Ecological subsidies alter the structure of marine communities

Abstract

Subsidies are usually the subject of purely economic debates. They represent flow of resources from one economic sector to another and can be fundamental to competitive ability of industries in different nations. Subsidies are not restricted to economics, however, and are a pervasive factor in the ecological commerce between ecosystems. In this issue of PNAS, Menge et al. (1) show the influence of subsidies on the pace of ecological interactions in an intertidal community in New Zealand. Subsidies come in the form of nutrients, organic particles, and larvae wafting in from oceanic water masses. Their impact is to dramatically alter the abundances of key animal species and ignite rates of ecological processes such as competition and predation. All ecosystems receive subsidies. Photons flood every leaf. Estuaries are fueled by nutrient flow from rivers. Amazonian forest soils are enriched by African desert dust that blows relentlessly across the Atlantic Ocean (2, 3). These additional inputs can greatly augment the supply of critical nutrients, such as phosphorus and nitrogen needed for plant growth, and, in so doing, they top up ecosystem fuel supplies. Subsidies like these represent a crucial link among ecosystems because the subsidies that flow into one ecosystem derive ultimately from another. Menge et al. (1) asked whether marine intertidal communities receive different subsidies on opposite coasts of the South Island of New Zealand. The west coast of this large, mountain-backed island is washed by currents that generate an intermittent oceanographic condition called upwelling. Surface currents that flow along coasts and bend out to sea tend to pull deeper waters up behind them. These upwelling waters, with origins in the deep sea, are colder and richer in nutrients than the surface waters they replace. Along the west coast, frequent periods of upwelling generate plumes of nutrient-rich seawater near the …