“No signs of saturation”: long-term dynamics of vascular epiphyte communities in a human-modified landscape

It is debated whether slowing human population growth and intense urbanization may result in a slowdown of deforestation and an acceleration of natural forest regeneration. In a fragmented landscape the structure and composition of developing secondary forests will strongly depend on the local species pool. Thus, the understanding how organisms cope with biotic and abiotic challenges outside pristine habitats is pivotal. Structurally dependent, vascular epiphytes are an important biotic component of tropical forests. In human-modified tropical landscapes potential hosts are often still present. We aimed to assess if human-modified landscapes offer a refuge habitat in which epiphytes can form metacommunities that are ultimately viable. Eight years after an initial assessment we recensused the epiphytes in pasture trees in western Panama along a strong rainfall gradient. We document a threefold abundance increase (ca. 20,000 vs. >60,000 individuals) and an increase in species-richness (66 vs. 86 mostly drought-tolerant species). This large net increase suggests a highly dynamic system. Although absolute abundances changed dramatically, the relative contribution of major taxonomic groups to overall diversity and abundance changed little. Neither rainfall nor tree growth had a significant effect on relative annual community growth rates. At the plot level (=metacommunities), abundance increase was mostly due to the species already present in the first census, at the tree level (=communities) the contribution of new and old species was comparable. The documented long-term trend in epiphyte metacommunities in a human-modified landscape suggests that a diverse set of species sustains viable metacommunities and is likely to provide structural diversity to developing secondary forest.