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Unlock This CourseImagine you're at a party where everyone has to pass chocolate bars to the next person, but they're allowed to eat 90% and only pass on 10%. By the time the chocolate reaches the fourth person, there's barely a crumb left! This is exactly what happens with energy in ecosystems - and it's why we get pyramid-shaped diagrams when we measure energy flow.
Key Definitions:
Only about 10% of energy from one trophic level gets transferred to the next level. The other 90% is lost through movement, heat production, waste and life processes like breathing and digestion. This is why energy pyramids always get smaller as you go up!
Energy enters ecosystems through producers (mainly plants) that capture sunlight through photosynthesis. From there, it flows through the food chain, but most gets lost at each step. Let's explore this process in detail.
Think of energy transfer like a leaky bucket system. Each time you pour water from one bucket to another, most of it spills out, leaving only a small amount for the next bucket.
Capture about 1% of sunlight energy through photosynthesis. The rest is reflected or converted to heat. Plants use most captured energy for growth, reproduction and cellular processes.
Herbivores like rabbits get only 10% of the plant's energy. They lose energy through movement, maintaining body temperature and producing waste. Much plant material can't be digested.
Carnivores receive just 1% of the original plant energy (10% of 10%). Energy continues to decrease as predators hunt, maintain territories and regulate body functions.
In the African savanna, grass produces 10,000 units of energy. Zebras get 1,000 units, lions get 100 units and decomposers get the remaining energy from waste and dead organisms. This explains why there are millions of grass plants, thousands of zebras, but only dozens of lions in the same area.
Unlike pyramids of numbers or biomass (which can sometimes be inverted), energy pyramids are always true pyramid shapes. This is because energy cannot be recycled - it flows in one direction and is constantly being lost as heat.
Energy disappears from food chains through several unavoidable processes:
All living processes produce heat as a waste product. Warm-blooded animals lose huge amounts of energy maintaining constant body temperature. Even cold-blooded animals lose energy as heat during metabolism.
Every movement uses energy - from a plant growing towards light to a cheetah chasing prey. Predators especially use lots of energy hunting, which is why they need large territories.
Not all food can be digested. Cellulose in plants, bones in animals and other materials pass through as waste, taking their stored energy with them. This energy feeds decomposers instead.
Breathing, circulation, brain function and cellular repair all require constant energy input. These essential processes consume most of an organism's energy budget before any is available for growth.
Ecologists use three types of pyramids to understand ecosystems. Each tells us something different about how energy and matter flow through food chains.
Always pyramid-shaped. Show energy flow and explain why food chains are short. Most accurate representation of ecosystem function.
Usually pyramid-shaped but can be inverted in aquatic ecosystems where small, fast-reproducing phytoplankton support larger zooplankton.
Often inverted. One large tree can support thousands of insects, or one predator might eat hundreds of small prey animals.
In ocean ecosystems, tiny phytoplankton (producers) have very short lifespans but reproduce rapidly. Their total biomass might be smaller than the zooplankton that eat them at any given moment, creating an inverted biomass pyramid. However, the energy pyramid remains normal because phytoplankton produce energy much faster than zooplankton consume it.
The 10% rule has massive implications for how ecosystems work and why they're structured the way they are.
Most food chains have only 4-5 trophic levels because energy becomes too scarce to support higher levels. After five transfers, only 0.001% of the original energy remains - not enough to sustain a viable population of top predators.
Humans get more energy by eating plants directly rather than feeding them to animals first. This is why vegetarian diets can support larger human populations on the same amount of land.
Large predators like eagles, tigers and sharks are naturally rare because they need enormous territories to find enough prey. This makes them vulnerable to extinction when habitats shrink.
Understanding energy transfer calculations helps us predict ecosystem capacity and understand conservation challenges.
Given: Oak trees capture 100,000 kJ of energy per square metre per year.
Calculate: Energy available to each trophic level
Solution:
• Producers (oak trees): 100,000 kJ
• Primary consumers (caterpillars): 10,000 kJ (10%)
• Secondary consumers (birds): 1,000 kJ (10% of 10%)
• Tertiary consumers (hawks): 100 kJ (10% of 1%)
This explains why you see thousands of caterpillars, hundreds of birds, but only a few hawks in a woodland.
While 10% is the average, actual transfer efficiency varies depending on several factors:
Cold-blooded animals are more efficient because they don't waste energy maintaining body temperature. Warm-blooded animals lose 80-90% of energy as heat.
Carnivores are more efficient than herbivores because meat is easier to digest than plant material. Herbivores struggle with cellulose and lignin in plants.
Human activities significantly affect energy flow in ecosystems, often reducing efficiency and disrupting natural patterns.
Chemical pollutants reduce photosynthesis efficiency in plants and increase energy costs for animals trying to detoxify harmful substances. This reduces energy available for growth and reproduction.
Breaking up habitats forces animals to use more energy travelling between food sources and suitable territories. This reduces the energy available for the next trophic level.