Food webs & trophic cascades: It’s more than just who eats whom.
Most of us are familiar with the concept of a food chain. Feeding relationships, however, are rarely as simplistic as a chain and are more accurately represented by a food web. A food web is a more realistic depiction of feeding relationships and is much more complex showing all possible energy paths (food chains) for organisms in an ecosystem. Food webs include the familiar terms producer, consumer or decomposer. Decomposers are organisms that break down organic material converting it to inorganic matter and include fungi and bacteria. Producers are organisms that are able to make their own food for energy like plants or chemosynthetic bacteria. Finally, consumers are organisms that must consume other organisms for food. A simple terrestrial food chain would be as follows:
These levels of a food chain, such as producer or consumer, are known as trophic levels. Certain trophic levels, like consumer, may also have multiple sublevels within it such as primary, secondary and tertiary. For example, below is a coastal aquatic food chain with consumer sublevels:
Terms may vary for sublevels; for example, a secondary consumer may also be called a mid-level consumer or a tertiary consumer may be called a higher-order consumer, but the meaning is the same. Another important consumer term is an apex predator. An apex predator is the top predator in a food chain and that has no other predators. A tiger shark, for example, is considered an apex predator.
Although a food web does not tell us the number of each organism present (i.e. its population), it does provide the basis for understanding various relationships between organisms. If one species declines or is removed, due to mortality, disease, fishing, etc., the removal will cause changes in trophic levels above or below the altered level, causing the food web to change. This can lead to an increase in its prey or a decrease in its predator. Since these changes causes effects elsewhere in the food web, they are often called trophic cascades. For example, in marine ecosystems, removal of apex predators such as sharks, through overfishing has had cascading effects on the scallop fishery. Some shark species feed on rays and when shark numbers decline there is increased consumption by rays on scallops. This illustration is an example of top-down control on a food web. Bottom-up control also occurs in marine and terrestrial ecosystems where a change at the primary producer level (e.g. increase in nutrients) cascades up the food web.
There are several techniques scientists use to construct a food web and determine trophic levels. One method is to examine gut contents. An organism is collected and the contents of its stomach analyzed to determine its diet. Another method scientists use is stable isotope analysis. We will talk more about this method later, but briefly tissue samples are analyzed to determine the relative abundance of specific isotopes. Isotopes, forms of a chemical element that differ slightly in molecular weight, behave differently as they are transferred through a food web. This difference allows scientists to determine the trophic level without conducting laborious and often inconclusive gut contents analysis. Another advantage to using stable isotope analysis over gut contents is that the whole organism does not have to be collected and sacrificed.
ACER scientists in the consumer group are investigating impacts of the oil spill on food webs and trophic cascades. Research is focusing on how top-down control from mid-level and higher-order consumers affected juvenile fishes and crustaceans in inshore and offshore waters.