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Not what you think

By: Dr Mandy Reid, Category: Science, Date: 09 Mar 2015

Mention ‘French Polynesia’ and palm trees and cocktails come to mind; but, think again.

Not what you think - Marotiri Rocks 83 km ESE of Rapa

Not what you think - Marotiri Rocks 83 km ESE of Rapa
Photographer: Mark McGrouther © Australian Museum

Our work colleagues had visions of us swanning on sandy beaches and more than once our ‘Pacific holiday’ was mentioned prior to our departure. However, aside from the physical effort that always goes with fieldwork, our experience of southern French Polynesia was far from grass skirts and suntans.

The truth is that French Polynesia encompasses a huge area, consisting of 118 volcanic and low coral reef islands spread over 5 million km2 covering a latitudinal range of 8° (truly tropical) to 28°S (that is about the same latitude as Byron Bay on the mid-east coast of Australia). The primary islands that we visited were at the southern end of this range. 

The islands are very diverse, covering a range of geological histories and ages. The youngest are volcanic islands without reefs, then, over time the islands become surrounded by reefs, then there is subsidence of the volcanic substrate leaving a large lagoon (such as at a site we visited called Morane Atoll) then there are true atolls. The diversity of animals at each of these different island types largely reflects this history, with the oldest atolls (such as the Tuamotu archipelago) being among the richest in terms of faunal diversity.

Rapa Island, where we spent most of our time, is an example of a volcanic island without a lagoon or barrier or fringing reefs, but with coral reef beds. Rapa is a horseshoe shaped island reflecting its origin as a breached caldera. It is known to have endemic marine species (species found on around Rapa and nowhere else on earth). In geological terms, Rapa is a very recent landform, emerging from the depths due to volcanic activity ~5 Mya. Its outer coast rises abruptly out of the sea towards dramatic jagged peaks, products of erosion around the caldera rim. The climate is cool, cloudy and windy with temperatures ranging from 20°C to 26°C. It was once covered by temperate rainforest but was completely denuded of forest at some point in its prehistoric past. No doubt, the marine environment would have been extensively harvested in historic times, but now there are only about 400 human inhabitants on this 35 km2 island.

A large latitudinal range with diverse histories makes for varied habitats and often unique faunal compositions among the islands. We sampled only a tiny part of this huge region.

More information:

Anderson, A., Kennet, D. J. and Conte, E. Archaeological research on Rapa Island, French Polynesia. http://press.anu.edu.au/wp-content/uploads/2012/10/ch013.pdf

Clouard, V. and Bonneville, A. (2004). Ages of seamounts, Islands and plateaus on the Pacific Plate. http://www.mantleplumes.org/WebDocuments/PacificSeamountAges.pdf

Hall, J. V. 1868. On the island of Rapa. Transactions of the New Zealand Institute 1, 128–134.

Neall, V. E., and Trewick, S. A. (2008). The age and origin of the Pacific Islands: a geological overview. Philosophical Transactions of the Royal Society B. 363(1508): 3293–3308. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2607379/