Tuatara: Volume 7, Issue 1, September 1958
Interrelationships of the Australasian* and South American Floras
Interrelationships of the Australasian* and South American Floras
Plant geography is the study of the pattern and mode of plant distribution over the earth's surface. Some of the chief problems of this field of study are the relationships between floras of widely separated areas such as those of eastern Asia and eastern North America. The present account is concerned with a similar problem in the Southern Hemisphere, namely the botanical affinities of Australasia and South America.
Historically, these affinities appear to have arisen during two distinct periods, the first and longest being before, and the second since the Pleistocene ice age. The more recent phase will be considered first.
Post - Ice Age Relationships
The land areas involved during this phase were most of the high latitude islands surrounding Antarctica, the Grahamland peninsula of Antarctica itself, and the southernmost portion of South America.
Since these areas are mostly separated by immense distances, it is surprising to find that their rather meagre floras are so closely related that they might reasonably be regarded as one.
Thus, one species, Azorella selago, of an otherwise entirely South American genus, is found in southern South America, the Prince Edward Islands, the Crozets, Kerguelen Island, Heard Island and Macquarie Island.
Acaena adscendens (Rosaceae), also of a largely South American genus, occurs in southern South America, the Falkland Islands, South Georgia, Kerguelen Island, the Crozets, the Prince Edward Islands and Macquarie Island.
Ranunculus biternatus (Ranunculaceae), of a cosmopolitan genus, is found in all localities of the last plus Amsterdam Island.
In total there are about thirty species with distributions of this kind in the south circum-polar region.
Some insight into the way in which the circum-polar flora, as it might be called, has become so widely distributed may be obtained by consideration of a particular area in the high southern latitudes. The example chosen for this account is Macquarie Island.page 2
Macquarie Island, lying 600 miles south-west of New Zealand, is separated by an ocean deep of more than 2,000 fathoms from a group of small, scattered islands on the New Zealand shelf. It measures approximately twenty miles by three miles, and is in the form of a narrow, elevated plateau bordered by steep cliffs. The total number of vascular plants present is thirty-five, of which about twenty are widely distributed in the south circum-polar region. In a recent account of the plants of Macquarie Island, Taylor (1955) quotes the following statement concerning glaciation from Mawson (1943). ‘There is no geological evidence that suggests even indirectly that there was any land in the region (i.e. Macquarie Island) not buried under ice.’ As there is no evidence of land connections since the ice age. Taylor concludes that the present flora has come to Macquarie Island since that time by long-distance dispersal. He suggests that the agents for this dispersal were various species of migratory birds, which are known to traverse great distances between the high-latitude land areas and islands.
Schimper (1905) reached a similar conclusion with regard to the flora of Kerguelen Island. He thought that only a few species survived the ice age, namely the monotypic, endemic Pringlea antiscorbutica, Luallia kerguelensis and probably Colobanthus kerguelensis. Most of the twenty-five other species are widely distributed in high southern latitudes, and Schimper suggests that they have spread from southern South America since the ice age by the agency of pelagic birds.
The scanty floras of Heard Island and South Georgia are almost certainly post-ice age in origin, and the same would also be true of the three species making up the vascular flora of Antarctica (Grahamland).
The circum-polar species were presumably derived from certain high-latitude land areas whose floras were not entirely destroyed during the ice age. The major source appears to have been southern South America, where most of the circum-polar species are to be found, and a secondary source was probably New Zealand and its sub-antarctic islands, where several of the circum-polar species occur, including two which have not been recorded from southern South America (Cotula plumosa and Juncus novae-zelandiae).
Pre - Ice Age Relationships
The land areas involved during this phase were much more extensive, ranging from New Guinea, or even further north, to South America. The relationships between the floras, however, are not so close, being at the generic rather than the specific level. These relationships can be best brought out by comparing the floras of the approximate geographic extremes, New Guinea and temperate South America, with an intermediate area, New Zealand.
A few examples of these genera follow.
The well-known genus Fuchsia is predominantly South American in its distribution, being represented there by sixty or more species. The only Fuchsias found naturally outside South America are three species in the lowlands of New Zealand. These are Fuchsia excorticata (a tree), F. perscandens (a vine) and F. procumbens (a creeping herb).
Freycinetia, a genus of root climbers closely related to Pandanus, has many species in the tropical forests of New Guinea and Malaysia generally. One species, Freycinetia banksii, is common in the lowland forests of New Zealand, where it often completely obscures the trunks of forest trees with a tangle of roots, stems and leaves.
Forests of the genus Nothofagus, a relative of the Northern Hemisphere beech, are found in temperate South America, New Zealand, Tasmania, south-east Australia, New Caledonia and the mountains of New Guinea. The discovery of Nothofagus in New Guinea is comparatively recent but a large number of species are now known from there.
