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Résumé :
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The number of the main macro-invertebrate species grazing on the microalgal film on rocky coasts was evaluated at six closely located stations on Mallorca ( Balearic Islands). The main bioerosive species at the sites studied were Melaraphe neritoides ( L., 1758), Patella rustica ( L., 1758), Monodonta turbinata ( Born, 1780) and Lepidochitona corrugata ( Reeve, 1848). The transects considered shared the same general environmental conditions and species pool. The aim of this study was to compare the effects exerted by the vertical gradient with those exerted by other sets of environmental variables. These sets were, namely, ( 1) inter-seasonal differences, ( 2) inter-transect differences ( comprising degree of wave-exposure and lithological differences) and ( 3) effects of the micro-morphology (i.e., crevices, basin pools and other small-scale structures). The main null hypothesis verified was that species composition remains constant after assessing the effect of the vertical gradient. This hypothesis is largely rejected here. The main factor modelling the species composition was the vertical gradient ( accounting for 31.6% of variability) whereas the percentages of variability yielded by the other sets individually were smaller but significant ( P
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