Energy Flow Through Ecosystems - Online Tutor, Practice Problems & Exam Prep

Energy flow in ecosystems involves understanding gross productivity, assimilated energy, net productivity, and energy efficiency. Gross primary productivity (GPP) represents the total energy captured by primary producers, while gross consumer productivity (GCP) applies to consumers. Assimilated energy (AE) is the portion of gross productivity used for cellular respiration and biomass production. Net productivity (NP) is calculated as NP = AE - R, where R is respiration energy. Energy efficiency, including net production efficiency (NPE) and trophic efficiency (TE), measures how effectively energy is converted into biomass across trophic levels.

Gross Productive Energy vs. Assimilated Energy

This video, we're going to begin our lesson on energy flow through an ecosystem by first differentiating between the terms gross productive energy and assimilated energy. Now gross productive energy or just gross productivity, we're going to abbreviate as GP moving forward in our course, and it's going to have this pinkish color coding to it. And gross productivity is really just the total amount of energy that is initially captured by an organism. Now the total amount of energy initially captured by a primary producer, we call the GPP, which is an abbreviation for gross primary productivity. And that's the main focus of this left-hand side of the image down below.

Now the total amount of energy initially captured by a consumer on the other hand, we call the GCP, which is an abbreviation for gross consumer productivity. And that's the main focus of this right-hand side of the image down below. It's also worthy of noting that in some textbooks, consumer productivity is also called secondary productivity. It turns out that not all of this total gross productive energy initially captured by an organism can actually be utilized by the organism because some of that energy will naturally be lost in the form of wastes and heat. And this leads us to the term assimilated energy, which we're going to abbreviate with the letters AE, and it's going to have this bluish color coding to it.

So, assimilated energy is the portion of gross productive energy that's actually utilized by the organism and specifically going to be used for the organism's cellular respiration and for the production of new biomass. By the term biomass, we really just mean the total mass of life, either the total mass of an individual organism or the total mass of multiple organisms. The production of new biomass can occur via either growth of the individual organism or via the organism's reproduction. For practical reasons and only for primary producers, not for consumers, we're going to assume that the gross primary productivity or GPP is going to be approximately equal to the primary producers' assimilated energy or AE. This is simply because primary producers like this plant down below simply don't generate significant amounts of wastes like consumers do.

Let's take a look at our image down below where we can start to piece some things together. Notice that on the left-hand side of the image, we're emphasizing once again that the gross primary productivity is going to be approximately equal to the primary producer's assimilated energy. The assimilated energy is, again, the energy utilized for respiration and for biomass. Here in this specific example, we're showing you that there's 1,000 kilojoules of the sun's energy that's initially being captured by this primary producer, this plant here, and so the 1,000 kilojoules represents the gross primary productivity, but it also represents the assimilated energy, some of this energy will be allocated to the plant’s cellular respiration, and some of this energy will be allocated to the plant's biomass. This is actually the plant's biomass that's available for consumption by the next trophic level.

Notice that we're indicating that it's the plant’s biomass that's being consumed by this consumer over here, this herbivore. More specifically, we're indicating that it's 450 kilojoules’ worth of the plant's biomass that's initially being captured and consumed by this herbivore. That's going to be this consumer's gross consumer productivity or its GCP. Again, not all of this gross total energy that's initially captured will be utilized by the organism because some of it is going to pass out the back end as waste and be lost as heat as well. Here, we're indicating that 150 kilojoules of the gross consumer productivity is being lost as waste and heat loss, and that's going to be not assimilated energy.

The leftover is actually going to be assimilated, so 300 kilojoules worth will be assimilated, and some of that 300 kilojoules will be allocated to this consumer's own cellular respiration, and some of it will be allocated to this consumer's production of biomass. Really, this here concludes this lesson, and to quickly recap, we've talked about the differences between gross productive energy or gross productivity and assimilated energy. We've talked about two different types of gross productive energy. We've talked about gross primary productivity or GPP for primary producers, and we've talked about gross consumer productivity or GCP for consumers. We've talked about how for primary producers, GPP is approximately equal to assimilated energy, but for consumers that's not the case because they generate significant amounts of waste and lose significant amounts of heat as not assimilated energy.

