A herd of zebras at Serengeti National Park.
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From Grolier's New Book of Knowledge
In the food web, energy flows through an ecosystem from one level to the next as food is consumed.
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Eric Isselée/iStockphoto
In a continuous cycle, water is evaporated from surface bodies into the atmosphere, only to be returned to the land as rain or snow, which ultimately finds its way back to lakes, streams, and the ocean again.
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Eric Isselée/iStockphoto

Some 93 million miles (150 million kilometers) away from Earth, in the heart of the star we call the Sun, 4 million tons of hydrogen fuse to form helium each and every second. A tiny fraction of the energy from this colossal nuclear reaction reaches our planet in the form of ultraviolet radiation. And a tiny subfraction of this falls on a sprout of grass in a patch of dirt wet by yesterday's rain. As the days pass, the sprout's pale, tender blades flush green, lengthen, and expand . . . before becoming lunch for a ravenous grasshopper. Within moments, the grasshopper is itself devoured by a garter snake, which falls victim to a keen-eyed hawk. When the hawk dies, years later, its body quickly becomes a metropolis of blowflies, flesh flies, carrion beetles, and mites. Meanwhile, mineral-rich body fluids from its moldering carcass seep into the soil. There they eventually nurture a new flush of green growth, coaxed forth by the continuing cycles of sunlight and rain.


In this simple scenario, we see energy and nutrients move through a series of living and nonliving elements. Biologists call such a dynamic system an ecosystem. The largest known to science is that of our entire planet, or biosphere. More practical for scientific study are the many more-or-less self-contained ecosystems that make up the larger biosphere: ponds, wooded groves, deep-sea vents, mountaintops, coral reefs, and sun-drenched fields. Even a carefully assembled terrarium can form such a fine-tuned, self-maintaining system.

When studying an ecosystem, biologists attempt to track the movement of energy and nutrients that make its continued existence possible. At first blush, these two factors—energy and nutrients—may seem synonymous. In fact, they represent two closely related but fundamentally different elements. They follow different courses and obey very different rules as they move through an ecosystem.


All activities associated with life, from the transport of a molecule across a cell membrane to the leaping of a playful dolphin, demand energy. The singular energy source for virtually all ecosystems on this planet is the Sun. The only known exceptions consist of isolated underwater pockets filled with unusual bacteria able to take energy directly from the atomic bonds of inorganic chemicals.

Earth receives a mere one-thousandth of one-millionth of the energy generated by the fiery nuclear-fusion reactor of our local star. Nonetheless, this tiny fraction amounts to about 170 trillion kilowatts—or 2 calories of radiant energy per minute for every square centimeter of Earth's surface. Some 30 percent of this energy reflects back into space after bouncing off clouds and dust in the atmosphere. Another 46 percent returns to space as heat after being absorbed and reradiated by Earth's surface. Still another 23 percent drives the evaporation of water that returns to Earth as precipitation.

Just 1 percent of the Sun's energy output to Earth actually enters the living portion of the planet's ecosystems. It does so by falling on the pigmented tissues of photosynthetic organisms such as plants on land, and algae (seaweed and phytoplankton) in aquatic ecosystems. Still, this infinitesimal fraction of the Sun's energy drives the production of an estimated 150 billion to 200 billion tons of organic matter. (The raw materials from which this living tissue is created consist of nutrients. They include small molecules of carbon, oxygen, water, and minerals.)

The Energy Relay. Solar energy captured through photosynthesis becomes available for use by the photosynthetic organism, its predators, and its predators' predators. In this way, energy flows through the living parts, or communities, of an ecosystem from one feeding, or trophic, level to the next. The first level is that of a producer—the photosynthetic plant or alga. All higher levels fall under the category of consumer.

On the first consumer level, we find herbivores, animals that eat plants. Each type of ecosystem has its particular array of herbivores. Blue whales, caterpillars, and cottontail rabbits all fill this same niche within their respective ecosystems. They share the primary consumer role with plant parasites, such as the fungi and microorganisms. Such parasites draw energy and nutrients from plant tissues without completely or immediately killing them.

Carnivores, or secondary consumers, are animals that eat other animals. This second level of consumer includes carnivores that eat herbivores, and those that also eat other carnivores. The hawk that eats a snake that has eaten a grasshopper is such a carnivore. Ecologists call this consumer a tertiary, or third-level, consumer. Straddling multiple consumer levels are omnivores. These include animals such as humans, that eat a variety of both plant and animal foods.

Decomposers, organisms such as bacteria and fungi, break down the remains of producers and consumers. They extract energy and release nutrients back into the ecosystem. Each stage of energy transfer involves some loss of energy. This fact is a basic law of thermodynamics. It applies to both living and mechanical systems. For example: when an insect chews a blade of grass, it obtains only a fraction of the solar energy originally stored inside the plant's tissues. The grass used some of the energy to drive its own growth and repair. It lost still more energy in the form of heat with each biochemical reaction. Nor could the insect extract all of the energy stored in the grass blade. Moreover, the insect had to expend energy in order to find, chew, and digest its food.

