Is trait-based ecology functional? A test from a climatically stable biodiversity hotspot
When Darwin visited the Cape in 1836 he recorded in his diary that ‘there was not even a tree to break the monotonous uniformity of the sandstone hills: I never saw a much less interesting country’. This unflattering comment is perhaps the first documented observation of the high structural and functional uniformity displayed by Fynbos vegetation across its range, despite encompassing multiple soil types and large climatic gradients (e.g. 200-3000mm rainfall) and comprising approximately 9000 (many spectacular!) plant species. Fynbos is characterized by rapid turnover in species, but little change in growth form or higher taxonomic composition along environmental gradients, with most communities containing representatives of each of the four major growth forms; shrubs, sub-shrubs, graminoids, and geophytes; and their characteristic families Proteaceae, Ericaceae, Restionaceae, and Iridaceae. This phylogenetic and functional uniformity poses a challenge for trait-based ecology in this system. Functional traits are currently viewed as the Holy Grail for unifying research at scales from genes to landscapes and across fields from biology to earth sciences and policy. A major attraction of trait-based ecology is the potential to establish links between community assembly processes and biotic effects on ecosystem function and benefits to society. Establishing these links would allow the development of predictive models that can inform policy or management, particularly in the context of global change impacts on biodiversity. Here we explore plant trait variation across several vegetation surveys sampled from the Fynbos Biome of the Cape Floristic Region, a hyperdiverse Mediterranean-Type shrubland.
Biotic effects on ecosystem function are likely driven by the amalgamated set of traits within a site. The relative uniformity of Fynbos vegetation across steep environmental gradients suggests that it should support similar ecosystem processes in terms of productivity, nutrient cycling, hydrology, habitat, and fire regime across its range, and that any impacts that alter composition are likely to drastically alter ecosystem function in this system. Notwithstanding, the uniformity in taxonomic composition, growth form diversity and trait variation suggests there must be significant ecophysiological divergence and specialization in habitat preferences amongst closely related species despite minor differences in traits. This advocates for phylogenetically explicit lineage-based approaches to discern community assembly processes in this system. The development of predictive models that integrate community assembly processes and biotic effects on ecosystem function for this system will require methods that can assimilate across phylogenetically explicit lineage-based approaches and whole-community analyses.