Theory and Implications of Habitat Size Influences on Stream Food Webs

Habitat size is a main axis of riverscape context, but the likely far-reaching influence of habitat size has been misplaced amongst modern models of river management.  Habitat size was an important part of early models of stream ecosystems such as the River Continuum Concept, but knowledge of its influence on the stability and characteristics of stream food webs has been poor.  We have shown through surveys and a stream fence manipulation experiment in Canterbury, New Zealand, that the scaling of habitat size with home range size fundamentally limits the maximum size of stream predators (i.e., fish).  Furthermore, comparisons of the food webs of drying streams and perturbed streams point to the stability of food webs decreasing with habitat size. Limitations on top predator size combined with the effects of habitat size on food web stability mean the capacity of streams to support stable predator biomass (i.e., fish) decreases with habitat size as we have observed over four orders of magnitude in the size of Canterbury streams.  This has important management implications.  In New Zealand, non-native fish (trout) are more likely to eliminate native fishes (galaxiids) from smaller streams.  More generally it means that many models of in-stream flow allocation probably underestimate fish carrying capacity because of the way habitat size scales with predator biomass.