Poisons and invaders in food webs

In Saginaw Bay on Lake Huron, walleye at the top of the food web carried more than four times as much PCB as the phytoplankton at the bottom.

Why poisons pile up at the top

Energy shrinks as it moves up a food web, but some substances do the opposite. A persistent chemical is one that the body cannot easily break down or get rid of, so it stays in fat and tissue. A small fish eats thousands of plankton and keeps most of the chemical from all of them. A larger fish eats many small fish and keeps theirs. At each level the concentration rises. OpenStax calls this biomagnification: the increasing concentration of persistent, toxic substances at each trophic level, from the producers to the apex consumers.

This simplified biomass example assumes that the chemical is absorbed and retained; its food-to-body-mass ratio is hypothetical, not a direct conversion of the 10% energy-transfer rule. If a fish needs about ten kilograms of food to build one kilogram of body, and the chemical is not lost along the way, the chemical from ten kilograms of food ends up in one kilogram of fish. Repeat that over several levels and the top predator carries far more per kilogram than anything below it.

DDT, PCBs and mercury

The best-known case is the pesticide DDT, described in Rachel Carson's 1962 book Silent Spring. In some waters, birds at the top of the food web took in enough DDT through the fish they ate that their eggshells became thin and broke during nesting, and their numbers fell. OpenStax notes that DDT use was banned in the United States in the 1970s. PCBs, chemicals once used in coolant liquids, were banned in the United States in 1979, yet they still turn up in lake fish today because they last so long.

Heavy metals such as mercury, lead and cadmium also build up. OpenStax summarises advice from the US Environmental Protection Agency: pregnant people and young children should avoid swordfish, shark, king mackerel and tilefish, which are large predatory fish with high mercury levels, and choose lower-mercury fish such as salmon, tilapia, shrimp, pollock and catfish. Advice differs between countries and is updated, so for your own diet check your national food safety agency.

A new species in the web

Food webs can also be changed by adding a species. Invasive species are organisms brought outside their native range that upset the balance of their new home. OpenStax tells the story of Asian carp, brought to the United States in the 1970s to clean ponds by filtering plankton. Some escaped, and by the 1980s they lived in many waterways of the Mississippi River basin. Silver carp eat the plankton that native mussels and snails feed on, and black carp eat native mussels and snails directly. In some parts of the Illinois River, Asian carp make up 95% of the community's biomass.

An invader with no predators in its new home behaves like the released prey in the food web lab, except that it also competes with native species for food. Electric barriers on the Chicago Sanitary and Ship Canal are used to keep the carp out of the Great Lakes. The examples in this unit share one lesson: whatever enters a food web, whether a chemical or a species, moves along the same feeding links.