Human activities have reorganized the earth's biota resulting in spatially disparate locales becoming more or less similar in species composition over time through the processes of biotic homogenization and biotic differentiation, respectively. Despite mounting evidence suggesting that this process may be widespread in both aquatic and terrestrial systems, past studies have predominantly focused on single taxonomic groups at a single spatial scale. Furthermore, change in pairwise similarity is itself dependent on two distinct processes, spatial turnover in species composition and changes in gradients of species richness. Most past research has failed to disentangle the effect of these two mechanisms on homogenization patterns. Here, we use recent statistical advances and collate a global database of homogenization studies (20 studies, 50 datasets) to provide the first global investigation of the homogenization process across major faunal and floral groups and elucidate the relative role of changes in species richness and turnover. We found evidence of homogenization (change in similarity ranging from 20.02 to 0.09) across nearly all taxonomic groups, spatial extent and grain sizes. Partitioning of change in pairwise similarity shows that overall change in community similarity is driven by changes in species richness. Our results show that biotic homogenization is truly a global phenomenon and put into question many of the ecological mechanisms invoked in previous studies to explain patterns of homogenization.

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Aim Species introductions and extinctions have reorganized the earth's biota, often leaving formerly spatially distinct assemblages more similar in species composition, a process termed biotic homogenization. The study of biotic homogenization has been almost entirely focused on the change in taxonomic similarity between assemblages through time. Here, we provide a trait-based method for calculating functional similarity through time and compare these trends in functional attributes with those trends generated from a taxonomic perspective.Location Data were produced through computer simulation and gathered from North American Breeding Bird Survey (BBS) data and published accounts of North American birds for 10 locations across the east and west coast of the United States.Methods We simulated change in assemblages with different trait types (binary and continuous), levels of trait overlap, number of traits and species richness to determine the relationship between change in taxonomic similarity (DTS) and change in functional similarity (DFS). We also assess the relationship between DTS and DFS for bird assemblages across 10 locales in the USA between 1968 and 2008. We used simple linear regression to determine the slope and correlation between DTS and DFS and used multiple regression to assess the influence of trait overlap, number of traits, species richness and the ratio of traits to species on the relationship between DTS and DFS. ResultsSimulations reveal that trait redundancy governs the relationship between DTS and DFS. A decrease in trait overlap increases the slope of the regression between DTS and DFS and an increase in the ratio of traits to species in the regional pool increases the correlation. The relationship between DTS and DFS for breeding birds is comparable to simulations with low trait redundancy. Main conclusionsWe show that often losing or gaining species from an assemblage tells us very little about the loss or gain of function, and that this scenario most often occurs when the two assemblages have high trait redundancy. It remains to be seen how prevalent this scenario is within empirical examples; however, the implications for the continued delivery of ecosystem functions in the face of species introductions and extinctions are large.

Biogeography has traditionally focused on the spatial distribution and abundance of species. Both are driven by the way species interact with one another, but only recently community ecologists realized the need to document their spatial and temporal variation. Here, we call for an integrated approach, adopting the view that community structure is best represented as a network of ecological interactions, and show how it translates to biogeography questions. We propose that the ecological niche should encompass the effect of the environment on species distribution (the Grinnellian dimension of the niche) and on the ecological interactions among them (the Eltonian dimension). Starting from this concept, we develop a quantitative theory to explain turnover of interactions in space and time -i.e. a novel approach to interaction distribution modeling. We apply this framework to host-parasite interactions across Europe and find that two aspects of the environment (temperature and precipitation) exert a strong imprint on species co-occurrence, but not on species interactions. Even where species co-occur, interaction proves to be stochastic rather than deterministic, adding to variation in realized network structure. We also find that a large majority of host-parasite pairs are never found together, thus precluding any inferences regarding their probability to interact. This first attempt to explain variation of network structure at large spatial scales opens new perspectives at the interface of species distribution modeling and community ecology.