Taxonomic homogenization and differentiation across Southern Ocean Islands differ among insects and vascular plants
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Aim To investigate taxonomic homogenization and/or differentiation of insect and vascular plant assemblages across the Southern Ocean Islands (SOI), and how they differ with changing spatial extent and taxonomic resolution. Location Twenty-two islands located across the Southern Ocean, further subdivided into five island biogeographical provinces. These islands are used because comprehensive data on both indigenous and non-indigenous insect and plant species are available. Methods An existing database was updated, using newly published species records, identifying the indigenous and non-indigenous insect and vascular plant species recorded for each island. Homogenization and differentiation were measured using Jaccard’s index (JI) of similarity for assemblages across all islands on a pairwise basis, and for island pairs within each of the biogeographical provinces. The effects of taxonomic resolution (species, genus, family) and distance on levels of homogenization or differentiation were examined. To explore further the patterns of similarity among islands for each of the taxa and groupings (indigenous and non-indigenous), islands were clustered based on JI similarity matrices and using group averaging. Results Across the SOI, insect assemblages have become homogenized (0.7% increase in similarity at species level) while plant assemblages have become differentiated at genus and species levels. Homogenization was recorded only when pairwise distances among islands exceeded 3000 km for insect assemblages, but distances had to exceed 10,000 km for plant assemblages. Widely distributed non-indigenous plant species tend to have wider distributions across the SOI than do their insect counterparts, and this is also true of the indigenous species. Main conclusions Insect assemblages across the SOI have become homogenized as a consequence of the establishment of non-indigenous species, while plant assemblages have become more differentiated. The likely reason is that indigenous plant assemblages are more similar across the SOI than are insect assemblages, which show greater regionalization. Thus, although a suite of widespread, typically European, weedy, non-indigenous plant species has established on many islands, the outcome has largely been differentiation. Because further introductions of insects and vascular plants are probable as climates warm across the region, the patterns documented here are likely to change through time.