Friday, 6 February 2009

Land of the Dodo

Part 1: Geography and colonisation

In order to understand how Mauritius came to be the home of the Dodo and many other unique animals, it is first necessary to consider how these animals arrived.

Mauritius is of volcanic origin, and lies at the end of a chain of islands and submerged plateau extending more or less due north towards the Deccan Traps on continental India. This is significant, as it explains the islands origin. As with the Hawaian island chain, the heat source is a volcanic hot spot sourced from deep under the earth, and as the movement of the Indian Ocean tectonic plate has carried India northwards over the last 66 million years first the Deccan, and then a series of oceanic islands, have erupted and then eroded away. South west of Mauritius is the current center of volcanic activity, the island of Reunion, and situated some distance east over a volcanic ridge is the small island of Rodrigues. Scattered around these main islands are various small outcrops, the most famous of which is Round Island near Mauritius. The entire group is known as the Mascarenes.

Situated to the north of Mauritius, and not part of this chain, are the Seychelles. These are not volcanic, but are instead a tiny fragment of India that broke off as it moved northwards. As a part of a continent, they have retained some animals that cannot cross salt water, such as unique endemic amphibians, and have been a major source of colonisations of other Indian ocean islands.

The oldest lavas on Mauritius are about 10 million years old, the youngest about 25,000, but Reunion erupts almost every year. The last major eruption was about 180,000 years ago, which must have devastated the island and probably wiped out many species. It is noteworthy for example that Reunion had no flightless birds, although the Reunion Ibis apparently was very reluctant to fly. The age of Rodrigues is unclear, but given the number of endemic species it is probably fairly old.

The problem for any creature getting to the Mascarenes is that both wind and sea currents blow from the east, and east of the Mascarenes there is basically nothing but open sea for many thousands of kilometres, until you reach Australia or Indonesia. For anything incapable of flying very long distances, such as seabirds, or which can survive long periods of travel on floating rafts of vegetation, such as some reptiles, arrival can only be from the north. Flying from Madagascar is a shorter trip, but involves flying into the prevailing wind, and does not seem to be the major source of Mauritian endemics.

The eroded remnants of older islands however would have been above sea level on numerous occasions during the ice ages, and species such as the fruit bats, parakeets, pigeons and several other groups probably reached Mauritius in these periods. When Mauritius first appeared above the sea the nearest reef, St Brandon shoals, would have been a high island, and it is probably from there that the ancestral dodo flew to the island.

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