LWE: Lesson - Ecosystem Energy Flow (Topic 1.10) đź“–
⏳ Estimated Reading/Watching Time: 7 - 9 minutes
Determine how the energy decreases as it flows through ecosystems.
Ecosystem Energy Flow
The flow of energy through our biosphere provides the basis for life-sustaining chemical reactions and other processes necessary for life on Earth.
Energy enters most ecosystems as light energy from the Sun. This energy is converted into chemical energy by producers, which demonstrates the First Law of Thermodynamics. The First Law of Thermodynamics states that energy can neither be created nor destroyed. In other words, "you can't get something from nothing." The chemical energy in plants moves into herbivores, then carnivores, and up the food chain. Energy can only flow in one direction through an ecosystem. It cannot flow backward.
The Sun provides plenty of energy – much more than we can ever use. It is also constant – we do not have to worry if the Sun will be working each day. Even on cloudy days, the Sun is still radiating energy to the Earth. The only time sunlight is limiting is during the winter and plants are adapted to this. In the winter, when the days get short, plants go dormant and lose their leaves. As the weather warms and the days lengthen, their leaves return so they can photosynthesize again.
Photosynthesis is a process where plants convert light energy from the sun into chemical energy. However, photosynthesis only captures 2% of incoming solar energy. Animals and plants use 60-90% of their food for energy. The remaining 10-40% is converted to body tissues and allows the organism to grow. If you’ve felt an animal, you know that animals are usually warmer than their environment. This is because cellular respiration, metabolism, and other cellular processes create heat energy, which cannot re-enter living organisms.
When an organism eats or a plant produces energy, only a fraction of that energy is passed on to the next trophic level. Because of this, much of the preceding trophic level is biomass that is not consumed. Much of what is consumed is used as energy to fuel the organism’s cells and tissues. Some is also undigested and passes through the organism as waste.
In general, when a consumer eats something, it receives about 10% of the total energy available to the previous trophic level. So, for example, if the producers have 1000 kJ of energy available to them, the primary consumers will have 100 kJ of energy available to them, the secondary consumers will have 10 kJ available to them, and the tertiary consumers will only have 1 kJ of energy available. You can see that as you move up the trophic pyramid, the organisms quickly run out of energy. This is why there are so many more producers in an ecosystem than tertiary consumers.
The loss of energy that occurs when energy moves from lower to higher trophic levels can be explained through the laws of thermodynamics.
All ecosystems depend on a continuous inflow of high-quality energy in order to maintain their structure and function of transferring matter between the environment and organisms via biogeochemical cycles.
The 10% Rule
For ease of calculating the energy available to each trophic level, we use something called the 10% Rule, which states that only about 10% of the energy available in a trophic level is available to the next trophic level.
Because only a fraction (10%) of the energy in each trophic level is available to the next trophic level, individuals at higher levels in the biomass pyramid represent a greater amount of the Sun’s energy for the same amount of body tissue. This means that top-order consumers require more water and other resources and take longer to create.
If you have an ecosystem that has 1000 kg of primary producers…
- Only 100 kg of that biomass will be available to herbivores, or primary consumers…
- Only 10 kg of that biomass will be available to secondary consumers…
- Only 1 kg of that biomass will be available to tertiary consumers…
You can see that we quickly run low on energy. This is why many terrestrial food chains tend to only have 3-4 links – it would take too much energy to sustain a quaternary consumer.
Watch the video below to learn how to calculate the amount of energy lost at each trophic level:
As you can see, losing 90% of the energy available in each trophic level explains why food chains are usually relatively short - eventually, there is not enough energy left to sustain another link in the chain. This is why environmental scientists often suggest that humans eat lower on the food chain to conserve energy. For example, it takes much less energy to create corn or wheat than it does to create the same amount of meat.
The 10% rule approximates that in the transfer of energy from one trophic level to the next, only about 10% of the energy is passed on.
You will need to know how to calculate the energy lost to each trophic level using the 10% Rule.
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