The Vegetation of Robinson Crusoe Island (Isla Masatierra), Juan Fernandez Archipelago, Chile

Abstract

Robinson Crusoe Island of the Juan Fernández Archipelago, as is the case with many oceanic islands, has experienced strong human disturbances through exploitation of resources and introduction of alien biota. To understand these impacts and for purposes of diversity and resource management, an accurate assessment of the composition and structure of plant communities was made. We analyzed the vegetation with 106 relevés (vegetation records) and subsequent Twinspan ordination and produced a detailed colored map at 1:30,000. The resultant map units are (1) endemic upper montane forest, (2) endemic lower montane forest, (3) Ugni molinae shrubland, (4) Rubus ulmifolius-Aristotelia chilensis shrubland, (5) fern assemblages, (6) Libertia chilensis assemblage, (7) Acaena argentea assemblage, (8) native grassland, (9) weed assemblages, (10) tall ruderals, and (11) cultivated Eucalyptus, Cupressus, and Pinus. Mosaic patterns consisting of several communities are recognized as mixed units: (12) combined upper and lower montane endemic forest with aliens, (13) scattered native vegetation among rocks at higher elevations, (14) scattered grassland and weeds among rocks at lower elevations, and (15) grassland with Acaena argentea. Two categories are included that are not vegetation units: (16) rocks and eroded areas, and (17) settlement and airfield. Endemic forests at lower elevations and in drier zones of the island are under strong pressure from three woody species, Aristotelia chilensis, Rubus ulmifolius, and Ugni molinae. The latter invades native forests by ascending dry slopes and ridges. It successfully outcompetes endemic taxa, including its congener Ugni selkirkii. The aggressive herb Acaena argentea severely threatens to overtake native grassland.

Oceanic Islands are fragile ecosystems that are easily modified through natural and human disturbance (e.g., Carlquist 1965, 1974, 1980, Nunn 1994, Kirch and Hunt 1997, Whittaker 1998). The eventual fate of such archipelagoes is disappearance under the surface of the sea after millions of years of erosion through wind and rain and the relentless pounding of the sea. Combined with these natural impacts on oceanic islands are the more recent, and often severe, human impacts. Easter Island provides an excellent, though tragic, case study of the negative effects of human intervention in an island ecosystem (Heyerdahl 1989, Zizka 1991). Both natural and human impacts not only modify the surface of an island itself, but they also bring about changes in the environment. These changes are mirrored in the plants and animals that inhabit the archipelagoes and reflected by the vegetation at any point in time. Water, soil characteristics, and erosion patterns are all important abiotic factors that interact with the plants to yield the resultant vegetation.

The Problem

Study of patterns and processes of evolution in island ecosystems requires a good understanding of the underlying vegetation. It is difficult to understand modes of speciation without viewing this process in the context of environmental parameters. Reproductive isolation, an important aspect of speciation, often has an ecological component. Further, adaptive radiation, so prevalent in oceanic islands, is driven in large measure by adaptations to the rapidly changing island environment (Schluter 2000).

A number of studies of the vegetation of Pacific oceanic islands have already been completed. The most recent summary is provided by Mueller-Dombois and Fosberg (1998). Through detailed descriptions of the vegetation, plus photographs of many of the dominant associations and endemic representatives of these fragile floras, a good understanding of the basic aspects of the vegetation can be obtained. This represents a good beginning for more detailed studies on these Pacific island archipelagoes, especially more precise vegetation mapping. Detailed vegetation maps of islands provide patterns of diversity that serve to stimulate evolutionary and biogeographic questions. They also allow more realistic assessment of conservation impacts and possible remedies.

Of the many oceanic island systems of the Pacific Ocean, the Robinson Crusoe ( Juan Fernández) Archipelago (Figure 1) is important for evolutionary and biogeographic reasons. This flora of 441 total species contains 125 endemic vascular plants, including the endemic family Lactoridaceae, which is of significance in the context of early angiosperm evolution (Stuessy et al. 1998c). Investigations in the archipelago over the past...