Timber plantations do not homogenize soil arthropod diversity but do alter species composition

Abstract

Landscape transformation for agriculture threatens global biodiversity through simplification of ecosystems. We determine whether large-scale timber plantations contribute to biotic homogenization of soil biodiversity relative to remnant natural vegetation. We examined ant and springtail alpha- and beta-diversity, and assemblage composition, among biotopes in two different climatic and soil regions in a subtropical area of South Africa. Surprisingly, we observed equally high levels of alpha- and beta-diversity in plantations (Eucalyptus and Pinus) as in natural biotopes (indigenous forest and grassland) for both taxa in both regions, except in the inland sites where ant species richness was highest in grassland. Furthermore, beta-diversity in all biotopes was mainly due to species replacement rather than nestedness. The high diversity in plantations is likely due to high environmental variation among plantation sites, likely providing diverse niche opportunities across plantations. A possible mechanism for the large proportion of shared species between natural areas and plantations is colonization of arthropods from natural areas into plantations and/or source populations surviving harvesting and tree planting and then their populations growing during long periods of tree maturation. Plantations and natural areas supported complementary assemblages, which builds on the idea that remnant natural vegetation in production landscapes is essential for conserving not only unique aboveground biodiversity, but also unique belowground biodiversity. Furthermore, timber plantations do not homogenize soil arthropod taxonomic diversity as expected. This contrasts with global trends on biodiversity response to land-use change and emphasizes that conservation principles for soil biodiversity may differ from those aimed at aboveground biodiversity.