We'll be able to learn more and apply these concepts moving forward in our course, so I'll see you on our next one.

Which of the following statements is true? 

• A. The energy assimilated by an organism is used for new biomass only.
• B. The energy assimilated by an organism is used for new biomass, respiration & reproduction.
• C. The energy assimilated by an organism is used for new biomass, respiration, & is lost as waste (feces).
• D. Consumers assimilate 100% of the energy they consume.

Net Productive Energy ≈ New Biomass

So now that we've covered gross productive energy and assimilated energy in our last lesson video, in this video we're going to talk about net productive energy and new biomass. And so net productive energy or net productivity is the portion of assimilated energy that's used for producing new biomass via growth or reproduction. And if we want to get the net productive energy or the net productivity, all we need to do is use this equation right here. So we can take the assimilated energy, which recall from our last lesson video is the energy available for respiration and for biomass, and then subtract off the respiration energy, which we're going to abbreviate with the letter r down below. And when you do that, you're left with only the energy available for biomass, and that's exactly what net productivity or net productive energy is the energy available for biomass, which is why we say up above in this title that net productive energy is about equal to new biomass.

And so net productivity can actually be expressed in units of energy, or it could be expressed in units of biomass. Now it's also very important to note that net productivity or net productive energy also represents the total amount of energy that's available to be consumed by the next trophic level. And this is critical for you to keep in mind as we move forward in our course as well. And so down below, we're going to introduce 2 types of net productivity, 1 for primary producers and one for consumers, just like what we did for gross productivity in our last lesson video. And so, the first here is going to be net primary productivity or just NPP for short, and the next is going to be net consumer productivity or net secondary productivity, which we're going to abbreviate as NCP.

And so net primary productivity or NPP as its name implies with the term primary is simply going to be a primary producer's net productive energy. And net consumer productivity or NCP as its name implies is simply going to be a consumer's net productive energy. And again, net productivity or net productive energy can be calculated just by taking assimilated energy and subtracting off respiration. So that's what we can see here in these equations. But notice that for the primary producer, we use GPP instead of assimilated energy, and that's because recall from our last lesson video, we said that for a primary producer's assimilated energy, it's approximately equal to its gross productive energy or GPP.

NPP = GPP − r respiration

So we can take GPP and just plug it in for assimilated energy, and that's exactly what we can see over here. And this is how your textbooks tend to show this equation. Otherwise, I would show this equation with assimilated energy right here in this position, just as what you can see down below. Now we can go ahead and plug in some numbers to calculate the net primary productivity here. We can use these numbers in the image.

NPP = 1000 − 150 = 850 kilojoules

Now we can do the same for the equation down below, except we're doing it for the consumer this time, so it's going to be NCP. And that's going to be equal to the assimilated energy given to us as 300 kilojoules here in this image, And we're going to subtract off the energy for respiration, r, given to us here as 200 kilojoules. And when do you 300 minus 200, of course, you get the answer for NCP, which is 100 kilojoules. And so we can go ahead and plug that in here in our image as well. And really, that's all there is to net productive energy.

NCP = 300 − 200 = 100 kilojoules

The big takeaway here is that there are 2 types, 1 for primary producers, 1 for consumers, just like what we saw with gross productive energy. And it's the energy available for biomass, and it's also the energy available to the next trophic level. So moving forward, we'll be able to learn more and apply these concepts in problems. See you all there.

Energy Flow Through Ecosystems Example 1

So here we have a pretty tricky example problem that says, in an experiment, scientists found that over a certain period of time, the total respiration of an ecosystem exceeded the total amount of energy captured in photosynthesis. How is this possible? And we've got these 4 potential answer options down below. Now to solve this problem, I've gone ahead and drawn a little sketch here of a tree that represents the ecosystem, and I've drawn a yellow arrow here going into the tree representing the energy captured in photosynthesis, or in other words, it represents the gross primary productivity or the GPP. And then I've drawn this purple arrow over here going out of the ecosystem, which represents the total respiration of an ecosystem.

