Geographical and Historical Comparisons of Neotropical Streams: Biotic Diversity and Detrital Processing in Highly Variable Habitats

Although many species that live in low-order rainforest streams obtain much of their energy in the form of terrestrial leaf detritus, marked regional differences in the quantity, quality, and seasonal timing of leaf fall occur at different latitudes. There are also major seasonal, elevational, and historical differences in the frequency of floods and droughts that influence patterns of leaf input, transport, and decomposition. These differences may provide a basis for comparing stream communities in terms of biotic diversity and modes of detrital processing. At present, regional patterns of community structure are apparent in temperate-zone streams, but no distinct general patterns of biotic diversity or functional feeding groups are clearly evident in stream communities across latitude. Diversity of stream-dwelling detritivorous fishes is apparently highest in the large, geomorphologically complex Amazon basin where environmental conditions have remained highly variable throughout the Pleistocene. Diversity of other neotropical fish communities is regionally influenced by proximity to marine coastal habitats. Some insect and decapod crustacean taxa show higher species richness in mainland montane streams; high levels of endemism characterize some insular streams. As field experimentation increases, the underlying bases for evolution of specific adaptations by benthic invertebrates in highly seasonal neotropical streams can be interpreted more reliably, and detritivory can be placed in a latitudinal perspective. Human-induced disturbances are rapidly increasing in frequency and intensity in tropical regions; the biotic richness of low-latitude streams is being altered and opportunities for research on community dynamics are being lost.