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Energy flow and primary productivity

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 about 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 problematic, 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 other organisms that have previously eaten plants. In this way, all the consumers—or heterotrophs, other-feeding organisms—of an ecosystem rely on the ecosystem's producers for energy. Consumers include herbivores, carnivores, and decomposers.
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 dissipates as heat as it moves through the ecosystem, and it must be constantly replenished.
Image credit: 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—or 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 that ecosystem 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% to 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 including 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 2,000 g/msquared/yr in highly productive tropical forests and salt marshes to less than 100 g/msquared/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.
Image 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 were as follows:cubed
  • Primary producers, such as plants and algae: 7,618 kcal/msquared/yr
  • Primary consumers, such as snails and insect larvae: 1,103 kcal/msquared/yr
  • Secondary consumers, such as fish and large insects: 111 kcal/msquared/yr
  • Tertiary consumers, such as large fish and snakes: 5 kcal/msquared/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 calculated below:
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?

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 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 credit: 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 energy 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, or inverted.
Image credit: 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 credit: modified from Energy flow: Figure 3 by OpenStax College, Biology CC BY 4.0

Summary

Primary producer, 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 about 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?

  • blobby green style avatar for user cb04132
    Why is it that only 10% is transferred and not more?
    (4 votes)
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  • starky sapling style avatar for user Isabella Nicolè
    What would the energy transfer efficiency be assuming that all individuals of each trophic level are eaten and therefore not wasted?
    (2 votes)
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  • starky seed style avatar for user oe616874
    Why does unit of biomass is g/m^2, why not kcal/m^2? and why do we need to use it for
    (2 votes)
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  • blobby green style avatar for user a c
    I was reading this article as supplementary information for my course and found this passage confusing: "Why is energy transfer inefficient? [...] Not all the organisms at a lower trophic level get eaten by those at a higher trophic level."

    I have never seen anything like this before. Why does this necessarily lead to energy waste? What's the difference between an organisms of different trophic levels eating something? Wouldn't the energy transferred between organisms be consistent despite their trophic levels?

    This also leads me to doubt my understanding of trophic levels. I thought that trophic levels had a rough structure of the producers at level one, herbivores at level two, omnivores at level three, and carnivores at level four (and that these two last levels were kind of the same thing anyway). Is the passage is referring to carnivores eating plant matter and thus excreting it as waste?
    (2 votes)
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    • duskpin ultimate style avatar for user Owen Friedman
      For your first question: if the organism dies without being eaten its energy is not passed to the next trophic level it is instead given to the decomposers. This loss results in a lower average energy transfer.

      Next: trophic levels are a way of mapping the path of energy and biomass through organisms, they do not have a ridged structure.

      And I believe the passage is referring to the inefficiency of our digestive systems. When you eat a meal your body can't collect all the energy from it, so energy leaves your body as poo without being taken up by your trophic level.
      (1 vote)
  • blobby green style avatar for user 0019860
    what happens to the other 90%?
    (1 vote)
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  • blobby green style avatar for user 001303405
    What are 3 things that affect primary productivity?
    (1 vote)
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  • blobby green style avatar for user yo mama
    What does the kcal/m^2/year mean in relation to the energy flow? Is it a measure of how much energy is stored in an organism?
    (1 vote)
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  • leaf green style avatar for user chinna.chilukuri
    Why does unit of biomass is g/m^2, why not kcal/m^2?
    (1 vote)
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  • aqualine sapling style avatar for user Yuliannis
    q looooo eeeeee no no no
    (1 vote)
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  • blobby green style avatar for user ayoub.laanaoui
    Are the ecological pyramids visual
    (1 vote)
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