Human impact on vegetation of the Juan Fernandez Islands, Chile

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The flora and fauna of the Juan Fernandez Islands (33o50'S 80o00'W) have developed in isolation, some 700 km west of Santiago in Chile, on three small islands of volcanic origin (Go to map). The most ancient of these islands, Robinson Crusoe Island (Left image, Robinson Crusoe Island viewed from the western  end),  is some 4 million years old and harbours plant communities with up to 70% species endemism, including survivors   of many ancient plant groups that were once much more widespread in the southern hemisphere (Right image, Lactoris fernandeziana, a weedy shrub species that is the only member of Lactoridaceae and is endemic to Robinson Crusoe Island).

There are 209 native species of vascular plants in the Juan Fernandez Islands, of which 126 are endemic (Left image, Robinsonia gayana at around 550 m altitude overlooking the steep southern cliffs of Robinson  Crusoe Island). A further 152 introduced species have so far been recorded. Below about 500 m altitude small patches of dry forest (Myrceugenia and Fagara) and extensive grasslands, including many introduced species, are found. (Right image, Fagara mayu, a large endemic tree found in some of the dry Myrceugenia forest communities on the islands)

At higher altitudes the open ridges and ravines are subject to more humid conditions and can support montane forests that is abundant in ferns.  (Left image) At around 650m altitude in the southwestern montane forests of Robinson Crusoe Island a wet environment results in dense forest cover (including Cuminia, Fagara, and Rhaphithamnus) and an abundant fern understorey (including Hymenophyllaceae, Thyrsopteris, and Blechnum).

The treeline is reached at around 950 m altitude only in the uplands of Alexander Selkirk Island. The treeline and alpine communities are characterised by a temperate Magellanic flora (Left image, Alpine vegetation at around 1050 m altitude on the southern side of Alexander Selkirk Island, close to the palaeoecological site,  Los Innocentes Abajo 1), including such genera as Acaena, Dicksonia (Right image, Dicksonia externa, an endemic fern forming a dense subalpine community.), Drimys, Empetrum, Gunnera, Myrteola, Pernettya, and Ugni.

The first human occupation of the islands was in 1574 when the spanish explorer Juan Fernandez discovered the islands. The relatively late colonisation provides an important opportunity to compare the prehuman ecosystem with changes that occurred after human occupation and to assess to what extent these changes have been exacerbated or driven by natural processes and climate variability.(Go to climate diagram).


Pollen Studies

The Los Innocentes Abajo 1 sequence provides the first palaeoecological record for the Juan Fernndez Islands that includes data on pre-occupation vegetation (Figure 5). The upland vegetation of Alexander Selkirk Island was little affected by fire before 1574. A subalpine heathland of Empetrum, Pernettya and Coprosma, including herbaceous (Wahlenbergia) and fern (Blechnum and Gleichenia) species, was growing near the site prior to human occupation. The strong regional input of Myrceugenia and Fagara pollen from lower altitudes, suggests that lower montane forests were much more extensive at this time.

Figure 5: Summary pollen data from Los Innocentes Abajo 1.

Subalpine heathland vegetation is replaced during the late 16th century by a more open fern-heath complex dominated by Compositae, Lophosoria and Dicksonia. Fire was still not a major factor in local vegetation dynamics, though regionally there were fires occurring (possibly around 1740 AD), encouraging the expansion of newly introduced weeds such as Rumex acetosella. Subalpine vegetation change, coupled with a reduction in regional forest taxa, is likely to be a result of trampling and consumption pressure from goats introduced by humans. The treeline may also have been distrupted or lowered due to herbivore pressure during this period.

The most dramatic change in vegetation from a fern-heath complex to fern-grass complex, coincides with peaks in local and regional charcoal abundance and the first appearance of exotic Eucalyptus pollen. The impact on regional vegetation is clearly illustrated by the decrease or complete loss of some forest and shrub taxa from the pollen record. This period of high charcoal abundance appears to be associated with the major phase of permanent human settlement between about 1890 to 1930.

Principal conclusions of the whole project

Project funded by a British Ecological Society Research Travel Grant, 1996, while the author was based at the Department of Plant Sciences, University of Cambridge, Cambridge, UK.

Permission granted by Corporation Nacional Forestal (CONAF), Chile (Permit No.1372-A/013).

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