GUAYANA HIGHLANDS VERSUS GUAYANA LOWLANDS, A REAPPRAISAL

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Abstract
Summary: During the last 40 years, an impressive amount of biological, especially botanical, information concerning the Venezuelan Guayana region has been accumulated. Both the lowlands, as well as the distinctive highland mountains (‘tepuis’), evidence an outstandingly rich and diversified flora and vegetation. It has been postulated that this biological richness and the high degree of endemism are the result of long‐term and highly effective isolation suffered by the flat summit areas during extremely long geologic epochs. This static view is being challenged presently by the results of recent intensive fieldwork made in the Venezuelan Guayana. A detailed analysis of two highly evolved Guayana taxa,Stegolepis(Rapateaceae) andBonnetia(Theaceae s.l.) shows a rather uniform distribution not only in summit habitats, but also in intermediate and lowland habitats. As a whole, both genera and several related taxa prove to have occupied successfully not only the ancient summit regions of the tepuis, but also the surrounding lowlands of much more recent geomorphological origin. Palaeoecological research also supports the hypothesis that the entire Guayana Shield area has been affected by climatic oscillations during the recent past as have the nearby Andes. This implies significant past fluctuations of the altitudinal vegetation belts in the tepui area and could well explain the wide altitudinal gradient of most of the Guayana Highland taxa. Also, the presence of convergent ‘paramoid’ vegetation types on the summits of the Chimantá Massif, indicates active speciation not only at the systematic level, but especially at the ecologic level. Thus, it is proposed that the Guayana highland and lowland biota have been and still are dynamically and genetically interdependent, not only within the Guayana region itself, but also with the adjacent complex of lowland and mountain biota.