Comparative analysis of a community of Drosophilids (Drosophilidae; Diptera) sampled in two periods widely separated in time

The present work represents a natural test with one replicate of the dynami cs of a community of Drosophilids, sampled in two periods widely separated in time. The aim has been to detect a possible loss of biodiversity with it s evolutionary implications. Monthly collections of Drosophilidae (Diptera) were carried out during a 2-year period (September 1979-August 1981) in Bo rdils (north-east of Spain). The site was sampled again during a later peri od (1996-97) to analyst the pattern of changes that had taken place in this community of Drosophilids. As a marked seasonal component of the diversity was detected during the first period as shown by the rarefaction method an d the contribution of seasonal variability to the niche component, only one collection per season was performed during the second period. A significan t decrease in diversity was detected. An increase in dominance of the most common species in the collections (Drosophila subobscura) was also observed , possibly due to a change in the behaviour of this species associated with an environmental change, Six species collected during the first period. we re not collected during the second period. Three of these were already rare in the first period. so their absence might be attributable to a sampling effect. The absence of one of the other three species (Drosophila picta) ca n most probably be attributed to a change in environmental conditions. A ch ange in the cosmopolitan species and the unusual behaviour of the species a ssociated with fungi were detected by analysis of the relative monthly nich e breadth.