Energy & Biomass Pyramids - Online Tutor, Practice Problems & Exam Prep

Energy and biomass pyramids illustrate ecosystem structure and trophic efficiency. Energy pyramids, always upright, show energy transfer between trophic levels over time, with only about 10% of energy passing to the next level. In contrast, biomass pyramids capture a snapshot in time and can take various shapes, reflecting the biomass at a specific moment. Trophic efficiencies are low due to incomplete ingestion, digestion, metabolic energy use, and heat loss. Additionally, biomagnification highlights how pollutants concentrate at higher trophic levels, impacting top predators significantly.

Energy & Biomass Pyramids

This video, we're going to talk about energy and biomass pyramids. So it turns out that energy and biomass pyramids are really just visual representations of ecosystem structure and trophic efficiency. Now as their name implies, energy pyramids focus on energy and how energy is transferred from one trophic level up to the next trophic level over some period of elapsed time. And because trophic efficiencies tend to be low, only a small fraction of the energy stored in one trophic level's biomass will actually be transferred up to the next trophic level's biomass. Just as what we can see in this energy pyramid down below, only a small fraction of the energy in each trophic level is going to trickle its way up the food chain.

And because this is the case, energy pyramids are forced to always have your normal, regular, upright shape to them. And, again, this is because it tracks the energy over time. And so notice that the units of the values in this energy pyramid are in units of energy per area per unit of elapsed time. In this case, units of joules per meter squared per month. Now, biomass pyramids on the other hand, of course, focus on biomass, but they tend to only show a single moment, instant, or snapshot in time rather than some elapsed period of time.

And so because that's the case, biomass pyramids can take on several different shapes, including being upright, inverted, or even diamond shaped. And so notice that down below in the image on the right hand side, we're showing you an example of a biomass pyramid that seems to have somewhat of a diamond shape to it, if you will. But if we were to exclude the top two trophic levels, the tertiary and secondary consumers, and only focus on the bottom two trophic levels, the primary, producers and primary consumers, then we'd have ourselves an inverted biomass pyramid since it's somewhat of an upside-down triangle. And in this inverted biomass pyramid, we're saying that 8 grams worth of phytoplankton is supporting 42 grams worth of zooplankton. And so you might expect that the 42 grams worth of zooplankton is consuming the phytoplankton at an incredibly high rate, and indeed it is.

But what isn't being captured by this biomass pyramid in this single instant in time is that the phytoplankton also have an incredibly high reproduction rate that allows them to sustain their own biomass despite the fact that it's being consumed at an incredibly high rate by this higher mass of zooplankton. And so again, it's the fact that biomass pyramids reflect only a single instant or moment in time that allows them to take on several different shapes. And so notice that the units of the values here only have units of mass per area, in this case, grams per meter squared, but it has no unit that reflects elapsed time because it's focusing on just a single instant or moment in time. However, if we focused on how the biomass changed over time, then it would always maintain an upright shape to it just as the energy pyramids do. Now once again, we know that trophic efficiencies are generally low, averaging right around 10%.

But why are trophic efficiencies so low? Well, it turns out that there are actually several different reasons for that, including the 4 that we highlight down below. The first being incomplete ingestion, meaning that not all of the energy that's available for consumption in one trophic level will always be consumed by the next trophic level. So there will always be some amount of energy that is just not ingested. Incomplete digestion, on the other hand, refers to the fact that even of the energy that is consumed, not all of it is going to be assimilated and used by the organisms because some of it will be lost in the form of waste.

Now metabolism and cellular respiration also require energy, and the energy going towards that is energy that will not be transferred up to the next trophic level, keeping trophic efficiencies low. And also, with every single energy transfer, there is heat loss, and the energy lost as heat is energy that will not be transferred up to the next trophic level, again keeping trophic efficiencies relatively low. Now the very last idea that I'll leave you all off with here is this idea of biomagnification. And so biomagnification is the process of pollutants becoming more and more concentrated as they move up the food chain to higher trophic levels. And so notice that in this biomagnification image on the far right, that towards the bottom trophic levels, the concentration of these pollutants is relatively low, and these pollutants are not able to be digested or excreted.

