Who’s Eating Whom?
Effects of Human Disturbance on Benthic Habitats

David Gillett in the lab

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

Dave is a Ph. D. student studying shallow water benthic (bottom) communities of Chesapeake Bay.  It all seems quite complicated as he shows me a model diagram that shows relationships among different kinds of species.  Various sized arrows in and arrows out, along with boxes for each trophic level, such as predators and their prey, make it look complicated.  In the real world, it is complicated, and figuring it out is a major challenge. The diagram is generated by Ecopath, a specialized computer program for the construction of food web models. Dave explains that the arrows and their magnitude show the pathways and amount of energy that is transferred from one group to another.  He wants to know how the sizes of the arrows and boxes change along with disturbances in the Bay ecosystem that are caused by nearby human activities, such as urbanization and farming.  “These models provide instantaneous snapshots of what’s going on at any one time”, says Dave.  We use the models to determine how efficient the food web is in terms of transferring food from the producers to the consumers.  “A happy ecosystem is an efficient ecosystem.” 

As I try to get a handle on the big picture, he says that his advisor, Dr. Linda Schaffner, has drilled into him that differences in how species respond to human disturbances affect food web structure.  According to Dave, this has important implications for food web efficiency, and in the longer term, just how many fish and crabs a system can produce.  Resilient species, including many benthic worms, are like the weeds in your lawn.  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 thinks the productivity of upper trophic levels is tied to the disturbance regime through the food web. 

There is always some sort of disturbance going on that is short term in nature and systems can bounce back.  But, increasingly the shallow water habitats of the bay are being influenced by changes in key factors, such as light availability, nutrients and contaminated sediments, due to human activities.  These disturbances affect the balance in the benthic food web and may favor bacteria and “weedy” worms over clams, crabs and fish.

Dave with benthic samples

Before I leave, Dave shows me sample jars from his field study.  They come from two parts of an undisturbed tidal creek, one left open and the other covered with a mesh cage that limits access to fish and crab predators.  Where predators can feed on the bottom, the number of worms and clams in the jars is low.  Where the bottom is protected by cages that exclude the predators, the jars are full of worms and clams.   These diverse invertebrate communities feed everything from juvenile blue crabs to adult spot.  He comments that similar jars from a creek heavily impacted by human activity would look very different.  The clams would be gone regardless of the cages.  And, while the small worms that flourish in these areas may support juvenile predators, but they do not seem to make very appetizing food for larger fish and crabs.   Potential changes in habitat affect everything from bacteria to fish. …the concept of the food web and its impacts on everything, including your seafood dinner, has just been made clear.