Ourisia, an attractive genus of the Scronhulariaceae, is found in alpine habitats in South America, New Zealand and Tasmania.
Oreomyrrhis, of the family Umbelliferae, also alpine, is found from Mexico through South America to New Zealand, Tasmania, south-eastern Australia, New Guinea, Borneo and Formosa.
Even among the cosmopolitan genera there is evidence that certain of the species in Australasia and South America are more closely related to each other than they are to the remainder of the genus elsewhere. Thus Epilobium pedunculare in New Zealand is very similar to E. conjugens in temperate South America, and there are several other such species pairs.
A recently observed case of natural hybridism between two giant grasses, namely pampas grass (Cortaderia selloana) from South America and toe-toe (Arundo conspicua) from New Zealand, provides a little direct evidence for the relationship of their respective floras. Pampas grass has become naturalised at various localities in New Zealand and at one of these places near Auckland, where toe-toe is also present, many plants have been observed which combine the characteristics of the two species.
How can these many links between the floras of Australasia and South America be explained? Is it a matter of dispersal by birds, wind and water page 5 over vast distances, or has there been some more tangible connection between the land areas? The majority of students of this problem believe that there must have been connections or at least close approximations between the land areas, although not necessarily all at one time. From the New Zealand viewpoint this would involve a former northward extension to New Guinea and a connection with southern South America through the intervening land mass of Antarctica. In view of the existence of submarine ridges between New Zealand and New Guinea and a chain of islands between South America and Antarctica, former land connections in these areas are a definite possibility. On the other hand, between New Zealand and Antarctica there is a wide gap of predominantly deep water which would make a former connection between these areas appear unlikely, even though a recent bathymetric map (U.S. Navy, 1955) shows a series of submarine eminences in this region which may be the sites of former islands.
Apart from the improbability of a complete connection between New Zealand and Antarctica, there is also the fact that the latter, being a frozen continent with virtually no higher plant life, would undoubtedly act as a barrier to plant migration. However, in the light of fossil evidence this state of affairs is found to be comparatively recent in a geological sense. Fossils examined in many parts of the world indicate that the early part of the Tertiary period in earth history was much warmer than the present, e.g. palms and corals were found as far north as Oregon in North America, and many tree species existed in Grahamland, Antarctica. At this time, then, parts of Antarctica at least supported forests, which may have been continuous with those of South America. A former Antarctic migration route is certainly very attractive as an explanation of Australasian-South American plant links, but it is still far from being an established fact, and still presents problems of which the Antarctic-New Zealand ocean gap is the most, outstanding.
This account cannot be concluded without reference to the problem of Hawaii in Pacific plant distribution. Several of the genera largely restricted to Australasia and South America are represented by one or a few species in Hawaii, e.g. Astelia, Uncinia, Acaena. Hawaii is separated from the nearest continental land mass by over 2,000 miles, and there is nothing in the configuration of the ocean bottom to suggest connection with other land areas in the past. In this case the only possibility seems to be that the seeds of these plants have reached Hawaii over great distances by the agency of birds, winds, or ocean currents.
A general conclusion from these floristic comparisons might be that no one agency has been responsible for the floristic links between widely separated areas in the Pacific region. In the cases of Hawaii and the south circum-polar flora, transoceanic dispersal is probably the explanation, while in the case of the Australasian-South American links probably land connections have also played a part.
Looking to the future, it seems that the most likely sources of new information bearing on this problem will be derived from fossil floras of page 6 Tertiary and later age in Australasia, South America and particularly Antarctica, and from detailed comparisons of living plants from all three areas.
BRASS, L. J., 1941— The 1938-39 Expedition to the Snow Mountains, Netherlands New Guinea. Journ. of the Arnold Arboretum, Vol. 22, 271-342.
GORDON, H. D., 1949— The Problem of Sub-Antarctic Plant Distribution. Report of the Australian and New Zealand Association for the Advancement of Science, 27.
SCHIMPER, A. F. W., 1905 (In Schenck)— Miss. erg. Tiefsee Exped. auf dem dampfer ‘Valdivia’ 1898-99. Vol. II, Part I.
SKOTTSBERG, C., 1956— The Natural History of Juan Fernandez and Easter Island. Vol. I, Part 3.
TAYLOR, B. W., 1955— The Flora, Vegetation and Soils of Macquarie Island. A.N.A.R.E. Reports, Series B, Vol. II.
U.S. NAVY, 1955— Sailing Directions for Antarctica (map). Hydrographic Office Pub. 138.
* In this account Australasia is taken to include New Zealand, Tasmania, Australia, New Caledonia and New Guinea.