It has generally been accepted that the length of a food chain (i.e., the number of trophic levels) in a given ecosystem is a function of the amount of food energy in the ecosystem. But a study published last year in Nature reported that the length of the food chain is instead determined by the size of the ecosystem. For example, a small lake should have a shorter food chain than a large lake.

The researchers suggest two explanations for why larger lakes have longer food chains: larger lakes have top predators that aren't found in smaller lakes; and some species that are found in both large and small lakes eat higher up on the food chain in larger lakes.

The study was significant because proper understanding of food chains can have an impact on conservation efforts. For example, since smaller ecosystems have shorter chains, shrinking ecosystems may risk losing their top predators. Also, because of how food chains work, top predators generally accumulate more contaminants than species lower on the food chain.

Perhaps even more distressing were the research results announced last month by biologists at Brown University. Experiments along the Virginia and Georgia coasts showed that over-harvesting of blue crabs by commercial fishermen can actually destroy entire salt marshes. The blue crabs eat periwinkle snails which, in turn, eat the cordgrass anchoring a salt marsh. Without the blue crabs, the periwinkle snails flourish, destroy the cordgrass, and the salt marsh quickly becomes a barren mudflat. These salt marshes are critical environments, acting as nurseries for important fish, filtering water runoff from the land, protecting the coast and helping prevent the erosion of barrier islands. Break one link in the chain — and everything falls apart.

Learn About the Problem
It is difficult to imagine just how tied together life is in a food chain or food web. An inability to understand this can lead to an inability to properly evaluate the importance of each individual link in that chain or piece of that web. You will find the following Logal Science Gateways and Science Explorer activities useful in teaching this subject to your students:

• Introduce students to food chains and webs with the Biology Gateways activity, Exploring Food Chains and Food Webs.

• Have students look at the vertical relationship in a food web using the Biology Gateways activity, Predator-Prey Relationships.

• Next have students explore a horizontal relationship in a food web in the Biology Gateways activity, Competition between Two Species.

• Finally, students can take a look at an entire ecosystem by simulating eutrophication and suggesting a lake management plan in the Biology Explorer activity, Maintaining a Balance.

• More advanced students can test their understanding in the Biology Explorer activity, Design an Ecosystem.