Energy Transfers in Ecosystems (College Board AP® Environmental Science): Study Guide
The 10% rule
The 10% rule explains how energy decreases as it moves through trophic levels in an ecosystem
Approximately 10% of the energy from one trophic level is passed to the next level
Example: If producers capture 1,000 kcal of energy, only 100 kcal is available to primary consumers
The rest of the energy is lost as heat, used in metabolism, or remains in undigested parts of organisms
The total organic matter transferred from one trophic level to the next is never 100% because:
Not all the food available to a given trophic level is harvested
Of what is harvested, not all is consumed
Of what is consumed, not all is absorbed
Of what is absorbed, not all is stored
For example, if we take the example of caterpillars (the primary consumer) eating the leaves of an oak tree (the producer):
The caterpillars do not eat every leaf available to them:
There may simply be too many leaves, not enough caterpillars, or some leaves may be in locations that are difficult for the caterpillars to access
The caterpillars may not eat the entire leaf:
They might eat only the softer, more nutritious parts and leave behind tougher portions or parts with toxins
Once the caterpillars eat the leaves, not all of the nutrients are absorbed by their bodies:
Some parts of the leaves may be indigestible or contain compounds that the caterpillars cannot process, which are then egested by the caterpillars
When the caterpillars digest the leaves and convert the nutrients into energy, not all of the energy from the leaves is stored for growth and development:
This is because some of that energy is lost as heat during cellular respiration
Laws of thermodynamics
The loss of energy that occurs when energy moves from lower to higher trophic levels can be explained through the laws of thermodynamics
The first law of thermodynamics
Energy exists in many different forms, including light energy, heat energy, chemical energy, electrical energy and kinetic energy
There are two laws of thermodynamics
The first law of thermodynamics is as follows:
Energy cannot be created or destroyed; it can only be transformed from one form to another
This is also known as the principle of conservation of energy
It means that the energy entering a system equals the energy leaving it
It means that as energy flows through ecosystems, it can only change from one form to another
The transfer of energy in food chains within ecosystems demonstrates the principle of conservation of energy
Energy enters the system (the food chain or food web) in the form of sunlight
Producers convert this light energy into biomass (stored chemical energy) via photosynthesis
This chemical energy is passed along the food chain via consumers as biomass
All energy ultimately leaves the food chain, food web or ecosystem as heat energy
The second law of thermodynamics
The second law of thermodynamics states that:
Energy transfers in systems are inefficient
This means that energy transfers in any ecosystem are never 100% efficient
The second law of thermodynamics explains the decrease in available energy within ecosystems:
In a food chain, energy is transformed from a more concentrated (ordered) form (e.g. light energy from the Sun) into a more dispersed or disordered form (heat energy lost by organisms)
Initially, light energy from the Sun is absorbed by producers
However, even at this initial stage, energy absorption and transfer by producers is inefficient
This is due to reflection, transmission (light passing through leaves) and inefficient energy transfer during photosynthesis
The energy that is converted to plant biomass is then inefficiently transferred along the food chain due to respiration and the production of waste heat energy
In ecosystems, the biggest losses occur during cellular respiration
When energy is transformed, some must be degraded into a less useful form, such as heat
As a result of these inefficient energy transfers, food chains are often short (they rarely contain more than five trophic levels)
Ecological pyramids & the 10% rule
Ecological pyramids include:
Pyramids of numbers
Pyramids of biomass
Pyramids of energy (also known as pyramids of productivity)
They are quantitative models usually measured for a given time and area
Pyramids of numbers
A pyramid of numbers shows how many organisms exist at each level of a food chain
The width of the box indicates the number of organisms at that trophic level
For example, consider the following food chain:
grass → vole → owl
A pyramid of numbers for this food chain would look like the one shown below
Often, the number of organisms decreases along food chains, as there is a decrease in available energy since some energy is lost to the surrounding environment at each trophic level
Therefore, pyramids of numbers usually become narrower towards the apex (the top)
Despite the name, a pyramid of numbers doesn’t always have to be pyramid-shaped
For example, consider the following food chain:
oak tree → insects → woodpecker
The pyramids of numbers for this food chain will display a different pattern to the first food chain
When individuals at lower trophic levels are relatively large, like the oak tree, the pyramid becomes inverted
Only a single oak tree is needed to support large numbers of insects (which can then support large numbers of woodpeckers)
Pyramids of biomass
A pyramid of biomass shows how much mass the organisms at each trophic level would have without including all the water that is in the organisms:
This is known as their ‘dry mass’
Following the second law of thermodynamics, the quantities of biomass generally decrease along food chains, so the pyramids become narrower towards the top
If we take our first food chain as an example, it would be impossible to have 10 kg of grass feeding 50 kg of voles feeding 100 kg of barn owls
Pyramids of biomass are usually pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like
However, they can occasionally be inverted and show higher quantities at higher trophic levels
These inverted pyramids sometimes occur due to marked seasonal variations
For example, in some marine ecosystems, the standing crop of phytoplankton, the major producers, is lower than the mass of the primary consumers, such as zooplankton
This is because the phytoplankton reproduce very quickly and are constantly being consumed by the primary consumers, which leads to a lower-standing crop but higher productivity
This can occur because phytoplankton can vary greatly in productivity (and therefore biomass) depending on sunlight intensity
Pyramids of energy
Pyramids of energy (also referred to as pyramids of productivity) show the flow of energy through trophic levels, indicating the rate at which that energy is being generated
Pyramids of productivity illustrate the amount of energy or biomass of organisms at each trophic level per unit area per unit time
The length of each box, or bar, represents the quantity of energy present
These pyramids are always widest at the base and decrease in size as they go up
This is because pyramids of productivity for entire ecosystems over a year always show a decrease along the food chain, following the second law of thermodynamics
The base is wide due to the large amount of energy contained within the biomass of producers
As you move up the pyramid to higher trophic levels, the quantity of energy decreases as not all energy is transferred to the biomass of the next trophic level (roughly 10 % of the energy is passed on)
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