No. 1 Feburary 1998
Conference Abstracts
IMPACTS OF HABITAT CHANGE ON ALPINE RANUNCULUS SPECIES: EXTINCTION THROUGH HYBRIDIZATION.
Tristan Armstrong, Division of Botany and Zoology, ANU , ACT 0200
In natural environments many species of plants are genetically isolated by environmental factors, even though they are fully reproductively compatible. Habitat changes can disrupt these isolating mechanisms leading to extensive hybridization and a serious loss of genetic integrity between the species as they become progressively more introgressed, potentially merging into a single panmictic gene pool.
This study is significant because it is one of the first to have investigated the long term genetic consequences of hybridization in a large species complex. The results not only contribute to our understanding of species boundaries and speciation processes but are also critical to the conservation of the species which are threatened increasingly by habitat alteration through climate change and land use impacts.
In the alpine region of mainland Australia, there are five species of Ranunculus belonging to the section Chrysanthe.: R.muelleri, Benth., R. dissectifolius F. Muell., R. graniticola, Melville, R. millanii, F. Muell. and R. niphophilus B. Briggs. Ranunculus dissectifolius and R. niphophilus are endemic to the Kosciusko alpine region (Briggs 1957), and have therefore been classified as rare by the Australian National Parks and Wildlife Service (Briggs, and Leigh 1988).
All five species are entirely cross-compatible, however, in undisturbed habitats where the species are parapatric, hybrids are restricted entirely to narrow contact zones. The stability of these hybrid zones is maintained purely as a result of habitat specialisation and intense disruptive selection against intermediate forms within parental habitat.
This was verified by extensive studies of potential barriers to gene exchange including several large scale habitat selection experiments using parental species and artificially produced hybrids. Of particular importance was the finding that hybrids and parentals appear to have equivalent fitness in disturbed sites and areas where habitat boundaries have recently changed. This raises important concerns about the management and conservation of these species under conditions of habitat change induced by climatic fluctuations.
Climate change is likely to have a profound effect on the current boundaries of alpine microhabitats such as snow banks and Sphagnum bogs. My results suggest that under these conditions, a massive expansion of the hybrid zones would be the most likely outcome, which would ultimately lead to a serious loss of genetic integrity, and in a taxonomic sense the extinction of the species, when hybrids dominate over parental forms.
This study has quantified the extent to which disturbance could modify the distributions of the five parental species and, on this basis, recommendations will be made for the long term survival of the group.