Who's Eating Whom?

Study shows food-web changes ripple from bottom to top

It may be mud and muck to you, but to David Gillett it’s like a super-sleuth game to see who’s eating whom and how energy moves from one part of a food web to another.

Gillett is a Ph. D. student studying the bottom-dwelling communities of Chesapeake Bay. The intricate relationships among the clams, worms, and myriad other invertebrates that make up these benthic communities, and between these organisms and their predators, makes figuring out the pathways that connect them a major challenge. A further challenge is to understand how these communities are affected by human activities such as urbanization and farming.

Gillett is helped in his endeavors by Ecopath, a computer program designed to construct food-web models. Ecopath generates box models that show the flow of materials and energy among organisms. “These models provide snapshots of what’s going on at any one time,” says Gillett. “We use them to determine how efficient the food web is in terms of transferring energy from producers to consumers. A happy ecosystem is an efficient ecosystem.”

Gillett’s advisor, Dr. Linda Schaffner, stresses that differences in how species respond to human disturbances can affect food-web structure. This has important implications for food-web efficiency, and in the longer term, for just how many fish and crabs a system can produce.

Species respond to disturbance in 2 main ways. Resilient species, including many benthic worms, are like weeds. They tend to reproduce quickly, grow fast, and may eventually monopolize an area that has been disturbed. Because most of these species are small, large predators may have to work harder for a meal. Resistant species, such as clams, grow more slowly and tend to live longer. They are able to withstand some disturbance due to their size and tolerance adaptations. They also tend to grow larger and are preferred food items for some larger predators, especially blue crabs.

Gillett says that nature can always bounce back from short-term disturbances. He warns, however, that long-term, human-mediated changes in light availability, nutrients, and other key environmental factors threaten the balance of the benthic food web, and may favor bacteria and “weedy” worms over clams, crabs, and fish.

Results from Gillett’s field study of an undisturbed tidal creek illustrate his concern. He’s covered some areas of the creek with mesh cages that limit access to fish and crab predators, and left other study areas exposed. Sampling of the exposed areas shows that they harbor just a few worms and clams, whereas the protected areas are full of these organisms. Gillett says sampling of a creek heavily impacted by human activity would look very different: the clams would be gone regardless of the cages, while unprotected areas would be dominated by small worms, which flourish in disturbed areas. The problem is that small worms don’t make very appetizing food for larger fish and crabs.

Gillett’s work shows that disturbances to the Bay’s benthic habitats don’t end there. Instead, they ripple to the very top of the food web—including your seafood dinner.