimen chronosequences (49–52%) than those of Jostedalsbreen (21–45%). Results support the addition and persistence model of succession, which is seen as being driven primarily by the individualistic behaviour of highly mobile (mainly carnivorous) species that apparently exhibit low dependence on the vegetation succession. Although this model differs substantially from models of primary plant succession, a relatively small element of replacement change increases with decreasing altitude. This contributes to a more complex community structure in subalpine and boreal zones and is attributed to greater biotic interaction, involving not only predators but also an increasing number of herbivores and omnivores.