An ecosystem is defined as a community of various organisms interacting with each other and their environment in a particular area. It accounts for all interactions and relationships between both biotic (living) and abiotic (nonliving) factors.

Energy is what drives the ecosystem to thrive. And while all matter is conserved in an ecosystem, energy flows through an ecosystem, meaning it is not conserved. Energy enters all ecosystems as sunlight and is gradually lost as heat back into the environment.

However, before energy flows out of the ecosystem as heat, it flows between organisms in a process called energy flow. It's this energy flow that comes from the sun and then goes from organism to organism that is the basis of all interactions and relationships within an ecosystem.

The definition of energy flow is the transfer of energy from the sun and up each subsequent level of the food chain in an environment.

Each level of energy flow on the food chain in an ecosystem is designated by a trophic level, which refers to the position a certain organism or group of organisms occupies on the food chain. The start of the chain, which would be at the bottom of the energy pyramid, is the first trophic level. The first trophic level includes producers and autotrophs that convert solar energy into usable chemical energy via photosynthesis.

The next level up in the food chain/energy pyramid would be considered the second trophic level, which is usually occupied by a type of primary consumer like an herbivore that eats plants or algae. Each subsequent step in the food chain is equivalent to a new trophic level.

Besides trophic levels, there are a few more terms you need to know to understand energy flow.

Biomass: Biomass is organic material or organic matter. Biomass is the physical organic material that energy is stored in, like the mass that makes up plants and animals.

Productivity: Productivity is the rate at which energy is incorporated into the bodies of organisms as biomass. You can define productivity for any and all trophic levels. For example, primary productivity is the productivity of primary producers in an ecosystem.

Gross primary productivity (GPP): GPP is the rate at which the energy from the sun is captured in glucose molecules. It essentially measures how much total chemical energy is generated by primary producers in an ecosystem.

Net primary productivity (NPP): NPP also measures how much chemical energy is generated by primary producers, but it also takes into account the energy lost due to metabolic needs by the producers themselves. So, NPP is the rate at which the energy from the sun is captured and stored as biomass matter, and it's equal to the amount available energy to the other organisms in the ecosystem. NPP is always a lower amount than GPP.

NPP varies depending on the ecosystem. It depends on variables such as:

• Available sunlight.
• Nutrients in the ecosystem.
• Soil quality.
• Temperature.
• Moisture.
• CO₂ levels.
  

Energy enters ecosystems as sunlight and is transformed into usable chemical energy by producers such as land plants, algae and photosynthetic bacteria. Once this energy enters the ecosystem via photosynthesis and is converted into biomass by those producers, energy flows through the food chain when organisms eat other organisms.

Grass uses photosynthesis, beetle eats grass, bird eats beetle and so on.

As you move up trophic levels and continue along the food chain, energy flow is not 100 percent efficient. Only about 10 percent of the available energy makes it from one trophic level to the next trophic level, or from one organism to the next. The rest of that available energy (about 90 percent of that energy) is lost as heat.

The net productivity of each level decreases by a factor of 10 as you go up each trophic level.

Why isn't this transfer 100 percent efficient? There are three main reasons:

1. Not all organisms from each trophic level are consumed: Think of it this way: the net primary productivity amounts to all of the available energy for organisms in an ecosystem that's provided by producers for those organisms in higher trophic levels. In order to have all of that energy flow from that level to the next, it means that all of those producers would need to be consumed. Every blade of grass, every microscopic piece of algae, every leaf, every flower and so on. That doesn't happen, which means that some of that energy doesn't flow from that level up to the higher trophic levels.

2. Not all energy is able to be transferred from one level to the next: The second reason why the flow of energy is inefficient is because some energy is incapable of being transferred and, thus, is lost. For example, humans cannot digest cellulose. Even though that cellulose contains energy, people cannot digest it and get energy from it, and it's lost as "waste" (a.k.a., feces).

This is true for all organisms: there are certain cells and pieces of matter that they cannot digest that will be excreted as waste/lost as heat. So even if the available energy that a piece of food has is one amount, it's impossible for an organism that eats it to obtain every unit of available energy within that food. Some of that energy will always be lost.

3. Metabolism uses energy: Lastly, organisms use up energy for metabolic processes like cellular respiration. This energy is used up and cannot then be transferred to the next trophic level.

Energy flow can be described through food chains as the transfer of energy from one organism to the next, beginning with the producers and moving up the chain as organisms are consumed by one another. Another way to display this type of chain or simply to display the trophic levels is through food/energy pyramids.

