Untangling a Food Web

Environmental & Science Education, STEM,Water, Watersheds, Biodiversity, Invasive Species

Ed Hessler

Somewhere in our education, grade school to high school to college, we encounter food chains and food webs often captured in lovely, stylized illustrations. Their purpose is to help us understand text descriptions on the passage of food through animal communities from their various plant bases until all are broken down by scavengers - saprophytic chains - and the much simplified constituents,the  inorganic residues are returned to the physical environment.

The study of food webs and food chains is still active and the methods various. In a recent Aquatic Sciences Chronicle (ASC), Issue 3, 2021, one of these, using immunochemistry. It is not a new technique in ecology with one of the investigators, John Berges (UW-Milwaukee) noting that the technique "is really the first time it's been used systematically in fresh waters."

The technique involves creating specific antibodies and the perfect incubators are New Zealand White rabbits. Suspected food of the prey, in this case, of spiny waterfleas, are ground up, homogenized, and a small sample is injected into the rabbit. This is followed by a 6-12-week period of waiting for the antibodies to be produced. Berges notes that after this period "you have a huge array...of antitbodies which now recognize the proteins that are in that ...potential prey item, that you injected into it."

This is followed by a blood draw and the antibody fraction of the blood (known as immunoglobulin, IgG.) to which is added to a  soup of spiny waterfleas. The IgG binds to the predators proteins and is removed. The leftover, now refined  chemicals, will react with prey soups and not the predator.

The research team investigated 12 different suspected prey. Some of these were negative and some positive. However, there were two surprises: 1) evidence that spiny waterfleas were "consuming the larvae of invasive Dreissenid mussels (aka Zebra mussels or quagga mussels). However, this flummoxed the investigators. Why would a large predator 9the spiny waterflea eating such tiny bites at a time. The authors think that "the spiny waterflea ate somethjng that itself first ate the larval Dressenid mussel. 

The second, involved a change in scale.A large zooplankton, a copeopod, Limnoclanus macrurus - to use a technical term, one of the "'big dogs in the lake.'"

Then you check "to determine if any of the markers from the rabbit blood overlap with the predator, the spiny waterflea. First a soup is made of waterfleas. Here, the investigators reasoned that the spiny waterflea was eating the juvenile and larval forms of the "big dog."

This is a great story about some of the complexity of food chains and food webs and make me appreciate the natural world all the more. And nature is a demanding subject of investigation, not always letting go of what is secret and unknown to us.

Advantages of the technique include its low cost and that it can be done in most research labs since it doesn't require highly specialized techniques, methods or expensive instruments.

You may read an abstract of the original paper in Limnology and Oceanography Methods. The oomplete paper is behind a paywall.