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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 10% 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.
The image shows energy flow in an ecosystem and is labeled Energy flows in as sunlight and out as heat. There are images of the sun, a tree and flower labeled plants, a rabbit and squirrel labeled herbivores, a fox labeled carnivores, and a worm, mushroom and bacteria labeled decomposers. There is an arrow from the sun towards the plants. There is an arrow moving from the plants towards the herbivores, and an arrow moving from the herbivores towards the carnivores. There are arrows moving from the plants, the herbivores and the carnivores towards the decomposers. There are arrows labeled heat moving from the producers, the herbivores, the carnivores and the decomposers.
Image based on similar image by J. A. Nilsson1.
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 1.3 - 1.6% 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% 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)2. 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 2,000 g/m2/yr in highly productive tropical forests and salt marshes to less than 100 g/m2/yr 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.
This is a graphics interchange format image of a world map. Below the map there is a colored scale titled Net Primary Productivity in grams per meters squared per day. The range on the scale is negative 1 to 6.5, and the color scale changes from beige to dark green. The date range on the animation changes monthly from March 2000 through December 2013 to demonstrate the seasonal and year to year changes in the 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% 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 were3:
  • Primary producers, such as plants and algae: 7618 kcal/m2/yr
  • Primary consumers, such as snails and insect larvae: 1103 kcal/m2/yr
  • Secondary consumers, such as fish and large insects: 111 kcal/m2/yr
  • Tertiary consumers, such as large fish and snakes: 5 kcal/m2/yr
Transfer efficiency varies between levels and is not exactly 10%, 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:
Transfer efficiency= 1103kcal/m2/yr7618kcal/m2/yr×100
Transfer efficiency=14.5%
A picture taken from a glass bottom boat ride in Silver Springs Florida. The picture shows multiple fish and plants in the water.
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)4,5.
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 kcal/m2/yr, or biomass units, such as g/m2/yr.
An energy pyramid for Silver Springs, Florida. It is titled Energy measured in kilocalories per meters squared per year. The energy pyramid has 4 levels. The bottom level of the pyramid is labeled plants 20,810. Above the plants, the label is insects, snails, 3368. The level above insects, snails is fishes, 383 and the top of the pyramid is labeled fishes, 21. There is a key at the bottom of the pyramid that shows that the fishes, 21 are the tertiary consumer, the fishes 383 are the secondary consumer, the insects and snails are the primary consumer, and the plants are the primary producer.
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).
A table with 2 images. The table is titled Biomass; dry mass measured in grams per meter squared. There is an image on the left labeled Silver Spring, Florida and an image on the right labeled English channel. The image labeled Silver Spring, Florida has 4 levels. The top level is labeled fishes 5 and the color key at the bottom of the table indicates they are the tertiary consumer. The next level is labeled fishes 11 and the color key at the bottom of the table indicates they are the secondary consumer. The next level is labeled Herbivorous insects and snails, 37 and the color key at the bottom of the table indicates they are the primary consumer. The bottom level in the image is labeled Plants 809 and the color key at the bottom of the table indicates they are the primary producer. The image labeled English Channel has 2 levels. The top level is labeled Zooplankton, 21 and the color key at the bottom of the table indicates they are the primary consumer. The bottom level is labeled Phytoplankton, 4 and the color key at the bottom of the table indicates they are the primary producer.
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.
A table with 2 images. The table is titled Number of individuals per 0.1 hectare. The image on the left is labeled Grassland; summer and the image on the right is labeled Temperate forest; summer. Both images have 4 layers and the layers are color-coded. The color-coded key shows that the top layer in both images is the tertiary consumer. The next layer in both images is the secondary consumer. The layer below the secondary consumer in both images is the primary consumer and the bottom layer in both images is the primary producer. The 4 layers of the Grassland; summer image all have labels. The top layer is labeled 1 bird, the next layer is labeled 90,000 predatory insects, the next layer is labeled 200,000 Herbivorous insects, and the bottom layer is labeled 1,500,000 grass plants. The 4 layers of the Temperate forest; summer image all have labels. The top layer is labeled 5 birds, the next layer is labeled 120,000 predatory insects, the next layer is labeled 150,000 Herbivorous insects, and the bottom layer is labeled 200 trees.
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 10% 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.....
      (20 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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  • 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!
    (4 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)
  • 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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  • 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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  • 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?
      (3 votes)
  • 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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  • 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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  • 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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