![An Australian Government Initiative [logo]](/images/austgovt_brown_90px.gif)
Case StudiesSouthern diversity The American liverwort specialist Rudolf Schuster has presented arguments as to why the Southern Hemisphere's liverwort flora is much more varied and interesting than that of the Northern Hemisphere. Pages 3-7 of the book listed in the next Reference button present a short version of his arguments. This website page will summarize that printed short version In the Northern Hemisphere the super-continent of Laurasia split into North America and Eurasia which are relatively close today (the Bering Strait and the North Atlantic being minor gaps) and were closer in geologically recent times. East-west winds could carry propagules and maintain at least sporadic gene flow between distant populations. By contrast the Southern Hemisphere has had a more complex geological history, dominated by the break-up of Gondwana. That break-up saw Antarctica, Africa, India, Australasia and South America eventually move far apart from each other. Highly significant in that break up was the virtual extinction of all Antarctic life. Antarctica had been the central area in Gondwana. There was no equivalent loss in the Northern Hemisphere. Once the separation between the surviving, ice-free Gondwanan remnants was great enough, gene flow between the different continents became very difficult. From that stage the bryophytes in the different Gondwanan continents evolved largely independently of each other. The movements and collisions of the various tectonic plates also saw the creation of imposing mountain ranges such as the Andes, the Himalaya and the mountainous spine of the island of New Guinea. Such mountain ranges created a great variety of new habitats between sea level and the alpine summits with striking differences in temperature and moisture levels. The Andes were particularly significant because of their great north-south extent, from chilly Tierra del Fuego to the heat of the tropics. Schuster writes that such mountain building:
He goes on to note that liverwort diversity cannot be measured by numbers of species, but that numbers of genera are significant. he noted 30 endemic liverwort genera for New Zealand, with its relatively young mountains, but only one for Africa, older and many times larger. Schuster argues that:
In the old Appalachian mountains of the USA there are about 250 liverwort species, with perhaps 4 or 5 endemic species, but no endemic genera. Finally, as well as creating new habitats, tectonically unstable areas seem to provide places where ancient liverworts seem more likely to survive so that Schuster's final sentence in a 1983 paper is:
|
.