Food Webs and Ecosystems

What is an ecosystem?

An ecosystem is all the living things in one place, plus the nonliving things around them, working together. A pond, a rotting log, a kelp forest, and a mountain rainforest are all ecosystems.

The living parts (plants, animals, fungi, bacteria) are called biotic factors. The nonliving parts (sunlight, water, temperature, rocks, soil) are called abiotic factors.

Every living thing in an ecosystem needs energy to grow, move, and stay alive. The big question of this page is: where does that energy come from, and where does it go?

Earthworms, termites, and millipedes help too. They are detritivores: animals that eat dead material and break it into smaller pieces, which makes the decomposers' job easier.

A food chain shows one path that energy takes. For example:

grass → grasshopper → mouse → snake → hawk

Notice the arrows. They point from the food to the eater. Think of them as meaning "is eaten by" or "gives energy to."

Real animals rarely eat just one thing, though. A hawk eats snakes, but also mice and rabbits. When you connect many food chains together, you get a food web. Food webs are messier, and much more realistic.

Energy moves through an ecosystem in one direction: from the sun, to producers, to consumers. It does not get recycled the way nutrients do. Every ecosystem needs a steady new supply of energy, usually from sunlight.

But energy does not pass along neatly. When a rabbit eats grass, it does not get all of the energy the grass captured from sunlight. Most of it is lost along the way:

• Some of the food is never digested and passes out as waste.
• A lot of energy is used to stay alive (breathing, moving, keeping warm) and ends up as heat.
• Some plants and animals die without being eaten at all.

Ecologists often summarize this with the 10% rule: on average, only about 10 percent of the energy at one level of a food chain gets passed on to the next level.

Try it: if the grass in a field captures 10,000 units of energy, about how much reaches the hawk at the end of a five-step chain? Grasshopper ≈ 1,000, mouse ≈ 100, snake ≈ 10, hawk ≈ 1. Only about one unit out of 10,000!

So little energy reaches the top that most ecosystems can support only about four or five levels. There simply is not enough left over to feed another level of predators.

Each step in a food chain is called a trophic level (TROH-fik), from a Greek word for food. Here is how the levels line up in a kelp forest along the Pacific coast:

Some animals don't fit neatly into one level. An omnivore that eats both plants and animals sits on more than one level at once. That is one more reason scientists draw webs instead of chains.

In a stone arch, the keystone is the wedge-shaped stone at the very top. Pull it out and the whole arch falls down.

Ecologist Robert Paine borrowed that idea in the 1960s, after studying sea stars on the rocky coast of Washington State. A keystone species is one that has a much bigger effect on its ecosystem than you would expect from how many of them there are. Remove it, and the ecosystem changes dramatically.

Keystone species are not always predators. Beavers build dams that create whole wetlands, and bees pollinate the plants many other animals depend on.

"Keystone" is not an official label, and scientists still debate which species count. Deciding takes evidence, which brings us to the most famous example of all.

Meet the players

Kelp is a large brown alga (AL-guh), not a plant, even though it looks like one. Kelp forests grow in cool, shallow coastal water and shelter thousands of kinds of animals.

Sea urchins are spiny, round animals that graze on kelp. Sea otters eat sea urchins, along with clams, crabs, and snails.

What went wrong

Before the fur trade, somewhere between 150,000 and 300,000 sea otters lived along the North Pacific coast. Hunters took them for their fur for about 170 years, starting in 1741. By 1911, when an international treaty finally protected them, fewer than 2,000 were left, scattered in a few small groups.

Without otters, sea urchin populations exploded. The urchins chewed through the kelp, leaving bare, rocky areas called urchin barrens. The fish, snails, and other animals that lived in the kelp lost their home.

The comeback

Protected by law, sea otters slowly returned to many areas. In California, a few dozen survivors near Big Sur grew into a population of around 3,000 southern sea otters. Where otters came back, they ate the urchins, and kelp recovered surprisingly quickly.

That is what makes the sea otter a textbook keystone species. Otters aren't the most numerous animal in the kelp forest, but they hold the whole system together.

The story isn't over

Since 2013, a disease called sea star wasting disease has killed huge numbers of sunflower sea stars, which also eat urchins. With fewer predators, urchins boomed again in some places. Along with a marine heatwave that left the water warm and low in nutrients, this helped wipe out more than 90 percent of the kelp forests along parts of northern California. Ecosystems keep changing, and scientists keep watching.

Food webs are everywhere, including high on the slopes of the Virunga Mountains, a chain of volcanoes where the Democratic Republic of the Congo, Rwanda, and Uganda meet. Mountain gorillas live there, at 8,000 to 13,000 feet, on the slopes of volcanoes that are dormant (quiet for a very long time). Two others in the chain, Nyiragongo and Nyamuragira, are still active. It is so cold that temperatures often drop below freezing, and the gorillas have extra-thick fur.

Mountain gorillas are huge, but in the food web they are primary consumers. Scientists who followed them found they eat almost entirely leaves, stems, and shoots of wild herbs and vines.

In 1967 the zoologist Dian Fossey founded the Karisoke Research Center in Rwanda's part of the Virungas. (She named it by joining the names of two volcanoes, Karisimbi and Bisoke.) Researchers there have studied the gorillas ever since.

Mountain gorillas were once feared to be headed for extinction. The most recent complete surveys counted at least 1,063 of them across the Virungas and Bwindi (a forest in Uganda), and they are the only great apes whose numbers are growing. In 2018 their status improved from Critically Endangered to Endangered. A newer count is still being analyzed.

How do scientists know otters protect kelp, instead of just guessing? They gathered evidence in several ways.

1. Compare places. In a study published in 1974, James Estes and John Palmisano compared islands in Alaska's Aleutian chain. Around Amchitka Island, where sea otters were common, kelp was thick and urchins were kept in check. Around Shemya Island, where there were no otters, urchins were everywhere and kelp was scarce. Same ocean, one big difference.
2. Watch what happens over time. When otters returned to places they had disappeared from, scientists recorded the kelp growing back. A cause that comes and goes along with its effect is strong evidence.
3. Watch what animals actually eat. Field biologists follow animals and record every meal. That is how we know mountain gorillas eat mostly leafy plants, and what sea otters pull up from the sea floor.
4. Measure the energy. Ecologists have measured how much energy each level of real ecosystems captures, like the Silver Springs study. Those measurements are where the "about 10 percent" average comes from, and why we know it is only an average.

Notice that no single study proves everything. Scientists trust an idea more when different kinds of evidence point to the same answer.

Seven links connect these seven living things. Can you find all of them? Pick a food, then the eater that eats it, then press Draw the arrow. Remember: arrows point from food to eater.

How scoring works: +10 points for each correct link, −3 for each wrong or backwards guess (your score never drops below 0). A perfect game is 70. Your best finished score is saved in this browser only.