Changes in stream food webs across gradients of ecosystem size, disturbance, and productivity: Adding a riverscape context to stream food webs

Instead of predictable changes in stream community structure driven by longitudinal variations in resource availability, New Zealand research has highlighted the role of unpredictable disturbances in structuring stream communities.  Biotic interactions have received less attention, but they may be just as important in shaping community structure, depending on the environmental context.  We studied how the length of stream food chains was influenced by the riverscape context in the Cass region using nitrogen isotopes to estimate maximum trophic position.  Ecosystem size, measured by stream cross-sectional area had little effect on food chain length, except where fish were completely absent from very small streams.  Food chain length was strongly influenced by local productivity measured by the standing crop of invertebrates.  However, that was primarily due to a strong link between disturbance and invertebrate abundance.  Streams with high invertebrate numbers generally contained eels which fed at a high trophic level.  In comparison to viewing rivers as a continuum, these results indicate that using a riverscape framework, which emphasises the importance of variations in environmental context, is likely to provide a more useful model for understanding stream food webs.