Food webs

The health of an ecosystem is tied closely to the health of its food webs. This article provides an overview of the concept, origin, and characteristics of a food web and how predator and prey relationships are shaped in the Salish Sea.  

Bear eats salmon. Photo: Robert Voors (CC BY-NC-ND 2.0)
Bear eats salmon. Photo: Robert Voors (CC BY-NC-ND 2.0)

The concept of the food web is one of the oldest in modern ecology, dating from Charles Elton’s landmark 1927 book, Animal Ecology. Elton was interested in those fundamental but perhaps overlooked processes that have the power to shape entire communities. One such process was getting food. “Animals are not always struggling for existence,” he wrote, “but when they do begin, they spend the greater part of their lives eating.” How an animal caught its food was not important. What mattered was the way energy moved from one level of organisms to the next— from, as Elton saw it, a plant, to an herbivore, to a carnivore.  

For Elton, those schematics were “food-chains”; all the food chains in a given community constituted its “food-cycle”. Over time, the metaphor evolved from a chain, with its reliance on single linkages, into its more modern iteration, the web. This better reflects the many roles a single organism can play, or the fact that the relationships between different trophic levels and functional groups are not necessarily linear. So, too, do scientists draw food webs in a number of ways, depending on the scale of what they are trying to show, or the nature of the interactions between species. Within the Salish Sea, for instance, there can be terrestrial and aquatic food webs; or, to parse more finely, freshwater and marine food webs; or to parse more finely still, a soft-bottomed nearshore food web, a pelagic (open marine waters) food web, and so on. 

The Salish Sea is rich in life, home to thousands of species of marine invertebrates, hundreds of species of plants, more than 200 species of fish, nearly 200 species of seabirds, and more than 30 species of mammals. All of them are part of at least one food web. They may be top-level predators, like seabirds or most of the marine mammals. They can be mid-level consumers, like juvenile fish, shorebirds, or sea stars, acting as links between the food web’s lower and upper levels. There are herbivores and detritivores near the base, which graze either on plants or other non-living organic matter. And there are the primary producers—the phytoplankton, algae, and vascular plants—that form the very base of the food web.  

All food webs are dynamic. Changes in the abundance of almost any species can cause strong ripples as the remaining organisms reshuffle themselves. Along the west coast of North America, one of the most famous examples of a so-called trophic cascade is that of the sea otter; or, more accurately, its absence. Sea otters are top-level predators in kelp forest food webs. Among other things, they prey on sea urchins, helping to control their numbers. When trappers hunted sea otters to near extinction from the mid-18th through the early 20th centuries— sea otters were completely extirpated from the state of Washington until their reintroduction in 1972— populations of urchins suddenly thrived, as they now had unfettered access to their preferred food, kelp. Heavily consumed, the kelp suffered as a result. But it wasn’t just kelp that paid the price for the absent sea otters: all the species that depended on kelp for shelter or protection were affected, too. More recently, the widespread loss of sea stars has left scientists watching to see how the rocky intertidal food web will reassemble itself, suddenly deprived of a keystone species.  

All throughout the Salish Sea, food webs are in a near constant state of flux, whether due to local or regional conditions, seasonal changes, or large-scale perturbations, the potential consequences of which often remain unknown. As scientists study ocean acidification, for example, they have begun to try to predict where in the marine food web it will have the greatest impacts. The lowering of pH is felt most keenly by species that build shells or other internal structures from calcium, such as mollusks, crustaceans, and echinoderms. Of these, mollusks have so far received the most popular attention. But ecosystem-based models show that changes to crustacean abundance—most especially copepods, a kind of zooplankton—will most likely have the strongest impact on overall food web structure.