Islands, floras and history : an environmental history of plant introduction and extinction on the Austral Islands, French Polynesia

Abstract

Jared Diamond has recently suggested that the unique societal and floral attributes of Easter Island (Rapa Nui) and Tikopia may be a result of their geographical and historical isolation. In what Diamond proposes as the 'orthodox' understanding of these islands, I suggest that the discovery of material remains of the extinct Easter lsland palm cf Paschalococcus disperta and the introduced Tikopian tree crop Canarium harveyi can be seen to represent fundamentally different social strategies for alleviating problems of isolation. In developing a synthesis of evidence for plant introductions and extinctions from the Austral Islands in French Polynesia, I question whether the botanical discoveries on Easter Island and Tikopia allow for the establishment of a robust model of environmental transformation in the Pacific Islands following human colonization. Despite the totality of explanation proposed in this orthodox picture of Pacific Islands, defining the course of events that led to the establishment and proliferation of human trans-located plant species or the extinction of indigenous plant species is by no means straightforward. There are geographical and historical biases that for many plant species render the relationship between human colonization and the geographic distribution of plants untenable particularly in relation to the downstream effects of human-generated disturbances. Just as some introduced taxa may become naturalized on islands once released, some indigenous taxa may decline or face extinction on their own accord in response to other environmental factors independently of human activity. The relationship between human colonization and phytogeographic patterning on the Austral Islands is assessed through the examination of multiple lines of botanical evidence. These include historical documentation from early explorer or missionary accounts, oral traditions, sub-fossil archaeological and palaeobotanical remains and the assessment of molecular phylogeographic patterning. Each line of evidence has a different chronological representation and each is preserved differently in different geographical settings. ln synthesizing this information I follow an historical method proposed by Femand Braudel which first establishes the duration of each line of evidence then explores the overlap between their chronological and geographical representations. It is at these points of overlap that a more robust history is revealed. By following this historical structure I argue that much of the Diamond's 'orthodox' model has a weak foundation. Material evidence from the Austral lslands suggests that the cases of Easter Island and Tikopia are not unique and that the chronological interpretation of plant extinction or introduction events may be distorted. Palm trees (Arecaceae: Iguanurinae type) have also become extinct on the Austral Islands and palm forest decline appears to correspond to the establishment and expansion of introduced tuber and tree crops. The chronology of these extinctions and introductions established from the examination of microfossil remains from sedimentary deposits on Rapa and Rimatara are generally synchronous. I suggest that changes in land-use following European contact directed the final extinction of some plant species including the Iguanurinae type palm on Rimatara. With the resolution of chronological evidence for the Austral Islands, particularly from palaeobotanical evidence, I suggest that most plant extinctions either occurred at the onset of human colonization or were delayed until after European contact. Show more