Plant response to habitat fragmentation : clues from species and functional diversity in three Cape lowland vegetation types of South Africa

[摘要] ENGLISH ABSTRACT: The South African Cape lowlands have been severely transformed and reduced to fragments embedded in matrices of various activities. With the need to prioritise conservation efforts, information on the conservation worthiness and management of these fragments is required. This study aimed to better understand how fragmentation affects the Cape lowland vegetation patterns and dynamics and more specifically, to determine if, and if so to what extent these fragments contribute to regional plant diversity and more importantly their functionality. The novel approach adopted focused on plant functional traits, which are better predictors of ecosystem response to global change than individual species.Species were sampled at four scales in four sites of decreasing sizes, including: a mainland and three fragments of three Cape lowland vegetation types i.e. Atlantis Sand Fynbos (ASF), Swartland Shale Renosterveld (SSR) and Langebaan Dune Strandveld (LDS). Traits such as dispersal, pollination, breeding mode and longevity were selected based on relevance to species' and plantfunctional types' (PFTs) responses to fragmentation. The findings revealed different effects on species richness and PFTs. The effect of reduced patch size on species richness was more evident in ASF where fragments below 600 ha had significantly fewer species than the mainland. This effect was not unequivocal in SSR and LDS due to several confounding factors (notably the grazing history of the sites). The SSR fragment grazed by indigenous herbivores had significantly more species than the ungrazed sites. Also, the largest LDS fragment grazed by livestock had significantly more species than the ungrazed mainland, indicating that grazing rather than fragment size influences species richness, although the smallest fragments of these two vegetation types had significantly fewer species than the larger fragments. Species turnover and complementarity were high for all three vegetation types, reflecting the degree of habitat heterogeneity and high contribution of beta diversity to overall gama diversity. The effect of reduced patch size was higher on PFT diversity than on PFT richness, with LangebaanDune Strandveld where habitat fragmentation was more recent being the least affected of the threevegetation types. This indicates a degree of functional redundancy in the Cape lowlands, which is important for ecosystem resistance and resilience. The ASF mainland and the largest fragment had higher PFT diversity than the medium-sized and the smallest fragments; the mainland had also higher PFT diversity than all the fragments combined. Similarly, the smallest SSR fragment had significantly lower PFT diversity and richness than the other sites. The grazed SSR fragment had higher PFT richness and diversity than the ungrazed mainland and smallest fragment, indicating the role of grazing in maintaining renosterveld vegetation. The PFTs absent from the different sites were mostly short-distance dispersed dioecious and non-dioecious species, and some with highly specialised pollination systems. This suggests that dispersal and pollination are vital functional attributes for the persistence of the studied fragmented ecosystems.Habitat fragmentation effects plant community composition and ecological functions in the Cape lowlands, a conclusion supported also by the revealed significant trait-convergence and divergence assembly patterns. These communities result from various fragmentation filters that operate at different spatial-temporal scales and selecting species with suitable responses. All three vegetation types are susceptible to fragmentation, albeit at varying degree. The fragmentation effect was confounded by the sampling and temporal scales, the nature of disturbance regime, and the traitmediateddifferences in species' response. The role of the surrounding matrix on fragment connectivity and gene flow appears to be of crucial importance, hence mitigation measures focusing on improving connectivity between patches, monitoring threatened taxa, and promoting dispersaland pollination have been recommended.

[发布机构] Stellenbosch University