Ecosystems

An ecosystem is a community of living things together with the nonliving environment around them, such as the weather, water and landscape, all interacting in one area. A pond is an ecosystem. So is a rotting log, a coral reef or the Amazon rainforest. Ecosystems have no fixed size; what makes something an ecosystem is that its parts interact.

Those parts come in two kinds:

The two kinds constantly shape each other. Abiotic factors decide which organisms can survive, and organisms change their surroundings in return: tree roots break rock into soil, and the shade of a forest cools the ground beneath it.

Every organism needs energy, and almost all of that energy starts as sunlight. Producers, also called autotrophs, capture it. Plants, algae and some bacteria use photosynthesis to turn light, carbon dioxide and water into sugar. (A few producers, mostly bacteria living in dark caves or at hydrothermal vents on the ocean floor, use chemical energy instead. They are called chemoautotrophs.) Everything else is a consumer that gets energy by eating.

A food chain is a single path of who eats whom, like grass → grasshopper → mouse → snake → hawk. Real nature is messier: mice eat more than one thing, and hawks hunt more than one animal. Linking all the food chains in an ecosystem gives a food web.

Two groups clean up after everyone else. Detritivores like earthworms and millipedes eat dead material, and decomposers, mainly fungi and bacteria, break it down into simple chemicals. Without them, dead matter would pile up and the nutrients locked inside it would never return to the soil.

Each step in a food chain is a trophic level. Producers are the first level. Primary consumers (herbivores) eat producers. Secondary consumers eat herbivores, and tertiary consumers eat those. An animal at the very top with no natural predators of its own, like an orca or a hawk, is an apex predator. Many animals feed at more than one level; a fox that eats both berries and rabbits is a primary and a secondary consumer.

Here is the catch: energy does not pass up the chain completely. An animal uses most of the energy it eats just to stay alive, to move, grow, breathe and keep warm, and much of it ends up as heat. Some of what it eats is never digested at all. On average, only about 10% of the energy at one level is passed on to the next. This is the 10% rule.

That steep drop explains a lot. It is why food chains are short: after four to six energy transfers, there is not enough energy left to support another level. It is also why a meadow has far more rabbits than foxes.

Energy makes a one-way trip through an ecosystem: in as sunlight, out as heat. Matter is different. The atoms that make up living things are not used up; they are recycled. The carbon, nitrogen and water in your body have been part of other living things before you, maybe ocean plankton or an ancient forest. These atoms move between living things, air, water and rock in biogeochemical cycles.

Carbon is the backbone of every living molecule. In the fast carbon cycle, plants and algae pull carbon dioxide (CO₂) out of the air during photosynthesis and build it into sugars. Animals eat the plants, and both plants and animals release CO₂ back to the air through respiration. When organisms die, decomposers break them down and release more.

The ocean is the giant in the cycle: it holds about 50 times more carbon than the atmosphere. CO₂ dissolves into seawater and comes back out, and some of the dissolved carbon combines with calcium to form calcium carbonate, the main ingredient of many sea shells. In the slow carbon cycle, carbon gets locked away for millions of years in limestone rock and in fossil fuels (coal, oil and natural gas), which formed from ancient buried life.

People have short-circuited the slow cycle. Burning fossil fuels, clearing forests and making cement release carbon that was stored underground for ages. Natural sinks, mainly the ocean and plants, soak up about half of the CO₂ humans emit each year. The rest stays in the air.

Living things need nitrogen to make proteins and DNA. There's no shortage of it: nitrogen gas (N₂) makes up about 78% of the atmosphere. The problem is that plants and animals can't use N₂ directly. Its two atoms are bonded too tightly. Turning it into a usable form takes special help, and most of that help comes from bacteria.

In 1909 chemists worked out the Haber-Bosch process, a way to fix nitrogen in factories. It made large-scale synthetic fertilizer possible. But fertilizer that washes off fields carries extra nitrogen (and phosphorus) into rivers, lakes and coastal water, which leads to the dead zones described in Human impacts.

The sun's energy evaporates water from oceans, lakes and soil, and plants release water vapor from their leaves through transpiration (together these are called evapotranspiration). The vapor cools and condenses into clouds, then falls as precipitation. Gravity takes over: water runs off the land into streams and rivers (runoff), or soaks into the ground (infiltration) to become groundwater.

About 96.5% of all Earth's water is salty ocean water. Of the small share that is fresh, more than 68% is frozen in ice caps and glaciers and about 30% is underground. People change the cycle too: pumping groundwater, building dams, paving land so rain runs off instead of soaking in, and warming the climate.

A biome is a large region with a similar climate and similar kinds of plants and animals, wherever on Earth it appears. The two abiotic factors that matter most in defining land biomes are temperature and precipitation. Biologists usually recognize eight major terrestrial biomes. There are aquatic biomes too, divided into ocean (marine) and freshwater.

Ecosystems are not frozen in place. After a disturbance, communities rebuild in a fairly predictable series of stages called ecological succession.

Back to Tatoosh Island. Paine coined the term keystone species for a species that holds its ecosystem together, the way the keystone at the top of a stone arch keeps the whole arch from falling. Remove it, and the ecosystem changes dramatically.

A famous land example is the gray wolf in Yellowstone National Park. According to the National Park Service, at least 136 wolves were killed in the park between 1914 and 1926, and the last wolf pack was killed in 1926. Without wolves, elk populations exploded and overgrazed the plants along streams. From 1995 to 1997, 41 wild wolves from Canada and northwest Montana were released in the park. National Geographic reports that since then, elk populations have shrunk, willows have grown taller, and beaver and songbird populations have recovered. A chain of effects like this, running from a predator down through the food web, is called a trophic cascade.

Keystone species aren't always predators. Kelp is called a keystone host: kelp forests give shelter to sea otters and food to the animals otters eat, such as fish and sea urchins.

People are part of ecosystems too, and we now shape almost every one on Earth. Each big impact connects to an idea from this lesson:

The same science points toward solutions. Restoring a missing species, as Yellowstone did, can help an ecosystem recover. Farmers can plant legumes to add nitrogen to soil naturally and use fertilizer more carefully so less runs off. And protecting forests and wetlands keeps the carbon and water cycles working in our favor.