Environmental controls on food web regimes: A fluvial perspective

Abstract

Because food web regimes control the biomass of primary producers (e.g., plants or algae), intermediate consumers (e.g., invertebrates), and large top predators (tuna, killer whales), they are of societal as well as academic interest. Some controls over food web regimes may be internal, but many are mediated by conditions or fluxes over large spatial scales. To understand locally observed changes in food webs, we must learn more about how environmental gradients and boundaries affect the fluxes of energy, materials, or organisms through landscapes or seascapes that influence local species interactions. Marine biologists and oceanographers have overcome formidable challenges of fieldwork on the high seas to make remarkable progress towards this goal. In river drainage networks, we have opportunities to address similar questions at smaller spatial scales, in ecosystems with clear physical structure and organization. Despite these advantages, we still have much to learn about linkages between fluxes from watershed landscapes and local food webs in river networks. Longitudinal (downstream) gradients in productivity, disturbance regimes, and habitat structure exert strong effects on the organisms and energy sources of river food webs, but their effects on species interactions are just beginning to be explored. In fluid ecosystems with less obvious physical structure, like the open ocean, discerning features that control the movement of organisms and affect food web dynamics is even more challenging. In both habitats, new sensing, tracing and mapping technologies have revealed how landscape or seascape features (e.g., watershed divides, ocean fronts or circulation cells) channel, contain or concentrate organisms, energy and materials. Field experiments and direct in situ observations of basic natural history, however, remain as vital as ever in interpreting the responses of biota to these features. We need field data that quantify the many spatial and temporal scales of functional relationships that link environments, fluxes and food web interactions to understand how they will respond to intensifying anthropogenic forcing over the coming decades.

Comparative use of side and main channels by small‐bodied fish in a large, unimpounded river

“…However, relatively little is known about the use of lateral habitats by small‐bodied fish in large rivers. Small‐bodied fish are key constituents of aquatic ecosystems (Pikitch et al ., ) because they influence food web regimes (Power, ), nutrient cycling (Stewart, ; Gido & Matthews, ) and ecosystem productivity (Hall, Jordaan & Frisk, ).…”

Light-mediated thresholds in stream-water nutrient composition in a river network

“…Changes in physical conditions often elicit nonlinear responses in species distributions and interactions that could lead to rapid spatial alterations of biological processes in river networks (Power and Dietrich 2002, Woodward and Hildrew 2002, Power 2006). Changes in physical conditions often elicit nonlinear responses in species distributions and interactions that could lead to rapid spatial alterations of biological processes in river networks (Power and Dietrich 2002, Woodward and Hildrew 2002, Power 2006).…”

“…While the presence of longitudinal environmental gradients is clear, their influences on ecological processes are less well understood. Changes in physical conditions often elicit nonlinear responses in species distributions and interactions that could lead to rapid spatial alterations of biological processes in river networks (Power and Dietrich 2002, Woodward and Hildrew 2002, Power 2006). Such environmental transitions or thresholds in processes are inherently difficult to detect in stream networks because of stochastic variability, natural and human-induced spatial heterogeneity, and the logistical challenges of working across multiple sites.…”

Ecological boundaries: a derivative of ecological entities

“…Examples span a broad range of ecological circumstances, from pitcher plant communities, small fish populations restricted to a desert spring (see Kodric-Brown and Brown, 1993), wetlands, riparian systems in arid areas to insular co-evolving florae and faunae (e.g., New Zealand or Lake Baikal). Food web compartmentalization is another area of ecology where strong habitat differences are seen to lead to spatially discrete networks of interactions (Power, 2006).…”