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Energy flow & primary productivity

Learn about primary productivity, the (in)efficiency of energy transfer between trophic levels, and how to read ecological pyramids.

Key points:

  • Primary producers (usually plants and other photosynthesizers) are the gateway for energy to enter food webs.
  • Productivity is the rate at which energy is added to the bodies of a group of organisms (such as primary producers) in the form of biomass.
  • Gross productivity is the overall rate of energy capture. Net productivity is lower, adjusted for energy used by organisms in respiration/metabolism.
  • Energy transfer between trophic levels is inefficient. Only \sim, 10, percent of the net productivity of one level ends up as net productivity at the next level.
  • Ecological pyramids are visual representations of energy flow, biomass accumulation, and number of individuals at different trophic levels.

Introduction

Have you ever wondered what would happen if all the plants on Earth disappeared (along with other photosynthesizers, like algae and bacteria)?
Well, our beautiful planet would definitely look barren and sad. We would also lose our main source of oxygen (that important stuff we breathe and rely on for metabolism). Carbon dioxide would no longer be cleaned out of the air, and as it trapped heat, Earth might warm up fast. And, perhaps most problematically, almost every living thing on Earth would eventually run out of food and die.
Why would this be the case? In almost all ecosystems, photosynthesizers are the only "gateway" for energy to flow into food webs (networks of organisms that eat one another). If photosynthesizers were removed, the flow of energy would be cut off, and the other organisms would run out of food. In this way, photosynthesizers lay the foundation for every light-receiving ecosystem.

Producers are the energy gateway

Plants, algae, and photosynthetic bacteria act as producers. Producers are autotrophs, or "self-feeding" organisms, that make their own organic molecules from carbon dioxide. Photoautotrophs like plants use light energy to build sugars out of carbon dioxide. The energy is stored in the chemical bonds of the molecules, which are used as fuel and building material by the plant.
The energy stored in organic molecules can be passed to other organisms in the ecosystem when those organisms eat plants (or eat other organisms that have previously eaten plants). In this way, all the consumers, or heterotrophs ("other-feeding" organisms) of an ecosystem, including herbivores, carnivores, and decomposers, rely on the ecosystem's producers for energy.
If the plants or other producers of an ecosystem were removed, there would be no way for energy to enter the food web, and the ecological community would collapse. That's because energy isn't recycled: instead, it's dissipated as heat as it moves through the ecosystem, and must be constantly replenished.
Image based on similar image by J. A. Nilssonstart superscript, 1, end superscript.
Because producers support all the other organisms in an ecosystem, producer abundance, biomass (dry weight), and rate of energy capture are key in understanding how energy moves through an ecosystem and what types and numbers of other organisms it can sustain.

Primary productivity

In ecology, productivity is the rate at which energy is added to the bodies of organisms in the form of biomass. Biomass is simply the amount of matter that's stored in the bodies of a group of organisms. Productivity can be defined for any trophic level or other group, and it may take units of either energy or biomass. There are two basic types of productivity: gross and net.
To illustrate the difference, let's consider primary productivity (the productivity of the primary producers of an ecosystem).
  • Gross primary productivity, or GPP, is the rate at which solar energy is captured in sugar molecules during photosynthesis (energy captured per unit area per unit time). Producers such as plants use some of this energy for metabolism/cellular respiration and some for growth (building tissues).
  • Net primary productivity, or NPP, is gross primary productivity minus the rate of energy loss to metabolism and maintenance. In other words, it's the rate at which energy is stored as biomass by plants or other primary producers and made available to the consumers in the ecosystem.
Plants typically capture and convert about 1, point, 3
1, point, 6, percent of the solar energy that reaches Earth's surface and use about a quarter of the captured energy for metabolism and maintenance. So, around 1, percent of the solar energy reaching Earth's surface (per unit area and time) ends up as net primary productivity.
Net primary productivity varies among ecosystems and depends on many factors. These include solar energy input, temperature and moisture levels, carbon dioxide levels, nutrient availability, and community interactions (e.g., grazing by herbivores)squared. These factors affect how many photosynthesizers are present to capture light energy and how efficiently they can perform their role.
In terrestrial ecosystems, primary productivity ranges from about 2, comma000 start text, g, slash, m, end text, squared, start text, slash, y, r, end text in highly productive tropical forests and salt marshes to less than 100 start text, g, slash, m, end text, squared, start text, slash, y, r, end text in some deserts. You can see how net primary productivity changes on shorter timescales in the dynamic map below, which shows seasonal and year-to-year variations in net primary productivity of terrestrial ecosystems across the globe.
Animation credit: "Net primary productivity," by NASA, public domain.

How does energy move between trophic levels?

