Are Tropical Streams Ecologically Different from Temperate Streams?

Abstract

If tropical streams differ ecologically from temperate ones, we must be cautious in our extrapolation of ecosystem models developed in temperate-zone streams. Similarly, approaches and techniques used routinely in management of temperate streams may not be applicable in the tropics. Despite considerable variability in geological history, flow regime and geomorphology, streams in the tropics typically receive higher insolation and more intense rainfall, with warmer water and often relatively predictable floods. For many groups of aquatic taxa, tropical streams also harbour higher biodiversity than their temperate equivalents. Nonetheless, there is little published evidence for consistent differences in food-web structure, productivity, organic-matter processing and nutrient dynamics, or responses to disturbance which would indicate that the term 'tropical' has special significance when applied to stream ecology. Instead, ecological processes in tropical streams appear to be driven by the same variables that are important in temperate ones. For example, biotic responses to drought and flooding are similar to those in temperate streams while in-stream productivity is limited by the same factors: nutrients, shading, disturbance, and trophic structure. Shredders are reputed to be rare in many tropical streams but this also is the case in many southern temperate streams, implying that models of leaf breakdown developed in the north-temperate zone may not have the universal applicability often assumed. Biome comparisons among temperate and tropical streams are confounded by the immense inherent variability of streams within both these zones, and the wide range of climatic and hydrological conditions – even in the tropics. Valid extrapolation of models and management strategies may be less a matter of tropical versus temperate streams but, instead, of ensuring comparability at appropriate scales and fuller understanding of ecological mechanisms, plus recognition of the magnitude and complexity of spatial and temporal variation in stream ecosystems at all latitudes.