Botanic Gardens Trust, Sydney, Australia

 

Decline of the Australian mesic zone biota

Dr Peter H. Weston - Senior Principal Research Scientist

The mesic biome, encompassing both rainforest and open sclerophyllous forests, is central to understanding the evolution of Australia’s terrestrial biota. The rainforest habitat has long been considered the ancestral biome of the continent, giving rise over the last 65 million years to the Australian sclerophyllous flora, which in turn contributed to the origins of the arid, savannah, grassland and alpine biomes. However, this received wisdom, based largely on fragmentary fossil evidence, along with a raft of associated assumptions, has not been explicitly tested using modern sources of phylogenetic evidence. In 2007, a large research group of botanists and zoologists associated with the ARC Australian Environmental Futures Research Network, including Senior Principal Research Scientist Peter Weston, decided to collaborate in testing several of these ideas, using phylogenetic trees, molecular chronograms and phylogeographic patterns for relevant taxa.

Dr Weston and his colleagues identified 5 hypotheses that had become tacitly accepted in the Australian paleobiological literature:

  • The ancestors of the biota of Australia were organisms of mesic environments
  • Rainforest organisms were the ancestors of the present Australian biota
  • Lineages of Asian origin became an important component of the Australian mesic biota in the last 20 million years
  • Rainforest communities suffered extinction and contraction to refugia during the Neogene, contrasting with expansion of sclerophyll lineages
  • Climatic fluctuations in the Quaternary influenced distributions of mesic biota

They then deduced predictions from these hypotheses that could be checked by comparison with the results of phylogenetic analyses. Reconstructions of inferred ancestral biomes of taxa containing rainforest, sclerophyll and non-mesic members were used to test the first two hypotheses. Reconstructions of inferred ancestral areas, combined with the inferred timing of dispersal events were used to test the third hypothesis. The fourth hypothesis was tested by contrasts in net diversification rates between rainforest and sclerophyll sister groups. Genetic variation within species was examined for phylogeographic patterning to test the fifth hypothesis.

These tests confirmed that the great majority of both plant and animal taxa examined were descended from mesic ancestors but not necessarily rainforest ones. Numerous examples were found of invasion of Australia over the past 20 million years by plant and animal taxa that originated in Asia. Contraction of the rainforest biome and expansion of the sclerophyll biome over the past 30 million years was confirmed by both plant and bird groups that have members in both rainforest and sclerophyll environments. Phylogeographical structuring showed strong genetic signatures of response to climatic oscillations of the past 2 million years in a variety of plant and animal taxa.

These results strongly corroborated the great antiquity of both rainforest and sclerophyll components of the Australian mesic biome. This study also documented the decline and fragmentation of this biome, and especially that of its rainforest component over the past 23 million years. A paper reporting these results was published in the Journal of Biogeography (Byrne et al. 2011).

 

 

Mesic biome
The mesic biome, which is ancestral to other Australian biomes has substantially contracted and fragmented over the past 23 million years. Photo: Peter Weston


 

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