Energy can pass from one trophic level to the next when organic molecules from an organism's body are eaten by another organism. However, the transfer of energy between trophic levels is not usually very efficient.
How inefficient? On average, only about 10, percent of the energy stored as biomass in one trophic level (e.g., primary producers) gets stored as biomass in the next trophic level (e.g., primary consumers). Put another way, net productivity usually drops by a factor of ten from one trophic level to the next.
For example, in one aquatic ecosystem in Silver Springs, Florida, the net productivities (rates of energy storage as biomass) for trophic levels werecubed:
  • Primary producers, such as plants and algae: 7618 start text, k, c, a, l, slash, m, end text, squared, start text, slash, y, r, end text
  • Primary consumers, such as snails and insect larvae: 1103 start text, k, c, a, l, slash, m, end text, squared, start text, slash, y, r, end text
  • Secondary consumers, such as fish and large insects: 111 start text, k, c, a, l, slash, m, end text, squared, start text, slash, y, r, end text
  • Tertiary consumers, such as large fish and snakes: 5 start text, k, c, a, l, slash, m, end text, squared, start text, slash, y, r, end text
Transfer efficiency varies between levels and is not exactly 10, percent, but we can see that it's in the ballpark by doing a few calculations. For instance, the efficiency of transfer between primary producers and primary consumers is:
start text, T, r, a, n, s, f, e, r, space, e, f, f, i, c, i, e, n, c, y, end text, equals start fraction, 1103, start text, k, c, a, l, slash, m, end text, squared, start text, slash, y, r, end text, divided by, 7618, start text, k, c, a, l, slash, m, end text, squared, start text, slash, y, r, end text, end fraction, times, 100
start text, T, r, a, n, s, f, e, r, space, e, f, f, i, c, i, e, n, c, y, end text, equals, 14, point, 5, percent
Producers (plants) and consumers (fish) of Silver Springs. Image credit: "Glass Bottom Boat ride, Silver Springs Florida," by Katie Yaeger Rotramel, CC BY-NC-SA 2.0.
Why is energy transfer inefficient? There are several reasons. One is that not all the organisms at a lower trophic level get eaten by those at a higher trophic level. Another is that some molecules in the bodies of organisms that do get eaten are not digestible by predators and are lost in the predators' feces (poop). The dead organisms and feces become dinner for decomposers. Finally, of the energy-carrying molecules that do get absorbed by predators, some are used in cellular respiration (instead of being stored as biomass)start superscript, 4, comma, 5, end superscript.
Want to put some concrete numbers behind these concepts? Click on the pop-up to see exactly where energy goes as it moves through the Silver Springs ecosystem:

Ecological pyramids

We can look at numbers and do calculations to see how energy flows through an ecosystem. But wouldn't it be nice to have a diagram that captures this information in an easy-to-process way?
Ecological pyramids provide an intuitive, visual picture of how the trophic levels in an ecosystem compare for a feature of interest (such as energy flow, biomass, or number of organisms). Let's take a look at these three types of pyramids and see how they reflect the structure and function of ecosystems.

Energy pyramids

Energy pyramids represent energy flow through trophic levels. For instance, the pyramid below shows gross productivity for each trophic level in the Silver Springs ecosystem. An energy pyramid usually shows rates of energy flow through trophic levels, not absolute amounts of energy stored. It can have energy units, such as start text, k, c, a, l, slash, m, end text, squared, start text, slash, y, r, end text, or biomass units, such as start text, g, slash, m, end text, squared, start text, slash, y, r, end text.
Image modified from "Energy flow: Figure 3," by OpenStax College, Biology CC BY 4.0.
Energy pyramids are always upright, that is, narrower at each successive level (unless organisms enter the ecosystem from elsewhere). This pattern reflects the laws of thermodynamics, which tell us that new energy can't be created, and that some must be converted to a not-useful form (heat) in each transfer.

Biomass pyramids

Another way to visualize ecosystem structure is with biomass pyramids. These pyramids represent the amount of energy that's stored in living tissue at the different trophic levels. (Unlike energy pyramids, biomass pyramids show how much biomass is present in a level, not the rate at which it's added.)
Below on the left, we can see a biomass pyramid for the Silver Springs ecosystem. This pyramid, like many biomass pyramids, is upright. However, the biomass pyramid shown on the right – from a marine ecosystem in the English Channel – is upside-down (inverted).
Image modified from "Energy flow: Figure 3," by OpenStax College, Biology CC BY 4.0.
The inverted pyramid is possible because of the high turnover rate of the phytoplankton. They get rapidly eaten by the primary consumers (zooplankton), so their biomass at any point in time is small. However, they reproduce so fast that, despite their low steady-state biomass, they have high primary productivity that can support large numbers of zooplankton.

Numbers pyramids

Numbers pyramids show how many individual organisms there are in each trophic level. They can be upright, inverted, or kind of lumpy, depending on the ecosystem.
As shown in the figure below, a typical grassland during the summer has a base of numerous plants, and the numbers of organisms decrease at higher trophic levels. However, during the summer in a temperate forest, the base of the pyramid instead consists of a few plants (mostly trees) that are vastly outnumbered by primary consumers (mostly insects). Because individual trees are big, they can support the other trophic levels despite their small numbers.
Image modified from "Energy flow: Figure 3," by OpenStax College, Biology CC BY 4.0.

