Scoop up a handful of soil from a forest floor and you are holding a crowded city. Fungal threads run through it, bacteria number in the billions in every gram, and mites and worms graze and hunt among the crumbs. Each creature depends on others and on the moisture, warmth and chemistry of the soil itself. Ecology is the science that tries to make sense of webs like this one.

A science of relationships

The German zoologist Ernst Haeckel coined the word in 1866, building it from the Greek oikos, meaning house or household. In his usage, and in ours, ecology is the study of how organisms interact with one another and with their physical surroundings. It asks why a species lives where it does and why its numbers rise and fall.

Ecology is often confused with environmentalism, but the two differ. Ecology is a science that describes and explains; environmentalism is a set of values and political goals. An ecologist can tell you what will probably happen to a river's fish if a dam is built. Whether the dam should be built is also a question of values, which ecology alone cannot settle. John Muir, in 1911, caught the spirit of the science: "When we try to pick out anything by itself, we find it hitched to everything else in the Universe." Ecologists spend their careers working out which of those hitches matter most.

Levels of organization

Ecologists study nature at several scales, each with its own questions. An organism is a single living thing, such as one oak tree. A population is a group of individuals of the same species living in the same area, such as all the oaks in a valley. A community is all the populations of different species that live and interact in an area: the oaks, squirrels, jays, beetles and fungi together. An ecosystem is a community together with its nonliving surroundings, including soil, water, sunlight and climate. The biosphere is the sum of all ecosystems on Earth, the thin film of the planet where life exists.

Living and nonliving factors

Everything that affects an organism falls into two groups. Biotic factors are living influences such as predators, competitors, parasites and food. Abiotic factors are nonliving ones such as temperature, rainfall, light, salinity and soil chemistry. In the 1840s the German chemist Justus von Liebig, studying crop growth, popularized what became known as the law of the minimum: growth is limited by whichever essential resource is scarcest, not by the total supply of all resources. A plant with plenty of water and nitrogen will still be stunted if it lacks phosphorus. Around 1911 the American ecologist Victor Shelford added a refinement, the law of tolerance: too much of a factor can limit a species just as too little can.

One of the clearest tests of limiting factors came from the Experimental Lakes Area in Ontario. In the early 1970s the Canadian scientist David Schindler and his colleagues divided a small lake, Lake 226, with a plastic curtain. Both halves received carbon and nitrogen, but only one half also received phosphorus. By late summer the half that received phosphorus had turned green with algae, while the other stayed clear. Photographs of the two-toned lake helped persuade governments to limit phosphates in detergents.

Habitat and niche

A species' habitat is the place where it lives. Its niche is its role and way of life there: what it eats, when it is active, what conditions it tolerates and what eats it. A common teaching line puts it simply: the habitat is an address, the niche is a profession. In 1957 the ecologist G. Evelyn Hutchinson gave the idea a sharper form, defining the niche as the full range of conditions and resources within which a species can survive and reproduce. He distinguished the fundamental niche, which a species could occupy in principle, from the realized niche, the smaller portion it actually occupies once competitors and predators push it out of some places.

How ecologists know

Ecologists use field observation, experiments, long-term monitoring and models. Observations reveal patterns but rarely prove causes. Experiments establish causes but usually at small scales, in a plot or a tank, which may not reflect a whole landscape. Because so many variables interact, ecological findings often depend on context, and a result from one place may not hold in another. That is why some of the field's most famous stories, as later levels show, remain under honest debate.

Worked example: a farm pond

Consider a pond on a farm. One bluegill sunfish is an organism; all the bluegills form a population; the bluegills, bass, frogs, algae, dragonfly larvae and bacteria together form a community; add the water, dissolved oxygen, sunlight and mud and you have an ecosystem. Now fertilizer washes in after a storm. Phosphorus, which had been the scarcest nutrient, is suddenly abundant, so the limit on algae lifts and the water turns green, just as it did in Lake 226. When the algae die, bacteria decompose them and use up the dissolved oxygen, and fish begin to suffocate. Nothing poisoned the fish directly: a change in one abiotic factor rippled through every level of the system.