*4.3. Phylogenetic Clustering*

The Sunshine Cast heath data found phylogenetic clustering (a high NRI) on one of the nine regional ecosystems, in the dry, high, wind-buffeted, leached sand dunes, and along with a high NTI (clustered), which provides evidence for single clusters of taxa on the species pools, as opposed to several clusters evenly distributed around the tree [93]. This clustering, also found in global dune plant communities, leads to the conclusion that environmental filtering is at play [92]. This is supported by this regional ecosystem (RE 12.2.13) having a distinct community composition but overlapping in composition with a structurally taller system (RE 12.2.9), both growing in highly nutrient leached sands [94]. No other heath regional ecosystems were found to have a clustered NRI, but six had a clustered NTI, suggesting several clusters evenly distributed around the tree [93]. Phylogenetic clustering, due to the presence of closely related species, could indicate environmental filtering [23] and may be disadvantageous in terms of species competition. Yet, in long term heath studies in New South Wales, phylogenetic clustering became more pronounced over time since fire, in contrast to the expectation that increased competition would inhibit the coexistence of species with high niche overlap [95]. Furthermore, it is argued that closely related species may not necessarily compete more closely than distantly related ones, that filtering may not be the dominant mechanism, and that facultative interactions may play a part in this [14,96,97]. The co-occurrence of closely related species may be beneficial, for example, when facilitated by mutualistic exchange, such as through mycorrhizal symbiosis shared by co-occurring members of a clade, increasing the host's tolerance to environmental change [98]. Mycorrhizal strategies have been suggested as drivers of heath assemblages in Western Australia and Brazil, with well-known strategies used by the distinctive heath families Ericaceae, Orchidaceae, and Myrtaceae [99,100]. For the Sunshine Coast heaths, whose phylogeny is consistent with the general theory of sclerophyll and heath derivation, there are indicators that environmental filtering is a driver of their assembly but hints that facilitation and mutualism may be factors contributing to their community assembly and their general pattern of phylogenetic clustering.
