Conservation of spider diversity within an agricultural mosaic : insights from the Greater Cape Floristic Region, biodiversity hotspot.

[摘要] ENGLISH ABSTRACT: Agricultural expansion is one of the main drivers of habitat fragmentation and land use change which negatively impacts biological diversity. The Greater Cape Floristic Region (GCFR), a biodiverse hotspot, has been recognised as a priority for conservation as its unique endemic diversity is threatened by historic land transformation and habitat fragmentation. Private nature reserves and proclaimed protected areas alone cannot conserve all biodiversity, especially with >80% of land not formally protected. Thus we must conserve biodiversity within production landscapes. Remnant patches of natural vegetation supports a wide variety of arthropod taxa. However, little information is available on spider diversity in remnant fynbos and even less on which environmental parameters drive this diversity. Furthermore, research on how the matrix impacts adjacent remnant patches, and how spiders respond to different matrix types, are needed for protecting spider diversity and the services they provide within the GCFR mosaic. This study aims to identify environmental parameters that shape spider diversity within fynbos remnant patches, and how spiders respond to different matrix types.Here, I sampled spider diversity within remnant fynbos patches of the GCFR mosaic to identify which landscape and patch variables are important for maintaining spider diversity. Fifteen environmental variables (at landscape and patch scales) were collected at each site and analysed to determine their influence on spider species richness and assemblage structure of the whole spider assemblage, and for different functional guilds. Local patch variables best predict spider diversity, particularly soil compaction and topographic complexity which negatively influenced overall and plant dwelling spider richness. This pattern of complexity is mainly driven by common spider species. Tree species richness (mostly alien trees) negatively influenced free-living spider richness. Lastly, level of site invasion by alien trees influenced overall and epigaeic spider assemblage structure. Spider diversity was more influenced by patch scale variables, which reflects local patch management, than the landscape context.I also assess how spider diversity responds to different land-use types, the magnitude of associated edge effects on spider diversity, and identify complementary habitat elements for enhancing spider diversity within agricultural mosaics of the GCFR. Spider diversity was sampled along replicated transects covering remnant fynbos vegetation into three different matrix types: old fields, vineyards and invasive alien tree stands. Fynbos remnants had significantly higher overall spider diversity than matrix sites with higher diversity in edge locations than at patch cores. Old fields had the highest spider diversity between all land-use types, as well as the greatest assemblage similarity to remnant vegetation assemblages. Lowest diversity was recorded within vineyards. Lastly, vegetation complexity enhanced spider diversity across all land-uses.In conclusion, I demonstrate that remnant vegetation is a critical landscape element for conserving spider biodiversity in GCFR mosaics, but that old fields can play an important role in increasing functional connectivity within the landscape mosaic. Increasing native vegetation diversity within the matrix helps improve spider diversity. Additionally, this work recommends alien tree removal from fynbos remnant patches within the GCFR for biodiversity conservation. Preserving remnant patches of all sizes in production landscapes, and softening the matrix, can increase heterogeneity which benefits spider diversity within the GCFR mosaic.

[发布机构] Stellenbosch University