Ruan van Mazijk is a Masters student at the University of Cape Town. He is broadly interested in the ecology and evolution of plants. Ruan shares his recent work on how environmental heterogeneity influences species-area relationships in floral species from the Greater Cape Floristic Region and the Southwest Australia Floristic Region.
Ruan watching the sunset over the foothills of the Cape Fold Belt Mountains (namely in Bainskloof Pass), home to the Cape Floristic Region’s rich montane flora. Photo credit: Hannah Simon.
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Institute. University of Cape Town (Department of Biological Sciences)
Academic life stage. Masters
Major research themes. (Plant) phylogenetic systematics, macroecology, biogeography and trait ecology
Current study system. I currently work in the Cape Floristic Region (CFR) on the evolution and ecology of its plentiful plant species. My BSc Honours work formed the basis for my recent paper in JBI. My Masters dissertation explores how plant physiology and ecology vary with species’ genome-size in two species-rich clades of sedges, Schoenus and Tetraria. Both these genera have their centres of diversity in the CFR. Schoenus exhibits a broad range of fairly large, polyploid genomes. Tetraria, however, is much less variable, with most species having much smaller genomes than Schoenus. We know that genome-size affects organismal biology in a variety of ways. Most strikingly, plant stomatal dimensions are positively related to the amount of genetic material in cell nucleii. My work aims to demonstrate how other physiological functions of plants related to water- and gas-exchange (via stomata) might be affected by polyploidy-driven variation in genome-size!
Recent paper in JBI. Van Mazijk, R., Cramer, M.D. & Verboom, G.A. (2021). Environmental heterogeneity explains contrasting plant species richness between the South African Cape and southwestern Australia. Journal of Biogeography. DOI: 10.1111/jbi.14118. Wiley Content Link: https://onlinelibrary.wiley.com/share/author/Y2XIBHJBZPVMBXIIUWPA?target=10.1111/jbi.14118.
Motivation behind this paper. Our recent paper was based on my BSc Honours project, albeit very loosely, as the analyses are completely different in this paper. The topic my supervisors and I grappled with was the puzzling similarity in species richness of the Greater Cape Floristic Region (GCFR) and the Southwest Australia Floristic Region (SWAFR), two Mediterranean-type biodiversity hotspots with very different environmental conditions. Despite their climatic similarity the GCFR is rugged and mountainous while the SWAFR is flat, with only gentle hills. We know that complex and heterogeneous environments can support more species through greater niche diversity, and can facilitate speciation through adaptive radiations. Thus, the question follows: how can two plant biodiversity hotspots be so different in topography? What other forms of heterogeneity might be relevant (e.g., soil, rainfall)?
The differences in topography between the GCFR (left, mountainous) and SWAFR (right, flat as a pancake) are really striking! Pictured: Table Mountain National Park, South Africa (left) and Fitzgerald River National Park, Western Australia (CC BY-SA 3.0 Yewenyi).
The plants of the GCFR (left) and SWAFR (right) have many of the same ecological adaptations and growth forms, nevermind the taxonomic affinities of both these regions’ floras. Pictured are four Proteaceae species (clockwise from top left): Leucospermum reflexum var. luteum, Anigozanthos manglesii (Red and green kangaroo paw) (CC BY 2.0 A. Chapman 2008), Banksia ahsbyi (CC BY 2.5 Gnangarra 2011) and Leucospermum cordifolium (CC BY-SA 3.0 Marco Schmidt 2008).
Methodologies. Using publicly accessible environmental and species occurrence data for both the GCFR and SWAFR, we compiled datasets of environmental heterogeneity that we could correlate to species richness. Not only did we explore the effects of heterogeneity in various environmental variables (e.g., elevation, rainfall, soil properties), but also a composite measure of environmental heterogeneity by summarising individual measures in a principal component analysis (PCA).