And notice that it's larger than the yellow arrow because, again, this exceeds the total amount of energy captured in photosynthesis. And this is unusual because usually, it's the other way around where GPP is going to be larger than respiration energy, but it is possible for this condition to exist under temporary circumstances. And when this does happen, when respiration is larger than GPP, what this means is that respiration is pulling energy from the existing biomass of the ecosystem. So some of the energy stored in biomass will be used as energy to power the metabolic needs of the ecosystem, used as energy to power cellular respiration. And so if biomass is being used as an energy pool for respiration under these conditions, what that means is that over time, the biomass is going to decrease.

And so notice that answer option b indicates that the ecosystem's total biomass is decreasing, and that's exactly what's going to be happening in this scenario. So option b is going to be the correct answer to this problem. Now a potentially tempting correct answer option would have been answer option c, which says that the ecosystem must be getting energy from another source. Now it could be possible that this ecosystem is getting energy from some other source, and that's how the respiration energy is able to exceed the energy captured by photosynthesis. However, most ecosystems are going to rely solely on the sun as an energy source and the process of photosynthesis, which captures the energy from the sun.

But, there are some other processes that can allow energy to enter the ecosystem. But this problem doesn't really tell us that or gives us any kind of indication that this is indeed the case, that the ecosystem is getting energy from another source. And then also this problem is specifically saying that the ecosystem must be getting energy from another source. Now it could be the case that it is, but it doesn't absolutely have to be, because it says must be really, that is what makes answer option c not the best answer option. Because again, it could be that the ecosystem is just pulling energy from its total biomass and decreasing its total biomass.

Now answer option a is not going to be correct because it says that the ecosystem total biomass is increasing and that's exactly opposite from the correct answer, answer option b. And then answer option d is also not going to be correct because it says animals must have been emigrating out of the ecosystem, but if animals are leaving the ecosystem, that means that the respiration is also going to decrease along with it since those animals respire. So, but here, that's not really what we're seeing. We're seeing kind of respiration being larger than photosynthesis, so we wouldn't expect something, you know, to decrease respiration to be the correct answer to that. So that's why option d is not correct.

So again, answer option b is the correct answer to this problem, and I'll see you on our next video.

Which of the following statements about NPP & GPP is true?

• A. GPP is the total energy captured by producers, NPP is the amount of energy they convert to biomass.
• B. Both GPP & NPP can be heavily influenced by climate.
• C. The NPP in an ecosystem equates to how much food is available for primary consumers.
• D. All of the above are true.

Energy Efficiency in Ecosystems

This video, we're going to talk about energy efficiency in ecosystems. Energy efficiency refers to the percentage of energy that's either used or transferred within an ecosystem. In this video, we're going to discuss two types of energy efficiency. The first is Net Production Efficiency or NPE, which is the main focus of this top box here. The second is Trophic Efficiency or TE, which is the main focus of this bottom box down below.

Net Production Efficiency or NPE represents the percentage of assimilated energy that is used for producing new biomass or used for producing net productivity. Net Production Efficiency really just tells us how efficiently an organism converts its assimilated energy into its own biomass. We can calculate NPE by using this equation that we have down below, which is taking the net productivity of the organism and dividing it by the organism's assimilated energy, and then multiplying that ratio by 100%. We can calculate the NPE for every single trophic level. In this image on the left-hand side, we're showing you two trophic levels.

The first is the primary producer represented by this plant, and the second is the primary consumer represented by this herbivore. To calculate the NPE for the plant, all we need to do is take the plant's net productivity, more specifically, its Net Primary Productivity or NPP given to us as 850 kilojoules and then divide that by the plant's assimilated energy, which is given to us as 1,000 kilojoules. We then multiply this ratio by 100%. The NPE value for this plant is 85%, which means that 85% of this plant's assimilated energy is converted into its own biomass or converted into the plant's net productivity. Additionally, the remaining 15% of the assimilated energy is used for cellular respiration.

