Metapopulation processes in epiphytes inferred from patterns of regional distribution and local abundance in fragmented forest landscapes

Abstract

Summary Deciduous trees within a coniferous forest landscape provide habitat for many organisms. Single trees in deciduous forests form dynamic patches that emerge, grow and fall, but the stands themselves are also dynamic patches established after disturbances and replaced by conifers during succession. Increased dispersal distance, as imposed by landscape fragmentation, may lead to extinctions and reduced biodiversity among species dependent on this dynamic network. We analysed regional frequency distributions, local abundances and spatial occupancy patterns of epiphytic bryophytes in 135 deciduous forest stands in a fragmented landscape in Sweden. We used generalized non‐linear models to test whether these patterns could be assigned to metapopulation dynamics of individual epiphytes by investigating the relative importance of stand size, habitat quality, connectivity and landscape history on species occupancies and local abundances. Most asexually dispersed species were regionally rare, and spatial species occupancy patterns suggest that this is caused by dispersal limitation. In sexually dispersed species, a strong rescue effect was indicated by a bimodal frequency distribution of the species, as well as by increasing local abundance with increasing connectivity to stands present today, or some decades ago. There was a strong positive relationship between regional frequency and local abundance of the species, and between species richness and forest stand size. Vicinity to forest edge negatively affected the local abundance of most species. Our results clearly indicate a metapopulation structure. Sensitivity of epiphytes to habitat fragmentation is caused by decreasing forest sizes, habitat alteration at forest edges and increasing dispersal distances. Even in assumed good dispersers, increasing distances can significantly alter regional dispersal processes. A lower rescue effect leads to smaller stand population sizes with a larger extinction risk. Rapid reduction of the amount of habitat during the last decades and the expected time‐lag in species extinctions suggest that epiphytes will further decline in the future, although there may still be time for restoration programmes to prevent extinction.