Diversity of the genus Seira (Collembola: Entomobryidae: Seirinae) in the Fynbos and Southern Afrotemperate forest
Thesis (MSc)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: Soil is one of the most complex and understudied terrestrial habitats, and it comprises a wide range of organisms that affect the functioning of terrestrial ecosystems. Despite the importance of these below-ground fauna, our understanding of this diversity remains limited, especially in the Cape Floristic Region (CFR), where higher plants and other more conspicuous invertebrates have been better represented in the literatures. The main aim of this thesis is to provide the first insights into the taxonomic, cryptic and spatial diversity of one of the more diverse Collembola genera, Seira (Collembola: Entomobryidae: Seirinae), in the Fynbos and Southern Afrotemperate forest habitats of the CFR. Collembola constitute an important component of terrestrial biodiversity and are essential for ecosystem functioning. The thesis is divided into two data chapters. The first data chapter examines the mitochondrial barcoding COI (Cytochrome-c oxidase subunit I) gene of 496 Seira specimens from 41 Fynbos and forest sites. Discrete Molecular Operational Taxonomic Units (MOTUs) are identified within Seira. Furthermore, divergence times are estimated and tentatively used to propose historical triggers for the diversification of Seira. Habitat specificity of Seira is assessed through phylogenetic reconstruction using Parsimony and Bayesian analyses based on the nucleotide and amino acid sequences. Genetic divergence indicates that Seira is composed of at least 91 MOTUs, suggesting that morphological taxonomy has vastly under-estimated the richness of this genus by at least four folds. Most of the MOTUs are highly habitat specific and geographically localised. The reconstruction of an evolutionary time frame of these lineages reveals several deep diversifications in the Miocene and a spate of more recent radiations in the Pleistocene and Holocene. Palaeo-environmental fluctuations and vegetation composition shifts are hypothesised as having increased the environmental complexity of the region and this may have influenced the diversification patterns of Seira. In the second data chapter, the spatial variation of Seira diversity is examined and compared between and within two major habitat types, Fynbos and forest, across nine study sites. Abiotic variables were recorded and compared for the investigation of environmental differences among habitat types. The habitat specificity of Seira is assessed, as well as the relative effects of habitat types and study sites on assemblage structure. Species richness of Seira is higher in Fynbos than in forest, and this could be due to the high heterogeneity of niches and resources created by the steep environmental and floristic gradients in the Fynbos habitats. Perhaps one of the most striking results is the high levels of beta diversity exhibited by Seira. The spatial turnover of the Seira assemblage is complete or nearly complete among and within habitat types across study sites within the CFR. Low dispersal abilities and consequently isolated evolutionary histories may account for the strong assemblage differences within the same habitat type of different study sites. However, the substantial differences in assemblage composition between adjacent habitat types in the same site are likely to be the result of the contrasting abiotic conditions exhibited by the Fynbos and forest habitats, as found by this study. Overall, the results of this thesis suggest that Seira species richness is much greater than previously thought. It follows similar diversity patterns to the well-described and hyper-diverse plant communities of the CFR. Here, for both CFR plants and Seira, the spectacular regional diversity displayed is not the result of high alpha diversity, but rather of substantial beta diversity. Because of the high beta diversity and the fact that only two CFR vegetation types were sampled from a limited number of study sites, I predict that the Seira species richness presented here is but the tip of the iceberg.
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