Ever think about how everything that lives on this planet manages to...well...live?  Just how does everything get its capacity to move and breathe and just generally exist?  To that extent, let's talk a bit about energy.  Not necessarily the kind that will power your home or your car.  The kind that will power you.  The kind that powers all living things on Earth.

Let's figure this out!

Our Source of Energy

When we stop to think about energy, we usually think about energies that power the things we use.  Fossil fuels, wind, nuclear, all the kinds of energy that power our fancy, dead, but still quite cool, gadgets.  However, when it comes to life energy, the energy that sustains life, there is only one source for our planet: the sun.  While this seems like a very basic thing to understand, to the tune of "of course we all need the sun to survive...it's the sun!" I would expect most people to not quite understand why or how.​

The truth in the flow of energy is that all things generally use energy in the same way.  They ingest, they assimilate, they produce, and inevitably excrete either something or themselves.  But what does this have to do with the sun?  "I don't eat the sun," you may say, and to that I say, "but you eat the things that do."

How Energy is Used

​Okay, so going off of what we just said, all our available energy comes from the sun.  We, as humans, don't live off of the sun, we live off of food.  We do this because we physically cannot ingest sunlight.  Our biology does not allow for this to happen, thus we rely on getting energy from other sources.  Let's start from the beginning...

The sun is Earth's one source of life energy. All organisms on the planet depend on it to exist.

​Solar radiation in the form of light travels from the sun to our planet, where it will enter the atmosphere and hit our planet's surface.  Not all of this energy makes it through the atmosphere, so we lose a bit of the energy before it even gets "into" our planet.  From there, we have a group of living things capable of using this sunlight directly.  These are our lovely green plants.  You may know the term photosynthesis, and this biochemical reaction is what allows the sun's energy to stay on Earth at all.

We, humans and all other life, depend on plants to intercept the sun's energy for us to use

The plant ingests that energy, and uses some of it while doing so.  It pulls the energy into itself and uses this energy to foster its own growth.  The act of utilizing this energy also gives off some of its absorbed energy in the form of heat, generally by way of respiration, a plant's way of exhaling chemicals it no longer needs after using and storing what it does need.

All of this action expends quite a bit of the energy the plant absorbed to begin with.  In addition, plants are not capable of absorbing 100% of the energy that hits them.  Between respiration and their inability to absorb all energy that comes their way, plants lose close to 50% of the energy that comes into the planet.  Still, the remaining energy is stored either for it to use...or for something to take.

How Energy Moves

That last bit, "or for something to take," is precisely how the energy that is stored on the planet moves around.  Plants will happily continue to grow themselves, but they don't generally move or give much.  For others to get at that energy, they need to take action themselves.  They ingest the ingestors.

Something really important to note is that plants are the most efficient consumers of available energy.  When we mentioned earlier that plants lose roughly 50% of the energy that hits them, everything beyond this point is worse than that.  These plant consumers (and even plant consumer consumers!) still have to expend energy to ingest the energy stored in what they're ingesting.  The further up the chain you go, generally the more energy they are using to "earn their keep."  As energy moves beyond plants, there is inherently less to go around.

While not on the topic of energy directly, let's take a look at the levels that energy travels through.  These are called trophic levels.

 As energy is passed from plants to animals, less energy is left to ingest, leaving top carnivores to expend more effort for less energy

Levels of Energy Flow

As we start, the first trophic level is that of the plant kingdom.  On our planet, they are the only organisms able to directly interface with sunlight in a way that can be transformed into energy for use.  While they are the first consumers of the energy, they are also the organisms that make the energy for the rest of life on Earth.  Due to this, they are called producers.

By Thompsma - Own work, CC BY-SA 3.0, Link

The amount of life at each trophic level is proportional to the amount of energy left for them as it is passed along the ecosystem. Less energy = less life

Now that producers have stored the sun's energy in a form consumable by others, we have our second trophic level, primary consumers.  This level is our planet's herbivores.  This level gets the most of the remaining energy stored in plants just by virtue of not having lost any of it due to going through many levels of ingestion already.  Beyond that we have secondary consumers, the consumers of consumers.  These are carnivores who feed on our herbivorous friends.  Humans tend to fall in this category.  We have one level beyond that called tertiary consumers, which feed on the carnivores that feed on the herbivores.  These animals tend to show how we exist in a food web rather than a food chain, since the energy expended to capture and consume other carnivorous creatures is so high, that they also find other sources of food, such as plants, to survive.

As an aside, there is one other level, but it depends on the excretion, or waste, of other organisms.  These are the decomposers, like fungi, that feed on the detritus of other organisms.  Anything dead that comes out of other living things (bark, leaves, poop, corpses, bones), these organisms will ingest, and generally output nutrients that feed back into the ecosystem.  This is a major factor in the planet's biogeochemical cycles.

At all these levels, the same principles of ingestion, assimilation, production, and excretion apply.  And at each level, the energy left available from the previous level is further removed from the source, leaving less to utilize.  As a result, the higher in trophic level you go, the less biomass for that level exists.  Where there are lots of plants, there are fewer mice, even fewer snakes, and fewer more birds of prey.  This correlation is directly proportional to the energy available to that trophic level.

Putting all this into perspective, it's almost crazy that so much life exists on this planet.  If there weren't an abundance of green plants, life on the scale that we see today would simply not be feasible.  Any change in the balances we see today would have rippling effects throughout all the levels that energy flows.  It's so important to be aware of what sustains us, and do what we can to ensure it continues to be there so we can stick around to live on this amazing world.

~ And, as always, don't forget to keep wondering ~

Sources:
Discussion based on H.T. Odum's Silver Springs model on energy flow.
* Odum, Howard T. Environment, Power, And Society For The Twenty-First Century. Columbia University Press, 2007.

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