Deterministic processes dominate nematode community structure in the Fynbos Mediterranean heathland of South Africa

In many ecosystems, the factors that determine landscape-scale community structure of soil nematodes are poorly understood. We were interested in discovering whether deterministic or stochastic factors dominate nematode community variation. We used a novel metagenetic approach to investigate variation in nematode community structure in the Fynbos vegetation of South Africa. We compared 23 samples of soil nematode communities from five different Fynbos landscapes. Nematode DNA was 454-pyrosequenced for the 18S rRNA gene. We investigated the community structure, diversity, and the relative role of both deterministic (niche-based) and neutral processes may play in delimiting the nematode phylogenetic community structure in different Fynbos vegetation types. Nematode diversity showed no relationship to any measured soil parameter. The phylogenetic signal showed that more closely related types of nematodes in the Fynbos tended to co-occur more often than would be expected by chance, demonstrating that (a) closely related lineages occupy similar niches spatially and (b) community variation is influenced more by determinism than stochasticity. The standardised beta mean nearest taxon distance (ses.βMNTD) index showed no association with vegetation type. Both ses.NTI (nearest taxon index) and ses.βMNTD deviated significantly from null models, indicating that deterministic processes were important in the assembly of nematode communities. Furthermore, at local scale, the ses.NTI was significantly higher than null expectations, indicating that co-occurrence of related nematode lineages is determined by the differences in environmental conditions across the sites. We conclude that in the Fynbos there is niche overlap between closely related types of nematodes, that nematode speciation tends to occur conservatively into closely related niches, and that the phylogenetic community structure reveals that deterministic (rather than stochastic) processes are more important in delimiting the community assembly.