We can then calculate the NPE value for the primary consumer. The NPE for the primary consumer is equal to this consumer's net productivity, more specifically the net consumer productivity given to us as 100 kilojoules, divided by the primary consumer's assimilated energy given to us as 300 kilojoules. Then we multiply this ratio by 100%. The net production efficiency for this primary consumer is about 33.33%, which means 33.33% of this primary consumer's assimilated energy is converted into biomass or net consumer productivity, and the remaining 66.67% is used in the consumer's cellular respiration.

Moving on to Trophic Efficiency or TE, one thing to note is that while NPE can be calculated for every single trophic level, Trophic Efficiency can only be calculated across trophic levels. Trophic efficiency represents the percentage of a single trophic level's net productive energy that is transferred to the next trophic level's net productivity. Trophic efficiency is calculated by the following equation: take the net productivity of the current trophic level and divide it by the net productivity of the previous trophic level and then multiply that ratio by 100%.

We can go ahead and do that, and when we multiply this ratio by 100%, what we get is a trophic efficiency of about 11.76%. Trophic efficiencies tend to be right around 10% on average, so it's no surprise here that this trophic efficiency is coming out really close to 10% at 11.76%. Later in our course, we'll discuss more about why trophic efficiencies tend to be so low. But for now, this concludes our lesson on energy efficiency and we'll be able to apply these concepts moving forward.

So see you all in our next video.

Energy Flow Through Ecosystems Example 2

So here we have a 2-part example problem where part number 1 is here asking us to calculate net production efficiencies or MPE. And part number 2 is down below here asking us to calculate trophic efficiencies or TE values. We need to recall those equations from our previous lesson video, so I'll go ahead and paste those in here into their corresponding sections.

Part number 1 says, a grasshopper's body mass contains a total energy of 600 joules after it has assimilated 4,000 joules of plant matter throughout its lifetime. A praying mantis then consumes the grasshopper and assimilates 475 joules of energy. It then gains 220 joules of biomass.

Part number 1 wants us to calculate the net production efficiency or MPE of both the grasshopper throughout its life and the praying mantis as it eats the grasshopper. Recall from our previous lesson videos that net production efficiency or MPE can be calculated by taking the net productivity of the organism, dividing it by the assimilated energy of the organism, and then multiplying that ratio by 100%. Let's calculate that for the grasshopper throughout its lifetime first. The MPE value is equal to 6004000 multiplied by 100%, which is 15%.

This value represents the net production efficiency of the grasshopper throughout its lifetime. We then do the same calculation for the praying mantis. The MPE for the praying mantis is 220475 multiplied by 100%, giving an answer of approximately 46.32%.

The net production efficiency tells us how efficiently an organism converts its assimilated energy into biomass. For the grasshopper, 15% of its assimilated energy is converted into its own biomass, with the remaining 85% used for cellular respiration. For the praying mantis, 46.32% of its assimilated energy is used in its biomass, with the remaining energy used for its cellular respiration.

Let's move on to part number 2, where we are asked to calculate the trophic efficiency between the primary producers and primary consumers, and between the primary consumers and the secondary consumers. Trophic efficiency is the ratio of the net productivity across trophic levels, calculated by dividing the net productivity of the current trophic level by the net productivity of the previous trophic level, then multiplying by 100%. For the primary producers (tree) and primary consumers (grasshoppers), the trophic efficiency is 6000100000 times 100%, which is 6%.

For the primary consumers (grasshoppers) and the secondary consumers (praying mantis), the trophic efficiency is 10006000 times 100%, resulting in a value of approximately 16.67%.

This concludes this example problem, and I'll see you in our next video.

Which of the following answers describes the production efficiency of an organism? 

• A. The amount of assimilated energy divided by the amount of energy ingested.
• B. The amount of energy ingested divided by the amount of assimilated energy.
• C. The amount of energy used for new biomass divided by the amount of assimilated energy.
• D. The amount of assimilated energy divided by the amount of energy used for new biomass.

Using the data in this table, calculate the: Net production efficiency of the grass.

• A. 5.4%
• B. 21.7%
• C. 2.17%

Using the data in this table, calculate the: Trophic efficiency between insects and grass.

• A. 1.2%
• B. 6.3%
• C. 4.6%

Using the data in this table, calculate the: Net production efficiency of insects.

• A. 19%
• B. 73%
• C. 27%