Because energy flow is inefficient, the lowest level of the food chain is almost always the largest in terms of both energy and biomass. That's why it appears at the base of the pyramid; that's the level that's the largest. As you move up each trophic level or each level of the food pyramid, both energy and biomass decrease, which is why levels narrow in number and narrow visually as you move up the pyramid.

Think of it this way: You lose 90 percent of the available amount of energy as you move up each level. Only 10 percent of the energy flows along, which cannot support as many organisms as the previous level. This results in both less energy and less biomass at each level.

That explains why there's usually a greater number of organisms lower on the food chain (like grass, insects and small fish, for example) and a much smaller number of organisms at the top of the food chain (like bears, whales and lions, for example).

Here's a general chain of how energy flows in an ecosystem:

1. Energy enters the ecosystem via sunlight as  solar energy. 
2. Primary producers (a.k.a., the first trophic level) turn that solar energy into chemical energy via photosynthesis. Common examples are land plants, photosynthetic bacteria and algae. These producers are photosynthetic autotrophs, which means they create their own food/organic molecules with the sun's energy and carbon dioxide.
3. Some of that chemical energy that the producers create is then incorporated into the matter that makes up those producers. The rest is lost as heat and used in those organisms' metabolism.
4. They're then consumed by primary consumers (a.k.a., second trophic level). Common examples are herbivores and omnivores that eat plants. The energy that has been stored in those organisms' matter is transferred to that next trophic level. Some energy is lost as heat and as waste.
5. The next trophic level includes other consumers/predators that will eat the organisms on the second trophic level (secondary consumers, tertiary consumers, and so on). With each step you go up the food chain, some energy is lost.
6. When organisms die, decomposers like worms, bacteria and fungi break down the dead organisms and both recycle nutrients into the ecosystem and take energy for themselves. As always, some energy is still lost as heat.

Without producers, there would be no way for any amount of energy to enter the ecosystem in a usable form. Energy must continually enter the ecosystem via sunlight and those primary producers, or else the entire food web/chain in the ecosystem would collapse and cease to exist.

Temperate forest ecosystems are a great example for displaying how energy flow works.

It all starts with the solar energy that enters the ecosystem. This sunlight plus carbon dioxide will be used by a number of primary producers in a forest environment, including:

• Trees (such as maple, oak, ash and pine).
• Grasses.
• Vines.
• Algae in ponds/streams.

Next come the primary consumers. In the temperate forest, this would include herbivores like deer, various herbivorous insects, squirrels, chipmunks, rabbits and more. These organisms eat the primary producers and incorporate their energy into their own bodies. Some energy is lost as heat and waste.

Secondary and tertiary consumers then eat those other organisms. In a temperate forest, this includes animals like raccoons, predatory insects, foxes, coyotes, wolves, bears and birds of prey.

When any of these organisms die, decomposers break down the dead organisms' bodies, and the energy flows to the decomposers. In a temperate forest, this would include worms, fungi and various types of bacteria.

The pyramidal "flow of energy" concept can be demonstrated with this example, too. The most available energy and biomass is at the lowest level of the food/energy pyramid: the producers in the form of flowering plants, grasses, bushes and more. The level with the least energy/biomass is at the top of the pyramid/food chain in the form of high-level consumers like bears and wolves.

While marine ecosystems like a coral reef are very different from terrestrial ecosystems like temperate forests, you can see how the concept of energy flow works in the exact same way.

Primary producers in a coral reef environment are mostly microscopic plankton, microscopic plant-like organisms found in the coral and free-floating in the water around the coral reef. From there, various fish, mollusks and other herbivorous creatures, like sea urchins that live in the reef, consume those producers (mostly algae in this ecosystem) for energy.

Energy then flows to the next trophic level, which in this ecosystem would be larger predatory fish like sharks and barracuda along with the moray eel, snapper fish, sting rays, squid and more.

Decomposers exist in coral reefs, too. Some examples include:

• Sea cucumbers.
• Bacterial species.
• Shrimp.
• Brittle starfish.
• Various crab species (for example, the decorator crab).

You can also see the concept of the pyramid with this ecosystem. The most available energy and biomass exists at the first trophic level and the lowest level of the food pyramid: the producers in the form of algae and coral organisms. The level with the least energy and accumulated biomass is at the top in the form of high-level consumers like sharks.