DISTURBANCE AND THE TEMPORAL VARIABILITY OF INVERTEBRATE ASSEMBLAGES IN 2 ANDEAN STREAMS

1. Monthly variation in patterns of invertebrate density was examined in two streams in the Venezuelan Andes that differed in elevation, geomorphology, and disturbance regime. 2. At one stream, located in the Andean piedmont, invertebrate densities displayed large fluctuations and between-month variation differed by as many as four orders of magnitude. Major declines in invertebrate numbers were generally associated with frequent disasters, defined as channel-shifting spates and drought. 3. At the other stream, a montane site located at higher elevation, invertebrate densities varied by little more than an order of magnitude over the duration of the study. Reductions in invertebrate numbers between successive months generally occurred during the rainy season. 4. Monthly invertebrate densities were inversely related to rainfall at each site. In addition, a highly significant positive relationship was observed between invertebrate density and the number of days elapsed since the previous major rainstorm event. Relationships between rainfall and invertebrate densities differed between streams, however, with significantly steeper regression slopes being found at the piedmont than the montane site. 5. We suggest that disturbance plays a seasonally important role in structuring neotropical stream communities in which changes in discharge are severe and unpredictable. Our data contrast with studies from tropical floodplain rivers in which rainy-season floods are predictable events that cannot be readily characterized as disturbances.