Energy flow & primary productivity

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

Kluczowe punkty:

  • 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%\sim10\% 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.

Wstęp

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)start superscript, 2, end superscript. 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 g, slash, m, start superscript, 2, end superscript, slash, y, r in highly productive tropical forests and salt marshes to less than 100 g, slash, m, start superscript, 2, end superscript, slash, y, r 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 werestart superscript, 3, end superscript:
  • Primary producers, such as plants and algae: 7618 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, r
  • Primary consumers, such as snails and insect larvae: 1103 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, r
  • Secondary consumers, such as fish and large insects: 111 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, r
  • Tertiary consumers, such as large fish and snakes: 5 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, r
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:
T, r, a, n, s, f, e, r, space, e, f, f, i, c, i, e, n, c, y, equals start fraction, 1103, space, space, k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, r, divided by, 7618, space, space, k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, r, end fraction, times, 100
T, r, a, n, s, f, e, r, space, e, f, f, i, c, i, e, n, c, y, 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:
Let's take a closer look at where the energy goes as it moves through the Silver Springs ecosystem. Energy flow is diagrammed in the chart below.
Image modified from "Energy flow: Figure 2," by OpenStax College, Biology, CC BY 4.0.
For instance, the uppermost box shows that primary producers capture 20, comma810 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, e, a, r of solar energy (gross primary productivity), but use most of this (13, comma187 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, e, a, r) for cellular respiration and body maintenance – energy that's dissipated as heat. The net primary productivity is the rate of primary productivity once this cellular respiration is accounted for: 7618 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, e, a, r.
So, we have 7618 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, e, a, r of energy captured in the bodies of plants, available to get eaten by primary consumers. However, most of this energy is not taken up by the bodies of primary consumers – 4250 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, e, a, r pass to decomposers, either as dead plant matter or as the feces of the primary consumers (herbivores). The remaining 3368 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, e, a, r are taken up and used by primary consumers (gross productivity), but only 1103 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, e, a, r are stored as biomass (net productivity), with 2265 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, e, a, r dissipated as heat in respiration.
We can see this pattern repeating as we move downward through the flow chart (to progressively higher trophic levels). Most of the net productivity of any level is lost to decomposers, and most of the gross productivity of any level goes to cellular respiration and maintenance, not net productivity. In the end, all of the energy captured through photosynthesis is dissipated as heat.

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 k, c, a, l, slash, m, start superscript, 2, end superscript, slash, y, r, or biomass units, such as g, slash, m, start superscript, 2, end superscript, slash, y, r.
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.
That's a good point! Heat is not useless for endothermic animals like humans, which generate metabolic heat to maintain a steady internal body temperature.
However, in a thermodynamics sense, heat is "not useful" because it represents energy that cannot be fully converted into other, work-performing types of energy. In most biological contexts, is not used to do work at all, and just goes to increase the entropy (disorder) of the surroundings.
You can learn more in the tutorial on laws of thermodynamics.

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.
Each type of pyramid provides slightly different information about an ecosystem and how energy is stored in, and moves through, that ecosystem's trophic levels. There is no one "best pyramid," and the pyramid's that's most useful will depend on what question we are asking about the ecosystem.

Podsumowanie

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%\sim10\% 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.

Autorstwo

Ten artykuł został napisany na podstawie następujących źródeł:
Zmodyfikowany artykuł może być używany zgodnie z licencją CC BY-NC-SA 4.0.

Cytowane prace:

  1. Jan A. Nilsson, "Energy Flow Through Ecosystems," General Biology Hub, accessed June 11, 2016, http://www.desertbruchid.net/4_GB2_LearnRes_fa11_f/4_GB2_LearnRes_Web_10Ecol.html.
  2. Valiela, Ivan. "Factors Affecting Primary Production." In Marine Ecological Processes, 36-83. New York: Springer, 1995. http://dx.doi.org/10.1007/978-1-4757-4125-4_2.
  3. Howard T. Odum, "Trophic Structure and Productivity of Silver Springs, Florida," Ecological Monographs 27, no. 1 (1957): 106-107, https://www.jstor.org/stable/1948571?seq=1#page_scan_tab_contents.
  4. F. Stuart Chapin III, Pamela A. Matson, and Harold A. Mooney, "Trophic Dynamics," in Principles of Terrestrial Ecosystem Ecology (New York: Springer-Verlag, 2002), 250-251.
  5. Peter H. Raven, George B. Johnson, Kenneth A. Mason, Jonathan B. Losos, and Susan R. Singer, "The Flow of Energy in Ecosystems," in Biology, 10th ed., AP ed. (New York: McGraw-Hill, 2014), 1216.

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