A similar "waste" of energy occurs at each trophic level, so that all the levels stacked on top of each other form an energy pyramid. At the bottom of this pyramid is a large biomass of producers—be it 1,000 grass plants, or the equivalent in the form of one massive tree. Studies of many different kinds of ecosystems suggest that roughly 10 percent of the energy stored in any level gets passed on in usable form to the next. In effect, for every 1,000 calories captured by an ecosystem's producers, 100 calories reach its primary consumers. Ten calories reach its secondary consumers. A single calorie is left for a tertiary consumer.

Ecologists use this "10-percent law" to track energy loss through an ecosystem. They can sometimes estimate how much habitat a particular species, say a top predator, needs to survive. For example, scientists might calculate how many acres of grassland are needed to support enough insects to support enough snakes to support one pair of hawks and their young.

Food Chains and Webs. Of course, the paths that food energy can take through an ecosystem are many and complex. A food chain describes how one morsel of food might "travel" through the system. A one-celled alga may be eaten by a tadpole. The tadpole in turn becomes food for a minnow. The minnow is eaten by a bass. The bass is eaten by a bear.

In nature, however, the transfer of food energy seldom travels such a straight line. A food web shows the many interrelated paths possible. For example, the algae floating on a pond may be eaten by a tadpole, or by one of many kinds of aquatic insects, crustaceans, or small fish. Any or all of these may be eaten by a bigger fish, or by a bullfrog, snake, duck, raccoon, and so on. These in turn may become hosts to a variety of parasites, and/or be eaten by still-larger animals, or become food for scavengers and decomposers. Omnivores, which feed at various levels, further complicate a food web. Consider the tangle that results when a carnivorous plant digests an insect. Such plants serve as both producers and consumers in the same ecosystem!

The trophic level of an organism is the number of steps it is removed from the ecosystem’s primary energy source. Many organisms cannot be assigned to just one trophic level. For example, mammals, such as foxes and humans, are omnivores. They eat both plants and other animals.

Detritus feeders include such familiar scavengers as vultures and catfish. But far more widespread are the smaller but ever-present insects, mites, earthworms, and nematodes that consume everything from animal waste and fallen leaves to dead bodies.

Decomposers include a variety of fungi, bacteria, and filter-feeding microorganisms. They break down dead organic matter by releasing digestive chemicals and absorbing the resulting ultrasmall food particles. They leave behind the raw nutrients (carbon, oxygen, and minerals) needed to start the cycle of life anew.

Because usable energy is lost in the form of heat at each consumer level, new energy must be constantly added for the ecosystem to continue to function. Fortunately, astronomers predict we have another 5 billion to 7 billion years of uninterrupted solar energy from our aging Sun.


Ecosystems do require a continuous supply of new energy. But they prove very efficient at recycling nutrients. Indeed, life on Earth has been reusing the same reservoir of nutrients for more than 3 billion years. In the biological sense, nutrients include the building blocks of life—the actual elements and small inorganic molecules out of which organisms assemble themselves. The term macronutrients describes those chemicals that organisms need in large quantities. These include oxygen, carbon, hydrogen, nitrogen, phosphorus, and water (H2O), and lesser amounts of potassium, sodium, sulfur, calcium, and magnesium. Micronutrients—needed only in trace amounts—include a variety of minerals such as iron, zinc, iodine, cobalt, and selenium.

Biologists refer to nutrient-recycling paths as biogeochemical cycles because they pass through both the biological, or living, parts of an ecosystem and its geologic, or nonliving, parts. The biological parts include an ecosystem's producers, consumers, and decomposers. Its geologic parts include the atmosphere, with its gases and suspended particles of water and dust; the lithosphere, or solid ground and rock; and the hydrosphere, which includes bodies of water such as oceans, lakes, rivers, and groundwater.

The major storage sites and sources of a given nutrient are called its reservoirs. Such reservoirs tend to be nonliving, or geologic. Most carbon, for example, exists in the form of gaseous carbon dioxide (CO2) mixed in the atmosphere and dissolved in the oceans, or in the form of fossil fuels buried underground.

The Water Cycle. The hydrologic, or water, cycle is among the simplest. Most water remains chemically unchanged as it moves through an ecosystem. More than 97 percent of the world's available water can be found in the enormous reservoir of the oceans. Oceans cover three-fourths of Earth's surface. The solar energy that drives photosynthesis also drives the water cycle by evaporating liquid water from the surfaces of oceans, lakes, and rivers. The evaporated water enters the atmosphere until gravity brings it down again in the form of rain, snow, sleet, or dew. Most such precipitation falls back into the oceans. That which falls on land can take a variety of paths. Heat evaporates some back into the atmosphere. Some runs off the surface. It forms trickles, streams, and rivers that make their way back to an ocean, lake, or underground reservoir. And some enters living systems, for example, by being absorbed by the roots of a plant or lapped up by a thirsty animal. On average, living things on this planet consist of about 70 percent water.

Producers such as plants and algae destroy some water during photosynthesis. (Photosynthesis involves the combination of water and carbon dioxide to produce energy-rich organic molecules.) Virtually all organisms also create small amounts of water when they respire, or "burn" organic molecules for energy. In the process, oxygen (O2) and hydrogen (H2) recombine to form water (H2O).

