Botanic Gardens Trust, Sydney, Australia

Phylogenetic biome conservatism on a global scale

Dr Peter Weston – Senior Principal Research Scientist

The habitats that organisms occupy and why they are limited to them have long intrigued evolutionary biologists. The tendency for species to retain ancestral ecological traits and environmental distributions (‘niche conservatism’) has been discussed by ecologists in recent years but has been demonstrated only on local and regional scales. The extent of ecological conservatism over tens of millions of years and across continents had not been rigorously assessed until a research group including Senior Principal Research Scientist Peter Weston published the results of such a study in 2009 in the general science journal, Nature.

Dr Weston and his colleagues assembled a data set comprising 11,064 species of vascular plants in 45 taxa from extratropical Africa, Madagascar, Australia-New Guinea, New Caledonia, New Zealand and South America, equivalent to an estimated 15% of the total flora of these regions. They used the results of phylogenetic analyses to reconstruct ancestral distributions and, by dating divergences, inferred which disjunctions were likely to be the result of long-distance dispersal and establishment (colonisation) across oceans. Each plant species was assigned to one or more of seven biomes (wet forest, sclerophyll, alpine, bog, temperate grassland, savannah and arid) and transitions among biomes were recorded.

Biomes are broad vegetation types defined by climate, life-form and ecophysiology and, hence, are useful units for investigating the large-scale pattern of ecological conservatism. Biome conservatism might be a major determinant of the global distribution of biodiversity, such as the latitudinal gradient, which could have arisen because relatively few ancestrally tropical taxa were able to colonise temperate biomes. Global comparisons across multiple lineages were needed to investigate the generality of conservatism and how much it has influenced the assembly of biomes.

Dr Weston and his colleagues found strong support for the hypothesis of phylogenetic biome conservatism because biome shifts were rare, being associated with only 396 (3.6%) of the approximately 11,000 evolutionary divergences. Of these shifts, 356 occurred within landmasses and 40 were linked to transoceanic colonisations. Investigating taxa individually, a consistent pattern was found in which closely related species were more ecologically similar than would be expected by chance, irrespective of whether the taxa have been considered Gondwanan relics (for example Araucariaceae, Casuarinaceae, Nothofagus) or otherwise (for example eucalypts, grasses and some legume tribes).

The results of this study are relevant to the understanding of conservation for plants and plant habitats under climate change. Conservationists cannot expect plants to dramatically change their ecological preferences and evolve to fit new habitats as the boundaries of these biomes change in response to climate change.

Catalepidia heyana from north east Queensland

Catalepidia heyana's closest living relative, Malagasia alticola from Madagascar, provide one of many examples of biome conservatism in the Proteaceae. Both are descended from a unique common ancestor that lived over 15 million years ago, yet both still occupy the same biome, tropical cloud forest. Photos: Peter Weston