Pattern and process of biotic homogenization in the New Pangaea

Abstract

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 -0.02 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.

Figure 1.

Map showing the countries where homogenization datasets used in this analysis were collected. Circles denote oceanic regions where archipelagos or single islands were part of a dataset. Studies with a global spatial extent (n = 2) are excluded.

Figure 2.

Box plot comparing the mean mantel R² of Δβ_cc versus Δβ₃ and Δβ_cc versus Δβ_rich for all taxonomic groups (all) and across taxonomic groups (birds, fish and plants).

Figure 3.

Box plot comparing the mean mantel R² of Δβ_cc versus Δβ₃ and Δβ_cc versus Δβ_rich across spatial extent (continental and regional). Global spatial extent was not included because only two of the 50 datasets studied homogenization at this scale.

Figure 4.

Box plot comparing the mean mantel R² of Δβ_cc versus Δβ₃ and Δβ_cc versus Δβ_rich across spatial grain (small <1 km², moderate 1–100 km², large 100–1000 km², to very large >1000 km²).