Environmental heterogeneity explains contrasting plant species richness between the South African Cape and southwestern Australia
Ruan van Mazijk, Michael D. Cramer and G. Anthony Verboom
- Department of Biological Sciences, University of Cape Town, Rondebosch, South Africa
- Corresponding author: RvM, [email protected]
This is an open access repository for (some) data-sets, reproducible analyses, conference slides and manuscript drafts for a publication based on my BSc Hons project, published in Journal of Biogeography (https://onlinelibrary.wiley.com/doi/10.1111/jbi.14118) in 2021. See the ResearchGate page for more.
The folder for-Dryad contains the versions of scripts and data files lodged formally on Dryad, here: https://doi.org/10.5061/dryad.8w9ghx3m8.
Aim: Given the importance of environmental heterogeneity as a driver of species richness through its effects on species diversification and coexistence, we aimed to account for the dramatic difference in species richness per unit area between two similar mediterranean-type biodiversity hotspots and whether this difference is explained by differences in environmental heterogeneity.
Location: The Greater Cape Floristic Region, South Africa (GCFR) and Southwest Australian Floristic Region (SWAFR).
Taxon: Vascular plants (tracheophytes).
Methods: Comparable, geospatially explicit environmental and species occurrence data were obtained for both regions and used to generate environmental heterogeneity and species richness raster layers. Heterogeneity in multiple environmental variables and species richness per unit area were compared between the two regions at a range of spatial scales. At each scale, richness was also regressed against these individual axes and against a major axis of heterogeneity, derived by principal component analysis (PCA).
Results: The GCFR is generally more environmentally heterogeneous and species-rich than the SWAFR. Species richness per unit area is significantly related to the major axis of heterogeneity across both regions, the latter describing ca. 38-50% of overall heterogeneity, the slope of this relationship differing between the two regions only at the finest spatial scale. Multivariate regressions, and regressions against the first axes of the PCAs (PC1), revealed variations in the dependence of species richness on environmental heterogeneity between the two regions.
Main conclusions: Notwithstanding some region-specific effects, we present evidence of a common positive relationship between floristic richness and environmental heterogeneity across the GCFR and SWAFR. This is dependent on spatial scale, being strongest at the coarsest level of sampling. The generally greater richness per unit area of the GCFR compared to the SWAFR is thus explained by the former’s generally greater environmental heterogeneity and is concordant with its greater levels of floristic turnover.
Keywords: biodiversity, environmental heterogeneity, fynbos, Greater Cape Floristic Region, kwongan, macroecology, species richness, species turnover, vascular plants, Southwest Australian Floristic Region
This work was funded by the South African Department of Science and Technology (DST) and the National Research Foundation (NRF) under the DST-NRF Freestanding Innovation Honours Scholarship (to RvM), and by the South African Association of Botanists (SAAB) Honours Scholarship (to RvM). Thanks go to the Department of Biological Sciences, University of Cape Town, for providing a 2TB external hard drive for local GIS data storage.
For results used in earlier drafts of this work and a conference presentation, many computations were performed using facilities provided by the University of Cape Town's ICTS High Performance Computing team (http://hpc.uct.ac.za).
