How much energy is then passed between each trophic level of the food chain/energy pyramid?

1. Introduction

At the end of the last module, you responded to this scenario:

1. You’re the director of a mission to colonize a planet in another solar system.
2. Your spaceship is to carry colonists on a decades-long journey to your new home.
3. You have to grow your food on the ship.

Decision: to maximize crew size, are you going to direct your crew to eat as vegetarians or carnivores?

There is an answer, and it lies in the principles of ecology. Let’s take a look.

2. The Easy Math of Energy in Trophic Levels

To keep the math easy, let’s set 2000 Calories/day as our average target calorie intake for each crewperson.

Remember that our ship can produce a lot of energy: enough to grow 2,000,000 food calories worth of food each day. What we’re growing, of course, are plants. If we stick to a strict vegetarian diet, then the math is straightforward: 2,000,000 calories of plant food/day, divided by 2000 calories/day/crewperson= 1000 crew people.

But what happens if we wanted to eat as carnivores? We would take the seeds, grain, leaves, and other plant food that we produced on our ship’s farms, and we would feed it to cows  (or other animals, such as chickens). Then, instead of eating plant foods like bread and apples, we’d eat steak. If we measured the amount of chemical energy available to us from the cows, how much energy would be available for the crew?

Make a prediction:

As, you’ve just seen, here are the numbers:

Harvestable energy means exactly what it says. Imagine taking all of the plants that we can grow on the ship, and measuring their energy. That would be the first row: harvestable energy available in plants. The second row is the energy available to us from the meat of the animals that ate the plants.

Note that the total food energy that we could harvest from the cows is only 10% of the energy in the plants that we fed to the cows. Or, using the terms related to trophic levels that we learned in the previous tutorial, the harvestable energy in the producers (plants) is ten times as great as the harvestable energy in primary consumers.

The loss of 90% of the available food energy from producers to consumers is a widely known rule in ecology. The rule is called the 10 Percent Rule. To make sure that you’re getting this, try to label the diagram below. Note that in this ecosystem, the energy available in the producers starts at 600,000 units of energy.

What the 10% rule says is this: If you gathered up all of the primary producers and measured their chemical energy, and then did the same with the herbivores/primary consumers, the herbivores would have only 10% of the total chemical energy that was available in the producers. And, if you gathered up all of the secondary consumers/carnivores, and compared the energy in that trophic level to the energy in the primary consumers, you’d find that once again, you’d only transferred 10% of the energy.

Let’s apply the 10% rule to the issue of largest possible crew size on our spaceship to Alpha Centauri.

To translate what’s above into actual crew numbers, here’s the math.

• Eating as primary consumers/vegetarians, you’d have 2,000,000 calories available for the crew. At an average of 2,000 calories/crewmember, that would get you a crew of 1,000.
• Eating as secondary consumers/carnivores, you’ve have 200,000 calories available for the crew. At an average of 2,000 calories/crewmember, your crew size drops to 100.

In terms of founding a colony, more colonists is better. It maximizes genetic diversity, keeping your population more adaptable. The vegetarian strategy is the wiser course of action.

Of course, the actual decision would be much more nuanced. Humans are omnivores: it’s probably wise for the crew to have access to some animal protein. And the energy equation becomes very different if you’re considering animal products (eggs from chicken, dairy from cows) as opposed to meat. But given the limits of size of ship and amount of energy it can produce, it makes sense to have the crew eat primarily as herbivores, instead of primarily as carnivores.

MISCONCEPTION ALERT: Before going further, let’s clear up a possible point of confusion. What we’re discussing does NOT mean that a pound of meat (steak, or chicken, for example) has less energy than a pound of rice or a pound of broccoli. This is NOT a comparison of energy between organisms, or types of food. It’s a comparison of the energy available between trophic levels.

3. Understanding the 10% Rule

Why is so little energy transferred from one trophic level to the next? One reason is that the work that cells do to keep themselves alive is not performed with perfect efficiency. As sugar is transformed to ATP, and as ATP is broken down to ADP and phosphate to perform cellular work, a lot of energy is lost as heat. It’s the same thing that happens in a car. The gasoline in the gas tank is not perfectly transformed into the kinetic energy that moves the car. Rather, much of the energy is lost as heat: heat that you can feel coming off a car’s hood. That heat is wasted energy that dissipates (disperses, diffuses) into the environment.

Similarly, many of the food calories that you feed to a cow, as they are used by the cow to keep itself alive, dissipate as heat into the environment. Add to that the fact that a cow is a mammal with a body temperature similar to yours, and you get an even greater loss of energy to heat. All of the chemical energy lost as heat is not going to be available to the next trophic level: the carnivores who eat the cow.

