Refugee settlements as rapid ecological filters: multi-taxa biodiversity, beta-diversity, and community turnover in northern Uganda savannahs

The degree to which turnover in biological communities is structured by deterministic or stochastic factors and the identities of influential deterministic factors are fundamental, yet unresolved, questions in ecology. Answers to these questions are particularly important for projecting the fate of forests with diverse disturbance histories worldwide. To uncover the processes governing turnover we use species-level molecular phylogenies and functional trait data sets for two long-term tropical forest plots with contrasting disturbance histories: one forest is older-growth, and one was recently disturbed. Having both phylogenetic and functional information further allows us to parse out the deterministic influences of different ecological filters. With the use of null models we find that compositional turnover was random with respect to phylogeny on average, but highly nonrandom with respect to measured functional traits. Furthermore, as predicted by a deterministic assembly process, the older-growth and disturbed forests were characterized by less than and greater than expected functional turnover, respectively. These results suggest that the abiotic environment, which changes due to succession in the disturbed forest, strongly governs the temporal dynamics of disturbed and undisturbed tropical forests. Predicting future changes in the composition of disturbed and undisturbed forests may therefore be tractable when using a functional-trait-based approach.

The introduction and extirpation of predators can have large impacts on food web structure and ecosystem function. The magnitude of these predator effects can be influenced by species diversity and turnover along environmental gradients in space or time, which have the potential to buffer or magnify the impact of trophic interactions on ecosystem functioning. In this study, we explored the individual species and community effects of fish predators on lake zooplankton in the Sierra Nevada, CA, USA. Local and beta diversity in zooplankton communities were resilient to fish, only marginally differing among fish and fishless sites, and were more structured along elevational gradients. Observed diversity changes were attributed to species turnover in zooplankton communities, which suggests elevational gradients and fish are acting as ecological filters excluding subsets of species rather than reducing species density. We found that fish significantly reduced densities of larger‐bodied zooplankton species. Further, as larger‐bodied species densities were lower in the presence of fish, we found that the community weighted mean of body mass decreased in the presence of fish in lakes. Although species turnover largely maintained diversity over elevational gradients and in the presence of fish, such changes in larger‐bodied taxa and community biomass may have important implications in the transfer of energy throughout the food web and to connected riparian and aquatic ecosystems that are dependent on the flow of biomass and nutrients.

Bryophytes include about 20,000 species characterized by their poikilohydric condition, high long-distance dispersal capacities, and cold tolerance. Despite these specific life-history traits, large-scale biogeographic patterns in bryophytes are consistent with those observed in other groups, wherein they have been interpreted in terms of historical factors. Comparative phylogeographic analyses in bryophytes and angiosperms suggest, however, that spatially congruent patterns may not necessarily arise from common processes. This is best illustrated by the strikingly lower rates of endemism at all taxonomic and spatial scales in bryophytes due to their failure to diversify in-situ and the rapidity at which they enlarge their distribution range. In particular, the striking transoceanic disjunctions that are typical for many bryophyte species and their low community turnover at broad geographic scales both point to the higher capacities of bryophytes for long-distance dispersal than angiosperms. Such high long-distance dispersal capacities are reflected in the lower spatial genetic structure of bryophytes as compared to angiosperms at large geographic scales. This explains why, as opposed to the expectations of MacArthur & Wilson's model, bryophyte species richness is not necessarily lower on islands than on continents, suggesting that community assembly is more constrained by ecological filtering than dispersal limitations in bryophytes. The low relevance of historical factors for global patterns of bryophyte species richness has contributed to the idea that, as opposed to the predictions of one of the most general rules in ecology, bryophyte species richness does not decrease with latitude due to a strong tropical niche conservatism. Recent evidence for the existence of a latitudinal species richness gradient in bryophytes raises, however, the question of why bryophytes diversified faster in the tropics. This and other avenues of research in bryophyte biogeography are discussed.