Ecological subsidies alter the structure of marine communities.

S ubsidies 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, mountainbacked 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 western coast of New Zealand (Fig. 1). In contrast, because of current patterns and the rotation of the Earth, the east coast is primarily affected by downwelling, and eastern coasts lack the nutrient subsidies provided by upwelled water masses. Upwelling has long been known to dramatically affect coastal conditions. For example, nearly half of the world’s fisheries production currently derives from the 1% of the coasts where upwelling occurs (3). Delivery of marine larvae by coastal currents (4) helped spark strong interest in ‘‘supply-side’’ ecology, a view of ecosystems that emphasized the importance of inputs from outside an area on local ecological processes (5). What is significant about the current work is the penetration of the upwelling effect through a coastal ecosystem, from the basal consumers to the abundance of top carnivores, and the experimental tools deployed to document these patterns. Menge et al. (1) show a strong correlation between the intermittent upwelling on western coasts and settlement of larvae of intertidal animals such as barnacles and mussels. These larvae feed off ocean plankton as they develop and may be much more likely to survive their juvenile phases where food supplies are high and development times are short. Settling in droves, the larvae metamorphose into rock-dwelling filterfeeders that pull upwelling-enriched planktonic food from the water. Thus subsidized, mussels and barnacles grow much more quickly and cover far more of the intertidal rock surface than in similar habitats on the nonupwelling east coast. These invertebrates are the link between the planktonic ecosystem of the oceans and the rock-bound ecosystems close to shore, so their thriving response to upwelling is expected. More surprising is that these subsidies at the level of the planktivores also extend to the next level of the food web. Where subsidies are high, coastal predators are also more abundant. Rates of predation are therefore much higher on the west coast than the nutrient-poor east, and overall the entire intertidal ecosystem seems to move at a faster pace on the west coast (Fig. 2). Starfish are the top predators in these systems, known to be able to exert strong control over the abundance and diversity of prey communities (6). Recent debates in ecology (7) have centered on discerning the times and places when communities are controlled by the action of predators at the top of the food web (so-called ‘‘top down’’ effects) rather than the availability of primary production at the base of the food web (so-called ‘‘bottom-up’’ effects). The Menge et al. article shows how intimately top-down and bottom-up effects can be connected: the extra subsidies available on upwelling coastlines add to the abundance of filter-feeding producers in the system but also increase levels of predation.