Together, all of these processes return roughly the same amount of water to the world's oceans and lakes as they draw from them.

The Carbon Cycle. Carbon makes up the bulk of the organic molecules found in living tissues. Carbon first enters living systems through photosynthesis. As mentioned above, photosynthesis chemically combines carbon dioxide (CO2) and water to produce high-energy organic molecules. Plants draw their CO2 from the air, where it makes up roughly 0.04 percent of all atmospheric gases. Algae and photosynthetic diatoms use CO2 dissolved in water. Consumers capture carbon when they eat or parasitize producers (or other consumers). When living things burn organic molecules, they release some carbon back into the air or water as carbon dioxide gas. When an organism dies, decomposers finish the job of splitting off the carbon (C) in organic molecules and combining it with oxygen (O2) to form CO2.

However, some carbon takes far longer to cycle through an ecosystem. Mollusks, such as snails and clams, combine CO2 with calcium to form calcium carbonate shells. These shells and their locked-away carbon often become buried. Over centuries, they convert into limestone. Water reaching the limestone can slowly dissolve out its carbon and calcium and return it to the nutrient cycle.

An even larger amount of carbon enters long-term storage through the process of fossilization. Occurring over millions of years, fossilization involves the deep burial and transformation of dead organisms into coal, oil, and natural gas. When people burn fossil fuels such as gasoline and methane, they release stored organic carbon back into the atmosphere as CO2. Indeed, over the past 200 years, the burning of fossil fuels has significantly increased the amount of CO2 in our atmosphere.

The Oxygen Cycle. The oxygen cycle remains closely tied to that of carbon. Free oxygen (O2) first entered our atmosphere about 2 billion years ago—thanks to phytoplankton, the first photosynthetic organisms. Indeed, free oxygen is a universal "waste" product of photosynthesis—specifically of the step that splits water (H2O) in order to use its hydrogen atoms to build carbohydrates.

Both plants and animals consume oxygen during the process of respiration. In respiration, virtually all organisms burn food molecules to produce energy.

By about 75 million years ago, the level of oxygen in our atmosphere approached its present, stable concentration. Air at sea level contains about 21 percent oxygen; at higher altitudes, slightly less. Oxygen can also be found dissolved in most bodies of water.

The oxygen cycle plays a small part in the water cycle as well. In effect, every 2 billion years, photosynthesis splits all Earth's water to yield oxygen and organic molecules. Respiration continuously reacts with oxygen and organic molecules to create water again.

The Nitrogen Cycle Nitrogen forms a central part of many organic molecules. These include proteins, nucleic acids, and some vitamins. So life as we know it would be impossible without this element. In its gaseous form (N2), nitrogen makes up nearly 80 percent of our atmosphere. The trick is, neither plants nor animals can extract elemental nitrogen from the air. It enters the food chain primarily through the work of certain bacteria capable of nitrogen fixation. Specifically, these microorganisms combine nitrogen with hydrogen to make ammonia (NH3) or ammonium (NH4+), which plants can absorb. Still other bacteria convert ammonia or ammonium to nitrate (NO3), a form even more readily absorbed by plants.

Some nitrogen-fixing bacteria live independently in either soil or water. But the most important exist in a symbiotic relationship in the roots of plants such as legumes. Animals, in turn, obtain the nitrogen they need to grow by eating plants or animals that eat plants. Just as all life on this planet depends on photosynthetic organisms for energy, organisms depend on nitrogen-fixing bacteria as their means for obtaining this vital macronutrient.

In addition, certain high-energy, nonliving processes—such as lightning, fire, and volcanic activity—combine atmospheric nitrogen with oxygen or hydrogen to make ammonia and nitrate. Today, such processes account for a small percentage of the nitrogen entering the food chain. But they would have been the primary source when life first arose on this planet. Today, humans add to the load of nitrogen to nature's cycle in the form of artificial nitrate and ammonia fertilizers.

Living things recycle nitrogen in two ways. Land animals excrete ammonia in their urine. More importantly, decomposers break down the proteins and nucleic acids of dead tissues back into ammonia and nitrates.

However, the nitrogen cycle proves to be not nearly as efficient as that of other nutrients. A huge amount of nitrogen leaves the nutrient cycle when it is carried out of the reach of plant roots by water percolating down through the soil. Local ecosystems also lose nitrogen through the erosion of topsoil and the removal of plants. This is why farmers must continually add nitrogen fertilizers to their fields to maintain large harvests.

Soil Mineral Cycles

Weathered rock that helps make up the uppermost layer of Earth's crust supplies many other nutrients needed by plants and animals. These include many trace elements such as copper, zinc, and selenium. Rock also supplies the macronutrient phosphorus, a part of such crucial biochemicals as nucleic acids, phospholipids, and ATP. Together with calcium and magnesium, phosphorus is also a major component of teeth and bones. When rainwater runs over exposed rock, it dissolves phosphorus in the form of phosphate (PO3), which plants and photosynthetic microorganisms can readily absorb. Phosphorus passes up through the food web before eventually returning to the soil in wastes and dead tissues.