Challenges. This paper went through so many different drafts and different versions of the analyses that I’ve almost lost count. However, changes in approach to answering one’s original question can only delay things so much. Rather, these changes acted in concert with a combination of circumstances, external disruptions and the blows these dealt to my mental health. Circumstances included: carrying on with this while doing my MSc, the unfortunate inconvenience of loadshedding in South Africa, the advent of the coronavirus pandemic, and (chronic-, pandemic- and non-pandemic-related) illness in my family. I don’t want to make excuses, but this was a lot! On the upside, these challenges taught me a lot about myself and how to navigate some very difficult times.
Challenges. This paper went through so many different drafts and different versions of the analyses that I’ve almost lost count. However, changes in approach to answering one’s original question can only delay things so much. Rather, these changes acted in concert with a combination of circumstances, external disruptions and the blows these dealt to my mental health. Circumstances included: carrying on with this while doing my MSc, the unfortunate inconvenience of loadshedding in South Africa, the advent of the coronavirus pandemic, and (chronic-, pandemic- and non-pandemic-related) illness in my family. I don’t want to make excuses, but this was a lot! On the upside, these challenges taught me a lot about myself and how to navigate some very difficult times.
A selection of study species for my MSc (clockwise from top left): Schoenus compar, Schoenus albovaginatus, Tetraria ustulata, Tetraria thermalis. The tribe Schoeneae (Cyperaceae) is super morphologically variable, with T. thermalis reaching up to 2m (ca. 6ft 7in) tall and little S. albovaginatus getting down to 20cm (ca. 8in). These plants might be plain-looking at first, but their intricate inflorescence morphology and impressive contribution to a given fynbos landscape’s biomass make them hard to ignore!
Major results. We found evidence for a common positive relationship between floristic richness and environmental heterogeneity across the GCFR and the SWAFR, although the GCFR was more environmentally heterogeneous and species-rich. There are, of course, some region-specific effects. And those are also slightly dependent on spatial scale, although almost always positive effects across scales. We can conclude that the greater richness per unit area of the GCFR compared to the SWAFR is explainable in terms of the GCFR’s greater environmental heterogeneity. This helps support the idea that environmental heterogeneity is important in driving plant biodiversity. However, more work is required to understand how the effects of environmental heterogeneity change at different spatial scales, and it would be great to replicate this study in other geographic regions.
Next steps. Quite intuitively, the next step would be to apply a similar analysis across all five Mediterranean-type ecosystems (the GCFR, SWAFR, Mediterranean Basin, California and central Chile), to see whether the richness-heterogeneity relationship is applicable to them all, and to determine what forms of environmental heterogeneity vary between these floristic regions.
Additionally, in the context of the broader question, “How important is environmental heterogeneity (and which kinds) across Mediterranean-type ecosystems?”, it makes sense to expand the analysis to include absolute environmental variables. This will allow us to account for the importance of heterogeneity-type variables relative to absolute environmental variables. Though, this starts to get more complicated, and changes what the models are asking from, “Does heterogeneity matter in X regions?” to “Which matters more: heterogeneity or absolute conditions?” or “Let’s predict species richness as best we can”. With the latter, it starts becoming important to incorporate spatial structure and spatial autocorrelation into the models. This is easier said than done, and even easier done than to interpret the results!
This is what makes me oh-so-happy to work in the fynbos around Cape Town. Clockwise from the top are the views from atop Observation Peak, in the crevices of the Silvermine region of Table Mountain National Park, and on the slopes of the Kogelberg.
If you could study any organism on Earth, what would it be? Plants, both great and small, are definitely my jam, having grown up in the fynbos of the southwestern Cape. But I’ve also got big soft spots for lichens, insects, birds, cnidarians, cetaceans and seals! If I’m studying something I find personally interesting and/or “beautiful”, then I’m happy.
Anything else to add? Within my lab, this paper took infamously long for me to get published, not least because of the reasons I described above. I know that isn’t particularly unique or exciting as an answer to an interview question, but I think it’s relatable for many. After many, very different drafts of this paper, I feel like (but am obviously not) a veteran of the scientific process. I guess I just want to send my thoughts and well-wishes out to anyone else trapped in “Manuscript Rewriting Hell”, and say this: you can look forward to an indescribable feeling of relief when it’s done!