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The primary production of aldabra atoll indian ocean with reference to habitats used by giant tortoises geochelone gigantea


Philosophical Transactions of the Royal Society of London B Biological Sciences 302(1109): 167-200
The primary production of aldabra atoll indian ocean with reference to habitats used by giant tortoises geochelone gigantea
The seasonal changes of standing crop and production in habitats used by a high-density population of giant tortoises on Aldabra atoll are reported. The study had 2 main aims: first to investigate the primary production of a raised coral atoll and secondly to provide base data for a study of the interactions of a large reptilian herbivore (the giant tortoise) with its food supply and environment. Environmental heterogeneity made it necessary to measure separately the standing crop and the above-ground net production of different components of the vegetation; these components were usually single species or small groups of species of plants. Measurements of these components were then combined with cover data for the same components in selected places to illustrate the seasonal and spatial variability of primary production on Aldabra. Standing crop biomasses were estimated from harvest samples. Methods for production estimates varied with the component studied, but included harvest difference methods, repeated clipping of the same plots and direct measurement of leaf turnover rates on marked shoots. These methods are compared where appropriate. Net annual above-ground production varied between plant types from 3165 kJ/m2 of plant for tortoise turf to 47,700 kJ m-2 for Cyperus ligularis, a robust perennial sedge. Total above-ground annual net production of different habitat types (bare ground and rocks between plants being taken into account) varied from 9100 kJ m-2 in a thinly wooded area with high tortoise turf cover (open mixed scrub) to 28,200 kJ m-2 in an area of thick scrub forest (groves). The seasonality of production and standing crop also varied considerably between habitats, owing to the role of different components of the ground layer and shrub cover. These results are discussed in terms of the roles of environmental and structural heterogeneity in setting primary production and of the tortoises themselves in their interactions with the vegetation via trampling and grazing. The effect of this heterogeneity on sampling strategies and results is also assessed.


Accession: 006740771



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