Changing a Food Chain: Ripple Effects and Biomagnification

Pull one thread

Imagine a knitted sweater. Pull one loose thread, and a whole row can come undone. Ecosystems can be like that. Removing one species, or adding a new one, can change many others, sometimes in surprising ways.

In the last lesson, you saw how energy moves up a food chain. Now you'll see what happens when a link in the chain changes.

Predicting the effects of a change

Use this chain: grass → hares → lynx.

• Remove the producer (grass, after a drought): hares have less food, so many starve or move away. Then lynx have less food, and their numbers fall too. A change at the bottom affects every level above it.
• Remove the primary consumer (hares, after a disease): grass grows more, because nothing is eating it. Lynx lose their food and decline or switch to other prey.
• Remove the top predator (lynx, by trapping): hares increase quickly. With too many hares, they overeat the plants, then many hares starve. The plants may take years to recover.

Real ecosystems are food webs, not single chains, so animals can often switch to other foods. The more connections, the smaller the ripple (you saw this as resilience in Grade 6).

A real trophic cascade: sea otters on the Pacific coast

Off the coast of British Columbia:

kelp (a large seaweed) → sea urchins → sea otters

• Sea otters eat sea urchins. Sea urchins eat kelp.
• Starting in the late 1700s, sea otters were hunted for their fur until, by the early 1900s, they had disappeared from British Columbia's coast.
• Without otters, sea urchin numbers grew. The urchins ate the kelp forests down to bare rock in many places. Fish and other animals that lived in the kelp lost their homes.
• In the late 1960s and early 1970s, sea otters were brought back from Alaska to the west coast of Vancouver Island. Where otters returned, urchins decreased, and kelp forests grew back.

A change at the top of the chain cascaded down to the producers. Sea otters are called a keystone species.

Coastal First Nations, whose ancestors harvested both sea otters and shellfish, have shared knowledge about how otters affect shellfish such as clams and urchins. The otters' return brought back kelp, but also meant competition for shellfish that are important foods for these communities. Scientists and First Nations work together to understand these changes.

Adding a species: the sea lamprey

Adding a species can change a food chain too. The sea lamprey is an eel-like fish that attaches to other fish with a sucking mouth full of teeth and feeds on their blood and body fluids. It is not native to the upper Great Lakes. It spread there through shipping canals in the early 1900s.

In the mid-1900s, sea lampreys helped cause a huge drop in lake trout, a top predator fish in the Great Lakes. Canada and the United States now control lampreys together, using barriers on streams, traps and a chemical (a lampricide) that targets young lampreys in the streams where they hatch. Lake trout numbers have recovered in some areas.

When poisons climb the food chain

Some toxic substances, like the pesticide DDT, the metal mercury and chemicals called PCBs, are persistent: they break down very slowly. They also tend to stay inside an animal's body (often in its fat) instead of being passed out in wastes.

• Bioaccumulation: an organism keeps taking in a little bit of the substance every day, and it builds up in its body over its life.
• Biomagnification: each predator eats many prey, and gets the substance from all of them. So the concentration gets higher at each trophic level. Top predators end up with the most.

Remember the energy pyramid? Energy is lost at each level, but these toxins are kept. That's why they pile up at the top.

Real examples from Canada

• DDT and birds of prey: DDT was sprayed to kill insects in the mid-1900s. It biomagnified up food chains to birds such as bald eagles, ospreys and peregrine falcons. It made their eggshells so thin that they broke when the parents sat on them, and these bird populations fell sharply. After Canada and other countries restricted DDT in the 1970s, these birds slowly recovered.
• Mercury in fish: mercury can enter lakes from industry, from burning coal, and from flooded soils behind new dams. Bacteria change it into a form that biomagnifies. Large, older predatory fish like walleye and pike can have the most. Ontario publishes a Guide to Eating Ontario Fish that tells people how much fish from each lake is safe to eat.
• Grassy Narrows and Wabaseemoong: in the 1960s, a chemical plant connected to the paper mill in Dryden, Ontario, released mercury into the English–Wabigoon River system. The mercury built up in fish, an important food and source of jobs for the Anishinaabe communities of Grassy Narrows First Nation and Wabaseemoong Independent Nations. Many people became ill. These communities continue to work today for clean-up, health care and justice.

Modelling with code

Scientists use computer models to test predictions. Here is a plan for a simple program that models the 10% rule and the effect of a smaller producer level. The program asks for the producers' energy, then uses a loop to pass 10% to each level.