It is widely recognized that, to understand local community assembly, it is necessary to consider the influence of the regional species pool, or the set of all species that could potentially colonize a local site. However, the properties of species pools that influence local community assembly remain poorly understood. We conducted a laboratory microcosm experiment to test whether the geologic age of a region from which species colonize local sites affected the relationship between regional and local richness. For this purpose, we used a culture collection of root-associated fungi isolated from a single species of a native ericaceous shrub found across 4-million-year soil chronosequence in Hawaiian islands. We hypothesized that fungal species pools originating from older sites would have greater functional diversity and result in greater local fungal richness. Individual microcosms were assembled with a sterile seedling and 2-30 fungal species. Six months after fungal inoculation, fungal community composition was assessed using molecular methods. To quantify functional diversity, we made functional profiles of individual fungi using phenotype microarrays of carbon substrate use. We also measured phylogenetic diversity to account for unmeasured, phylogenetically conserved traits.
We found that fungal richness in an individual seedling began to saturate with increasing species-pool richness. In support of our initial hypothesis, the relationship between species-pool richness and realized local richness was significantly influenced by the mean age of the sites where species were initially collected. Specifically, species pools originating from older sites accumulated greater local richness with increasing species pool richness. However, contrary to our prediction, species pools from older ecosystems also had lower phylogenetic and functional diversity, suggesting that coexistence of ericaceous root-associated fungi in older ecosystems may be facilitated by equalizing fitness differences, rather than niche differentiation. Our results suggest ecosystem age is linked to local diversity not just through increased regional diversity but also the composition of the species pool.