18.1 Energy Flow and Productivity
Key Takeaways
Energy flows one way through an ecosystem and leaves as heat, while matter cycles so the same atoms are reused.
Producers are autotrophs: most capture light, and chemoautotrophs capture chemical energy instead.
Gross primary productivity is all the energy producers fix; net primary productivity is what remains after their respiration and is available to consumers.
A food chain is one feeding path and a food web is the real network; about 10 percent of energy reaches the next trophic level only as an approximation.
Biomass pyramids can be inverted in some aquatic systems, but energy pyramids are not, and decomposers recycle atoms rather than sunlight.
18.1 Energy Flow and Productivity
Energy and matter do not follow the same path through an ecosystem. Energy flows in one direction and leaves as heat. Matter cycles, so the same atoms of carbon, nitrogen, phosphorus, and water are used again and again. This section follows the energy. The atoms come back in the cycles that follow. Usable energy does not come back as light.
Producers capture energy once
Producers, also called autotrophs, build organic molecules from inorganic raw materials. Most producers are photoautotrophs. Green plants, algae, and cyanobacteria use pigments such as chlorophyll to capture light and store part of that energy in sugars. A smaller group, the chemoautotrophs, does not need sunlight for the capture. They oxidize inorganic chemicals such as hydrogen sulfide or ammonia and use the energy released to fix carbon. Chemoautotrophs are the producers at deep-sea vents and in some soils and hot springs. Whether the source is light or a chemical reaction, the producer is the entry point for energy that the rest of the community can eat.
Productivity measures that capture in a given area over a given time. Gross primary productivity is the total energy producers fix. It includes everything they capture, before they spend any of it. Producers are alive, so they respire. Cellular respiration uses a large share of the sugars they just made and releases that share as heat. Net primary productivity is what remains after the producers' own respiration. It is the energy stored in new leaves, wood, roots, and algal cells. That remainder is what is available to consumers. A forest can show high gross productivity and still offer herbivores only a modest net figure if the trees respire most of what they fix.
Warm, wet, well-lit places such as tropical rainforests tend to have high net primary productivity. Deserts, and the open ocean per square meter, are often low. When a question asks how much food consumers can take, answer with net primary productivity. When it asks how much energy producers fixed before their own respiration, answer with gross primary productivity.
Consumers and the detrital path
Consumers, or heterotrophs, take in organic molecules that another organism already built. Herbivores eat producers and are the primary consumers. Carnivores eat other animals. A carnivore that eats a herbivore is a secondary consumer, and a carnivore that eats that animal is a tertiary consumer. Omnivores eat both plant and animal matter, so one species can feed at more than one consumer step depending on the meal. A bear eating berries is acting as a primary consumer. The same bear eating a fish is acting higher in the web.
Detritivores and decomposers feed on dead bodies, leaf litter, and wastes. Earthworms, many insects, and some crabs shred detritus. Fungi and bacteria finish the chemical breakdown and release inorganic nutrients into soil and water. A forest is not buried in dead wood because of that work. Detritivores and decomposers recycle atoms. They are not a trophic level that recycles energy back to sunlight. The energy they spend in respiration still leaves as heat.
Chains, webs, and pyramids
A food chain is a single path, such as grass to grasshopper to frog to snake. It is a useful sketch and a poor portrait of a community. A food web is the real network of overlapping paths. One plant feeds several herbivores. One predator takes prey from more than one level. Omnivores and decomposers tie the lines together. A web shows why removing one species can ripple in more than one direction.
Ecologists compress a web into ecological pyramids. A pyramid of energy shows how much energy passes through each trophic level over a period of time. A pyramid of biomass shows the dry mass of organisms present. A pyramid of numbers counts individuals. In the usual introductory rule of thumb, only about 10 percent of the energy at one trophic level is transferred to the next. The rest is lost as heat from respiration, left in material nobody ate, or left in material that was eaten but not assimilated. The 10 percent figure is an approximation, not a law for every ecosystem. Some transfers are closer to 5 percent, and some are higher. The pattern is still steep, because much of the energy is spent on maintenance and lost as heat, and some material is never eaten. Top predators stay uncommon compared with plants.
A worked energy pyramid
Picture a meadow whose producers store 10,000 kilocalories of net production in a season. Apply the approximation. Primary consumers account for about 1,000 kilocalories. Secondary consumers account for about 100 kilocalories. A tertiary level, if the meadow supports one, holds only about 10 kilocalories. That patch of ground cannot feed a large group of top carnivores on local production alone.
| Trophic level | Who is feeding | Energy in the worked meadow |
|---|---|---|
| Producers | Grasses and wildflowers | 10,000 kcal |
| Primary consumers | Grasshoppers and voles | about 1,000 kcal |
| Secondary consumers | Shrews and spiders that eat those herbivores | about 100 kcal |
| Tertiary consumers | Hawks that eat the shrews | about 10 kcal |
Biomass pyramids usually narrow toward the top, but they can be inverted in some aquatic systems. Phytoplankton may weigh less, at one moment, than the zooplankton eating them, because the algae grow and are eaten quickly. The energy pyramid for that same water is not inverted. Over time, producers still handle more energy than consumers store. A numbers pyramid can look odd on land as well. One oak may feed thousands of insects, so a count of individuals seems upside down. Add the energy and the pyramid is upright again.
Warning
Detritivores and decomposers recycle atoms from dead matter and wastes. They do not send usable energy back to producers as sunlight. An inverted biomass pyramid does not invert the energy pyramid.
Most communities support about three to five trophic levels because each transfer loses usable energy as heat.
Which statement best describes energy and matter in an ecosystem?
Energy moves one way through organisms and leaves as heat, while atoms of matter are cycled and reused.
Matter is lost as heat at each trophic level, while energy cycles through rock and air without loss.
A food chain returns usable light to plants, and a food web stops matter from being reused.
Decomposers convert heat back into sunlight so producers can reuse the same energy.
What is net primary productivity?
The heat already lost from secondary consumers, fixed at exactly 10 percent in every ecosystem.
The total energy producers fix before they pay the cost of their own respiration.
The sunlight decomposers rebuild from dead leaves and animal wastes.
The energy left in producer biomass after producers respire, which is the energy available to consumers.
A lake has more zooplankton biomass than phytoplankton biomass at one moment. Which conclusion is sound?
The energy pyramid must be inverted, because consumers hold more energy than producers captured.
A biomass pyramid can be inverted in some aquatic systems, while the energy pyramid stays upright.
Decomposers have recycled consumer heat into sunlight and rebuilt the producer level.
The 10 percent transfer is a law, so an aquatic biomass pattern like this cannot occur.
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