Summary

Primary producers, which are usually plants and other photosynthesizers, are the gateway through which energy enters food webs.
Productivity is the rate at which energy is added to the bodies of a group of organisms, such as primary producers, in the form of biomass. Gross productivity is the overall rate of energy capture. Net productivity is lower: it's gross productivity adjusted for the energy used by the organisms in respiration/metabolism, so it reflects the amount of energy stored as biomass.
Energy transfer between trophic levels is not very efficient. Only \sim, 10, percent of the net productivity of one level ends up as net productivity at the next level. Ecological pyramids are visual representations of energy flow, biomass accumulation, and number of individuals at different trophic levels.

Want to join the conversation?

  • piceratops tree style avatar for user Jannah Rahman
    My name is Jannah, my question is are the decomposer's the main part of the food chain/ the food web?
    (10 votes)
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    • purple pi pink style avatar for user Amina
      @Jannah Hello, Hope you're having a great day!
      here is a close answer to what you were questioning....
      Actually, in some cases, They may be considered the main part of a food chain/ food web because they recycle elements (minerals) which help plants grow and plants transfer their energy to the consumers and so on.....
      (19 votes)
  • leafers tree style avatar for user Madiha
    Why is productivity an important factor when considering the stability of an ecosystem?
    (6 votes)
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  • leafers sapling style avatar for user tobi
    If energy gets dissipated as heat by the factor of *0.1 it will never reach 0. Is the remaining energy stored in the atoms of the molecules, that the decomposers produce (and so in fact recycled, because used by producers)?
    (6 votes)
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  • aqualine ultimate style avatar for user M D
    Hello! Where do decomposers fit in on the energy pyramid, and what percent of the energy of which level do they receive (i.e. tertiary consumers receive 10% of the secondary consumer's energy)? Thank you!
    (3 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      Decomposers are placed in separate trophic level.

      A separate trophic level, the decomposers or transformers, consists of organisms such as bacteria and fungi that break down dead organisms and waste materials into nutrients usable by the producers.


      You see that they work entirely different than producers or consumers.
      (2 votes)
  • aqualine ultimate style avatar for user a35220
    In the graphics, particularly the energy, biomass, and number pyramids, why do the authors feel the need to label the producer a primary producer? Is there a such thing as secondary producer?
    (3 votes)
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  • piceratops seedling style avatar for user Christopher Joshua Edrosolo
    Is there a way to increase/maximize the transfer efficiency of energy between trophic levels or in general?
    (2 votes)
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  • blobby green style avatar for user adriennefan.arroyo
    Hi! I need some help reconciling the reasons why energy is "lost" between trophic levels. In paragraph 3 of the section titled, "Producers are the energy gateway," you talk about how "(energy is) dissipated as heat as it moves through the ecosystem" and then explain in the "Energy pyramids" section that "(An energy pyramid's upright) pattern reflects the laws of thermodynamics, which tell us that new energy can't be created, and that some must be converted to a not-useful form (heat) in each transfer." However, the penultimate paragraph of the section titled, "How does energy move between trophic levels?" lists the reasons for inefficient transfer of energy, and it isn't clear to me how the previously described heat dissipation factors into this list. Can you clarify? Thanks!
    (2 votes)
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    • duskpin ultimate style avatar for user sisipanda
      Okay so basically, imagine a human and you're eating food. Not all of that energy gets put into your biomass, some of it goes to maintaining homeostasis (like your blood temperature and stuff.) If you eat a human (which you probably won't...), the energy you get is going to come from the biomass and the temperature of blood wouldn't give you any energy. Therefore, the energy is lost.
      (2 votes)
  • marcimus red style avatar for user 227012
    So I was wondering when you find NPP, you do GPP - Respiration Loss, right? But I now I get the respiration loss in a percent (For example: 20%) If you were trying to do the subtraction, how would you subtract 20% from 0.012 grams/cm2/day if 0.012 is the GPP and 20% is the respiration loss. The question also gives that 1 gram of rice is 1000 calories.
    (2 votes)
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  • leafers tree style avatar for user Vikram Kaushik
    If energy level decreased as the trophic level increases, doesn't this mean plants and other primary consumers are more energy dense than secondary and tertiary consumers? Then why is it that animal based foods are more energy dense than the plants we eat?
    (2 votes)
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    • female robot grace style avatar for user tyersome
      Total energy decreases, but as you noted energy density tends to increase. The difference is because organisms in lower trophic are typically much more numerous!

      If you weighed all the plants in an ecosystem and compared that to the mass of herbivores what would you find?

      Think about a grassland — how much grass is there compared to grazing animals?

      Does that help?
      (2 votes)
  • blobby green style avatar for user Rania Chihadeh
    If we measure the available biomass for a patch of a forest at 10 kgc/m2 per year , and the amount of CO2 given off into the atmosphere as 5kgc/m2 per year, what is GPP?
    (2 votes)
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