Habitat size influences on river food web structure and subsequent stability

Background/Question/Methods

Habitat reduction is a major driver of biodiversity loss, and is particularly important in rivers where ongoing global and local human influences on flow are dramatically altering habitat size. We investigated the mechanisms underlying the influence of river habitat size on food web structure and stability. Reductions in habitat dimensions are likely to have strong effects on larger organisms, particularly top predators, due to their greater requirements for spatial resources. Because of the metabolic efficiencies associated with larger body size and the far-reaching effects of large top predators in food webs, we predicted reductions in predator size associated with habitat contraction would alter both food web structure and stability. The effects of extreme, but localised, flow gradients (i.e., river drying) on food web structure were assessed by measuring stream size (discharge, cross-sectional area), sampling communities, and constructing isotope-based food webs at several locations along the perennial–intermittent continuum of fourteen streams in Canterbury, New Zealand. We also conducted a stability test across a continuum of habitat size to measure community resilience and resistance to a local river bed disturbance.

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.  Associated change in biological communities was dramatic and we found consistent and significant scaling relationships between habitat size and isotope-based measures of web size, food chain length, and trophic area. Moreover, the top-heavy food webs with less trophic diversity, and fewer connections associated with smaller streams are likely to be less stable as a consequence. These findings were supported by our stability assay, where we observed more rapid community recovery (resilience) in moderate to large streams than in small streams, despite a similar response to the initial perturbation across the stream size gradient (= resistance) . These findings illustrate that food webs may respond predictably to habitat loss and that there are substantial structural changes associated with habitat contraction that reduce community stability.