Testing the ‘addition and persistence model’ of invertebrate succession in a subalpine glacier-foreland chronosequence: Fåbergstølsbreen,southern Norway

Abstract

Invertebrate primary succession is investigated across a chronosequence in the subalpine (birch woodland) zone at Fåbergstølsbreen using pitfall traps. Presence and abundance of taxa, functional groups and communities are examined using a geo-ecological approach including mapping, graphical analysis, two-way indicator species analysis (TWINSPAN) and non-metric multidimensional scaling (NMS). Twenty-nine of the 67 recorded taxa, and 14 of the 37 epigeal (surface active) taxa colonize terrain deglacierized for <20 years and catches of most of these pioneer taxa attain an early peak on terrain deglacierized for <40 years. Catches of most later colonizers peak in the mature (‘climax’) stage where 86% of the pioneer taxa and 79% of the epigeal pioneers are also recorded. The number of taxa increases across the chronosequence as new taxa, predominantly predators, appear but relatively few taxa drop out of the succession, and as the dominant species in the traps changes from the harvestman, Mitopus morio, to the wood ant, Formica lugubris. The invertebrate communities are interpreted as loosely organized groupings of individualistic species, the distributions of which are strongly related to terrain age. Changes during succession are interpreted as driven primarily by competition between the carnivorous taxa, though herbivores and omnivores comprise up to 24% and 11% of the taxa, respectively, in both pioneer and mature zones. Hence the ‘addition and persistence model’ of invertebrate succession, previously proposed for the alpine zone, is largely substantiated but partially modified for the subalpine zone, where succession is generally more rapid, overall changes during succession are greater, communities are better organized, food webs appear better developed, and there is an element of replacement change in the later stages.