Evolution and conservation

Dr Maurizio Rossetto - Senior Research Scientist

‘Species distribution and ecological community assembly in a changing environment’

The size and connectivity of rainforest populations in Australia has been affected by long-term climatic trends such as the overall aridification of the continent (in the last 10 million years) and the glacial cycles of the Quaternary. These major environmental events have left distinct signatures on the distribution of species and genes across the landscape.

An important challenge in biogeography and evolutionary biology is to gain insight into the relationship between the ecology of species and their evolution. A combined reconstruction of long- and short term population histories from diverse co-distributed taxa can yield generalities about evolutionary constraints and differential success, particularly if variability in life-history traits and environmental niches is taken into account. A recent study by M. Rossetto and collaborators (including A. Ford from CSIRO Atherton, and D. Crayn from JCU Cairns) aimed to recognize broad phylogeographic patterns across 11 Elaeocarpus species from the Australian Wet Tropics (AWT), and identify the lifehistory traits and environmental factors associated with them.

The study analysed a combination of allelic and flanking region sequence data from nuclear microsatellite markers, and evaluated the relative influence of environmental preferences and functional traits on genetic diversity and gene flow. Our study confirmed the importance of some of the previously recognised upland refugial areas in the maintenance of rainforest tree diversity, and corroborated recent suggestions that lowland areas also played an important role in the preservation of rainforest diversity. However, most of the elaeocarps investigated did not show strongly differentiated monophyletic groups of closely related haplotypes originating from opposing sides of a biogeographic barrier, the Black Mountain Corridor (BMC). Although some common patterns exist, these rainforest trees do not partition genetic diversity in similar ways due to subtle trait- and habitat-related differences prompting independent gene flow and migration across the heterogeneous AWT landscape.

Trees restricted to upland rainforests appear to have been subjected to greater constraints in population size and gene flow than species that are also (or only) distributed at lower altitudes. Interestingly, the selective processes operating on upland species appear to be associated with relative competitiveness on poorer soil types rather than climate variables alone (such as low temperatures and high Plant Science Page 29 rainfall). Also, in contrast to previous observations within southern rainforests (including other elaeocarps; eg. Rossetto et al. 2008 Amer. J. Bot.), we determined that genetic differentiation appears to be associated with small - rather large-fruited species. However, we suggest that in order to understand the influence of seed dispersal on community structure we need to take into account a combination of factors, and that it can be premature to attribute to dispersal alone, expectations of relative abundance, distribution and community-level coexistence

Overall, our findings are more in agreement with the concept that the assembly of plant communities from the local species pool is dependent on a range of potential filters which may admit or exclude species and traits. As comparative studies correlating genetic, environmental, and life-history data become more common, increasingly accurate predictions of functional group-level responses to environmental change will be possible. Such knowledge can underpin management strategies that attempt to anticipate the likely response of ecological communities to environmental and climatic change. This study is part of a broader research theme that uses genetic and genomic approaches to investigate how historical and adaptive differentiation affects the current distribution of biodiversity. Exploring evolutionary interpretations of functional diversity in community assembly is a critical approach for understanding the influence of local-factor variability on biodiversity.

Elaeocarpus angustifolius inflorescence (photo: J Plaza) .

Elaeocarpus angustifolius, one of the study species, in subtropical rainforest. (photo: M Rossetto)

Elaeocarpus angustifolius fruit.

These are one of the main food sources of cassowaries in the AWT as shown by the seedling germinating after ingestion. (photos: M Rossetto)