Print PageEcological theory predicts that habitat size is a fundamental driver of community and food web structure, with species richness and food chain length hypothesised to increase in larger habitats. For streams, little is known about how these structural properties are influenced by discharge reductions, despite the importance of this information to freshwater biodiversity conservation and flow management. Working in a high-country alluvial environment, we examined the effects of extreme, but localised, flow gradients on food web structure. We measured stream size (discharge, cross-sectional area), sampled communities, and constructed isotope-based food webs at several locations along the perennial–intermittent continuum of fourteen streams in Canterbury, New Zealand.
Results/Conclusions
Flow decreased sharply along survey reaches, often transitioning from a perennial state (e.g., wetted channels ranging 2-6 m in width) to a dry river bed over distances of 200 m or less. Biological communities exhibited similarly dramatic longitudinal change across this continuum, particularly in fish-inhabited streams: species richness, the likelihood of fish occurrence, and predatory:prey richness ratios decreased as surface flows declined between perennial and intermittent sample stations. Furthermore, we found consistent scaling relationships for stream size and isotope-based measures of web size and food chain length; however, we also observed an influence of isolation (i.e., distance from nearest perennial water body) and upstream catchment area on the rate of food web change due to flow reduction. These findings illustrate that food webs may respond predictably to flow loss, but also suggest that the full range of variation in response can only be understood when the local biogeographic context is also considered. Results further indicate that an ecosystem size-based framework will be useful in identifying important ‘tipping points’ or thresholds for managing flow reductions.
