THE MACROINVERTEBRATE FAUNA OF ECUADORIAN HIGHLAND STREAMS IN THE WETAND DRY SEASON

The community structure and functional feeding groups of the macroinve rtebrate fauna were examined in eight Ecuadorian highland streams in t he wet and the dry season. The abiotic environment of the streams was highly unstable with great variability in discharge during the wet sea son, but relatively constant during the short dry season. Overall, the number of individuals and species were significantly higher in the dr y season than in the wet season. In all streams the composition of the fauna differed markedly between the two seasons, but no consistent ch ange in community structure and proportions of functional feeding grou ps was found for the eight streams. In both seasons Baetidae, Simuliid ae, Chironomidae and Elmidae were the four most numerous families. The equitability of the communities was, however, lower in all streams du ring the dry season. Collector-browsers were the most numerous organis ms while filterers, predators and shredders were much less important. Maximum stream temperature was the single environmental parameter that best explained the variability in community structure among the strea ms. Otherwise the fauna was weakly correlated to site characteristics. The instability of these tropical Andes streams is probably the main feature structuring the invertebrate fauna. The streams may be alterna ting between a stochastic community structure during the wet season an d early successional stages dominated by organisms with a fast growth and high colonisation capacity during the dry season.