A comparison of ungrazed and livestock-grazed rock vegetations in Curacao.
Five of the largest remaining patches of livestock-inaccessible rock vegetation of Curacao, Netherlands Antilles, were described and compared to rock vegetation which has experienced centuries of livestock grazing. Study rocks (maximum diameters: 20-47 m) were located at two sites differing in rock type and altitude. At the St. Christoffelberg site, study rocks were siliceous and at altitudes of 240-310 m while at the Tafelberg site the rocks were of limestone and at altitudes of 35-150 m. The main vascular species on livestock-inaccessible rocks at both sites was Tillandsia flexuosa. a bromeliad. At the St. Christoffelberg, Tillandsia was principally accompanied by the grass Paspalum secans, the orchid Brassavola nodosa and the herb Portulaca venezuelensis, while at the Tafelberg it was principally accompanied by the vine Serjania curassavica. On livestock-accessible rocks Tillandsia ground cover was reduced to insignificant levels and mature plants were virtually eliminated from the population. Grazed vegetations also showed reduced vascular cover and were principally dominated by the annual grass Aristida adscencionis (St. Christoffelberg site only), the prickly pear Opuntia wentiana and the shrubby tree Acacia tortuosa. None of these weedy species, all of which are widely distributed on the island, were of any significance in ungrazed rock vegetations. It is hypothesized that Tillandsia-dominated ground cover may have been a common feature of the rock vegetation of the island prior to the introduction of livestock.