In my Food Chain Song (embedded below), I refer to the heat loss as “metabolic rent.” But there are more reasons why energy transfer between trophic levels is so inefficient. The diagram below, which shows a caterpillar harvesting energy from a leaf, shows heat loss, and one additional source of energy loss (note that this diagram uses Joules (J) instead of Calories. The proportions would be the same).

The 67 J arrow on the right side (cellular respiration) is the heat loss that we’ve described above: it’s the energy that the caterpillar is burning just to keep itself alive. But note the 100J arrow to the left labeled “feces.” Not everything that an organism ingests goes making more of that organism. Some ingested food is indigestible, and passes right through you. On an ecological level, all of that defecated matter doesn’t become available to the next trophic level.

So, in the diagram above, what’s left to be transferred to the next trophic level? Only the 33J labeled as “growth.”

The fact that 33/200 is 16.5%, as opposed to to 10% might seem confusing. But remember that this an analysis of one animal eating a part of a plant.  If you widen the analysis to thinking about the actual organisms living in an ecosystem, you can find additional reasons for energy loss between trophic levels. We’ll do that in a moment, but first, answer the questions below.

The primary consumers are not going to be able to eat every last piece of the plants below them. Just think of yourself as a primary consumer. If you eat rice, for example, you’re eating only a tiny portion of that plant. Or consider eating an apple: there’s the whole rest of the tree that you’re not consuming. Similarly, when you eat as a carnivore, you eat only a tiny portion of the animal that was killed to feed you. When you eat chicken, you mostly eat the muscle tissue around the breasts, back, and legs. All the rest of the chicken (bones, innards, head, feet, etc). doesn’t get transferred to you.

Here’s how I summarize this in my Food Chain Song.

Each level in a food chain yields only ten percent
Of the former level’s energy you might ask where it went
There are bones and leaves you can’t digest that really make a dent
And don’t ignore the weighty cost of metabolic rent.

Ecologists have organized this notion of energy loss in ecosystems into a pyramid of energy. In the pyramid, you can see available energy dropping by 90% from one trophic level to the next. Remember: 90% of the energy is lost. Only 10% is transferred.

There are other ecological pyramids that show other aspects of ecosystem structure. One is a pyramid of numbers. This pyramid shows the actual number of organisms at each trophic level. In the pyramid of numbers below, you see that 1,500,000 producers support 200,000 herbivores. These herbivores support 90,000 secondary consumers, who in turn support one top-level carnivore.

But unlike a pyramid of energy, which is always widest at its base, the pyramid of numbers can have a base that’s narrower than the levels above. Consider a forest ecosystem where one tree might support thousands of insects, which are preyed upon by birds. That pyramid would look like this:

Ecosystems can also be represented through a pyramid of biomass. Biomass is living matter, and a biomass pyramid shows the amount of living matter in each trophic level. Here’s a typical biomass pyramid.

Like a pyramid of numbers, there are biomass pyramids that are inverted, with the base more narrow than the layers above. But that goes beyond the scope of our efforts here. If you want to read more about pyramids of biomass, you can follow this link to Wikipedia.

Let’s check your understanding of ecological pyramids with this quiz.

5. Biological Magnification: Interactive Reading

6. Concluding Thoughts

The key idea from this module is that while matter in ecosystems is endlessly recycled, energy dissipates as it flows from one trophic level to the next.  Here’s a haiku that captures this:

In ecosystems
Energy will dissipate
But matter cycles

The energy doesn’t disappear. It just becomes less useful. In almost every ecosystem on Earth, energy starts as sunlight. If the sun stopped shining, energy would stop flowing, and life would cease. Producers transform this light energy into chemical bond energy through photosynthesis, capturing it as carbohydrates. As this chemical energy moves from trophic level to trophic level, most of the energy diffuses away as heat. Consequently, fewer and fewer organisms can be supported at higher trophic levels.

I’ve tried to capture a lot of these ideas in my Food Chain song. If you are watching it in a classroom setting, please use headphones or earbuds so as not to disturb the students around you.

If you feel confident that you’ve mastered the ideas in this module, take this quiz.

7. Trophic Levels and Energy Pyramids Quiz

Next steps

This ends this series of tutorials about energy and ecology.

1. Return to the Ecosystems, main menu
2. Choose another module using the menu above
3. Watch a music video in which Mr. W presents a utopian version of humans colonizing Mars