Food Webs Show How Ecosystems Connect

Most food webs begin with producers such as plants and algae. They capture light energy through photosynthesis and store it in sugars. Herbivores gain some of that energy by eating producers, and predators gain some by eating other animals.

At every step, organisms use much of the energy for movement, growth, repair and keeping their bodies working. Some energy leaves as heat, so less is available to the next feeding level. This is why ecosystems usually support many producers, fewer herbivores and still fewer top predators.

Decomposers such as fungi and bacteria have a different but essential role. They break down dead material and waste, returning nutrients to soil and water. Energy does not cycle back in the same way, but matter does. New plants can use those recycled nutrients, so decomposition keeps the web supplied with raw materials.

A rabbit may eat grasses, clover and young shoots. A fox may eat rabbits, rodents, birds or insects. Draw all those links and the single chain quickly becomes a network.

The arrows in a food web usually show the direction of energy transfer, from the organism being eaten towards the eater. This can feel backwards at first, so ask where the energy is going. Grass to rabbit means the rabbit receives energy stored by the grass.

Food webs can also include omnivores, which feed at more than one level. A bear might eat berries, insects and fish, so it does not fit neatly into one box. These flexible diets are one reason feeding levels are useful models rather than rigid labels. Nature often crosses the lines we draw to understand it.

Because species are connected, a change in one population can affect many others. If a disease reduces rabbits, foxes may switch to other prey, while some plants may face less grazing. The final result depends on the other links in the web.

More connections can sometimes make an ecosystem resilient because consumers have alternative food sources. But not every species is easily replaced. A species with an unusually large effect is sometimes called a keystone species. To read a web calmly:

• •Find the producers first.
• •Follow arrows in the direction of energy.
• •Notice organisms with many connections.
• •Predict more than one result from a population change.

The clearest demonstration that a web is connected came from Yellowstone National Park. Wolves were gone by 1926, elk numbers grew, and young willow and aspen along the rivers were grazed down before they could mature. After wolves were reintroduced in 1995, elk began avoiding the exposed valley bottoms, the trees recovered, beavers returned to build dams in the regrown willow, and the ponds behind those dams brought back fish, amphibians and songbirds.

Ecologists call a chain of effects running down through the levels like this a trophic cascade, and while later studies have argued about how much of the change the wolves caused, the direction is not in doubt. The same pattern appears in the sea, where sea otters keep urchins in check and so protect kelp forests, and in reverse whenever a top predator is lost. Drawing the web before and after the change is the quickest way to predict which arrows will thicken and which will fade.

Food webs carry more than energy. Some substances, such as the pesticide DDT and certain forms of mercury, dissolve in fat rather than water, so animals store them instead of excreting them. A small fish eats many tiny organisms, each containing a little of the chemical. A larger fish eats many small fish, and a predatory bird eats many large fish. At each step the concentration in body tissue rises, a process called biomagnification, so top predators can carry levels far higher than those found in the water.

DDT showed the danger in a striking way. After it was widely sprayed in the mid twentieth century, birds of prey such as peregrine falcons, ospreys and bald eagles laid eggs with thin shells that cracked under the parents' weight, and their numbers fell sharply in many regions. Many countries restricted or banned DDT from the 1970s, and populations of these birds later recovered. The same idea explains why health advice suggests limiting how much large predatory fish, such as swordfish or some kinds of tuna, people eat.

Ecologists often use a rough guide called the ten per cent rule: on average, only around a tenth of the energy stored at one feeding level ends up stored at the next, although real values range from a few per cent to about twenty. Suppose the plants in a meadow store 10,000 kilojoules of energy over a season. The rabbits and other herbivores that eat them might store about 1,000 kilojoules in their bodies, the foxes and owls that eat the herbivores about 100, and any predator feeding on those hunters only about 10.

These numbers explain why food chains are short, usually with no more than four or five levels. After a few steps there is too little energy left to support a population of large animals that must roam widely to find enough food. The same arithmetic applies to people. A field of grain eaten directly feeds far more people than the same grain fed to cattle and eaten as beef, because the animals use most of the grain's energy for living, moving and keeping warm rather than storing it as meat.

Drawing a food web sounds simple until you try to fill in the arrows. Watching animals feed works for some species, but many eat at night, underwater or in places people rarely see. Ecologists therefore examine what animals leave behind. Barn owls cough up pellets containing the fur and bones of the small mammals they swallowed, so a single pellet can show how many voles, mice and shrews an owl ate. Stomach contents and droppings serve the same purpose for other animals. The catch is that soft-bodied prey digests quickly and leaves few traces, so these methods can underestimate it.

Chemistry adds a longer view. Nitrogen comes in a heavier and a lighter form, and animals retain slightly more of the heavier one than their food contains, so each step up a food chain raises its share by a fairly predictable amount. Measuring it in a sample of muscle, feather or hair reveals roughly which feeding level an animal occupies, averaged over weeks or months rather than one meal. Carbon forms can show whether the energy came originally from land plants or marine algae. Today, DNA in droppings can also be read to list prey species directly, including ones too small or too digested to recognise.

Food webs reveal that ecosystems are networks, not tidy ladders. Follow the flow of energy, look for multiple connections and remember that one change can travel through many species.