So they accumulate in biomass. And so as these pollutants make their way up the food chain, they start to become more and more concentrated in the higher trophic levels. And so biomagnification has its greatest impacts and effects on organisms that are towards the top of food chains. So this here concludes our lesson, and we'll be able to apply these concepts and problems moving forward. So I'll see you all in our next video.

Energy & Biomass Pyramids Example 1

So here we have an example problem that asks, why is it more energy efficient for humans to eat a plant-based diet acting as a primary consumer rather than eating meat and acting as a secondary or tertiary consumer, for example? And we've got these four potential answer options down below. Now when it comes to energy efficiency, it's important to recall the energy pyramids that we talked about in our last lesson video. So here I've drawn a quick sketch of an energy pyramid, which we know is always going to have an upright formation to it just as you see here. We've got the primary producers on the bottom here, which have the most amount of energy stored in their biomass.

But then as you move up the food chain to higher and higher trophic levels, notice that the amount of energy significantly drops off as we get to these primary, secondary, and tertiary consumers. And so we know that the energy that's stored in one trophic level's biomass is not going to be efficiently transferred to the next trophic level's biomass. And because that's the case, energy transfers within an ecosystem are not efficient. And so if we want to maximize the energy efficiency and be more energy efficient, then we need to minimize how many energy transfers there are before the energy actually gets to us. And so if we're serving as tertiary or secondary consumers and eating meat, well then we're not minimizing how many energy transfers there are before the energy actually gets to us.

And so therefore, it's not going to be very energy efficient. However, if we serve as primary consumers and eat plant and plant-based materials, well, then we're minimizing the energy transfers that are needed before the energy gets to us, and that allows us to maximize the energy efficiency. Now keeping this in mind, notice that option A says humans can obtain more energy from the same mass of plants than they can from meat. Now in some cases, this statement may be true, but in other cases, this statement may not be true, especially if the meat has a large percentage of fat content in it. And so option A is not a statement that holds true in every single circumstance, and really it's not the energy content that determines the efficiency.

It's really the energy transfers and how much energy is transferred that determines the energy efficiency. So that's why answer option A is not really the best answer option here, so we can cross it off. Then notice answer option C says, animal-based products are more expensive. Now although this also tends to be true, animals require more resources to upkeep, and that's why they tend to be more expensive. This doesn't really explain our energy efficiency problem here, and so for that reason, we can eliminate answer option C.

And if we can eliminate option A and C, then we can also eliminate answer option D, which says all of the above. And this leaves us with answer option B as the only option, which says a given amount of solar energy can produce far more plant biomass than animal biomass. And that is going to be true. So if we are tracking some subset of solar energy through the ecosystem, it's going to lead to the production of more plant biomass and lead to the production of less animal biomass, simply because of how energy transfers through the ecosystem, and energy is lost along the way with every energy transfer. So answer option B is the correct answer to this problem, and I'll see you all in our next video.

True or false: Most of an ecosystem’s net primary productivity is ultimately used for the growth of primary consumers within the ecosystem.

• A. True because this is the only food source available to primary consumers.
• B. True because primary consumers are the most efficient organisms at converting their food into biomass.
• C. False because primary consumers consume most of the plant matter, but don’t convert most of it to biomass.
• D. False because primary consumers don’t consume most of the plant matter, & they do not convert most of the food they ingest into biomass.

Which of the following is a reason that such a small fraction of the energy that reaches the Earth as solar radiation ends up at the top of the food chain?

• A. Plants only absorb a very small fraction of the total solar energy that reaches Earth.
• B. Lots of energy is dissipated as heat at all trophic levels.
• C. Incomplete ingestion – consumers do not eat all the food available to them.
• D. All of the above.

Due to biomagnification, there are certain species with a high concentration of heavy metals (e.g., mercury), leading to dietary advisories for vulnerable demographics (e.g., pregnant women and young children) to avoid these species. Given that these advisories are often most critical for species at the top of long marine food chains, which species would this advice most likely apply to?

• A. Cows (primary consumers, primarily eat grass).
• B. Shrimp (secondary consumers, feed on zooplankton).
• C. Alligators (secondary/tertiary consumers, feed on many species).
• D. Tuna (top predators, feed on predatory fish & invertebrates).