Ecosystems and Food Webs

Watch a meadow for an hour and you might see a bee on a clover flower, a spider in the grass, a swallow swooping overhead. It looks like a collection of separate creatures going about separate lives. It is not. The clover feeds the bee and is pollinated by it; the spider eats flies that fed on the clover; the swallow eats the bee. Remove any one and the others change. Ecology is the study of those connections, and the unit it studies is the ecosystem.

What an ecosystem is

An ecosystem is a community of living things together with the non-living environment they share: the soil, water, air, sunlight and climate. It can be as small as a rotting log or as large as the Amazon basin. Within it, every organism has a role. Plants, algae and some bacteria are the producers, capturing energy from the Sun and building it into sugar. Everything else is a consumer, getting its energy by eating producers or eating things that ate them. Fungi and bacteria are the decomposers, breaking down the dead and returning their nutrients to the soil, where plants take them up again. Energy flows through an ecosystem once, from Sun to plant to animal to heat. Matter goes round and round.

Chains and webs

A food chain is the simple version: grass is eaten by a rabbit, which is eaten by a fox. Real ecosystems are webs, not chains. The fox also eats mice, beetles and berries; the rabbit is also eaten by owls and stoats; the grass is grazed by insects as well. A food web diagram for even a small pond can have hundreds of links, and that complexity is a strength. If one food source fails, most animals have others.

A rule of thumb governs the whole structure. Only about a tenth of the energy stored at one level of a food chain makes it into the bodies of the next; the rest is spent on living, moving and keeping warm, or is lost as waste. A field can feed thousands of grasshoppers, which feed hundreds of mice, which feed a few owls. This is why plants are everywhere and tigers are rare, and why a food chain almost never has more than four or five links. There is simply not enough energy left at the top to support another.

Keystones

Some species matter far more than their numbers suggest. Sea otters eat sea urchins; when otters were hunted to near extinction along the Pacific coast, urchins multiplied and devoured the kelp forests, and the fish, seals and birds that lived in the kelp vanished with it. When otters returned, the kelp came back. Such species are called keystones, after the stone at the top of an arch that holds all the others in place.

The most famous example is Yellowstone National Park. Wolves were exterminated there by 1926. With no predators, elk grazed the valley bottoms bare, young willows and aspens never grew, and the beavers, songbirds and fish that depended on riverside trees declined. In 1995 wolves were reintroduced. Elk numbers fell and, as importantly, elk stopped lingering in open valleys where they were vulnerable. Trees regrew along the rivers, beavers returned and built dams, the dams created ponds for fish and amphibians, and the stabilised banks changed the rivers' very shape. One predator rearranged a landscape.

Biodiversity

The number and variety of species in an ecosystem is its biodiversity, and it is not evenly spread. Tropical rainforests cover about six percent of the land yet hold more than half of all known species; a single tree in the Amazon can host more ant species than the whole of Britain. Coral reefs are the marine equivalent. Diverse ecosystems tend to be more stable, because many species doing overlapping jobs means no single loss is catastrophic.

Scientists have named about two million species but estimate the true total at nearly nine million, most of them insects, fungi and microbes yet to be found. Many will go extinct before they are described. Habitat destruction, especially the clearing of tropical forest, is the main cause; climate change, hunting, pollution and invasive species carried around the world by people add to it.

Why it matters

Human life runs on ecosystems. Forests and plankton make the oxygen; wetlands clean the water; insects pollinate three quarters of our crops; soil organisms grow the food; mangroves and reefs blunt the storms. None of these services is sent a bill. Ecology's central lesson, learned again and again from otters, wolves and vanished forests, is that everything is connected, and that pulling one thread can unravel far more than anyone expected.