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 dynamics 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 its evolutionary implications. Monthly collections of Drosophilidae (Diptera) were carried out during a 2‐year period (September 1979‐August 1981) in Bordils (north‐east of Spain). The site was sampled again during a later period (1996–97) to analyse 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 and the contribution of seasonal variability to the niche component, only one collection per season was performed during the second period. A significant 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, were 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) can most probably be attributed to a change in environmental conditions. A change in the cosmopolitan species and the unusual behaviour of the species associated with fungi were detected by analysis of the relative monthly niche breadth. Zusammenfassung Vergleichende Analyse einer Gemeinschaft von Drosophilaarten (Drosophilidae; Diptera) über einen langen Zeitraum hinweg Die vorliegende Arbeit beschreibt einen in der Natur durchgeführten Test über die Dynamik in einer Gemeinschaft von Drosophilaarten, bei dem die Stichprobennahmen eine sehr lange Zeitperiode auseinanderliegen. Ziel der Untersuchung war, eine mögliche Reduktion der Biodiversität und deren evolutionäre Bedeutung aufzudecken. Monatliche Aufsammlungen von Drosophiliden wurden in einer Zweijahresperiode (September 1979–August 1981) in Bordil (Nordost‐Spanien) durchgeführt. Am selbe Standort wurden dann nochmals während einer späteren Periode (1996–97) Stichproben genommen, um das Muster einer Veränderung, die sich in der Drosophilagemeinschaft ergeben hatte, analysieren zu können. Da sich in der ersten Untersuchungsperiode eine deutliche jahreszeitliche Komponente ergeben