EPHM7110 Chap.2 Energy Flow and the Loss at Every Trophic Step
Energy Flow and the Loss at Every Trophic Step
One source in, one form out
Solar radiation reaching the troposphere arrives as visible light, infrared and a little ultraviolet. It warms the atmosphere, evaporates and recycles water, generates winds and supports plant growth. What leaves the system is heat.
Between those two facts sits every feeding relationship the course describes, and the whole of it is governed by one figure: ninety per cent of the energy is lost at each transfer between trophic levels.
The series that explains more management decisions than any argument
The course sets out four levels with the usable energy at each: ten thousand kilocalories at the producers, one thousand at the primary consumers, one hundred at the secondary consumers and ten at the tertiary.
Biomass, the dry weight of all organic matter at a level, falls for the same reason, because both producers and consumers spend the energy stored in glucose on their own living processes and that use releases heat. The consequence is arithmetical rather than ethical.
A system can support a large mass of grazers or a small mass of top predators, and protection does not change the exchange rate between them.
Roles are positions, not properties of species
Producers are self feeders that obtain food from the environment, most by photosynthesis and some, in dark environments, by building sugars from simple compounds such as hydrogen sulphide.
Consumers obtain food by feeding on other organisms or their remains, and divide into plant eaters, predators and those that eat both. Decomposers and detritivores draw from every level and release nutrients back into the environment.
A trophic level is a position in a food chain rather than a fixed attribute, which is why an animal that eats both plants and animals occupies more than one at the same time.
Why a web and a chain give different answers
A chain is a single path and a teaching device. A web is the real structure, with several routes between any two points.
Ask what happens if one species is removed and the chain gives a mechanical answer while the web gives a conditional one: it depends how many alternative routes exist around the missing node. That route count, not the species count, decides whether a removal propagates through the system or is absorbed by it.
What this chapter covers
- 01
Solar energy as a one way flow, and heat as the only output
- 02
The four level energy series and the ninety per cent lost at each step
- 03
Biomass and why it falls at higher trophic levels
- 04
Producers, photosynthesis and chemosynthesis
- 05
Herbivores, carnivores and omnivores as positions rather than labels
- 06
Decomposers and detritivores drawing from every level
- 07
Food chains against food webs, and what route count decides
- 08
Aerobic and anaerobic respiration, and the methane question
Choose between two stocking proposals using the loss figure alone
- 2Count the transfers above the algae for each proposal.
- 2Apply the loss figure and state both energy figures.
- 2Say what the arithmetic does not settle.
Key terms
- Trophic Level
- A position in a food chain, counted from the producers upward.
- Producer
- A self feeding organism that obtains food from the environment, usually by photosynthesis.
- Chemosynthesis
- The building of complex nutrient compounds from simple compounds by specialised bacteria in dark environments.
- Consumer
- An organism that feeds on other living things, or on what is left of them.
- Decomposer
- An organism that obtains nutrients from the waste or remains of others and releases those nutrients back to the environment.
- Biomass
- The dry weight of all organic matter at one feeding level of a chain or web.
- Food Web
- The set of feeding relationships in a community, carrying several routes between any two points.
Energy Flow and the Loss at Every Trophic Step FAQ
Why do top predators stay rare even in a healthy system?
Because abundance follows the same tenfold fall as usable energy. Each step upward retains about a tenth of what the step below held, so the mass available to whatever feeds at the summit is a small fraction of what the plants fixed.
Scarcity there is the normal condition rather than a symptom of mismanagement, and it also explains why a target expressed as a number of large animals has to be read against how productive the system underneath them is.
Is it ever safe to reason about a removal using a single food chain?
Only as a first pass. The chain assumes one path, so it predicts that everything above a lost node loses its supply, which is the worst case rather than the likely one. Real communities carry several routes between the same two points, and whether a loss propagates depends on how many of those routes survive it.
Diet data for the animals concerned settles the question; abundance data does not, because it reports what is being eaten rather than what could be.
Why does methane come up whenever wetland restoration is proposed?
Waterlogged ground runs out of oxygen, and some organisms break down organic matter without it, producing methane among other end products. Restoring saturated conditions therefore restores that process along with everything else.
The reply that survives scrutiny accepts the point and sets it beside its mirror image, since the same lack of oxygen is what slows decay enough for peat to build up and hold carbon over long periods.
Exam move
Write the four energy figures out from memory, then take a food web from your own reading and mark which of its animals sit at more than one level. Finish by asking what a ten per cent transfer efficiency implies for a fishery that targets the top of that web.
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