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Unlock This CourseEvery living thing needs energy to survive, grow and reproduce. In nature, energy flows through ecosystems in a very organised way, passing from one organism to another through feeding relationships. This creates what we call trophic levels - different feeding positions in a food chain or web.
Think of it like a giant energy highway where the sun provides the fuel, plants capture it and animals pass it along by eating each other. But here's the catch - not all energy makes it to the next level. Most gets lost along the way!
Key Definitions:
These are the foundation of all food chains - mainly green plants, algae and some bacteria. They make their own food using sunlight through photosynthesis. Examples include grass, trees, seaweed and phytoplankton. They capture about 1-2% of the sun's energy that reaches Earth.
Imagine a pyramid where each level gets smaller as you go up. That's exactly how trophic levels work! Each level contains less energy and fewer organisms than the one below it.
Also called herbivores, these animals eat only plants. Examples: rabbits, deer, caterpillars, zebras. They form the second trophic level and get their energy directly from producers.
These are carnivores that eat primary consumers. Examples: foxes eating rabbits, birds eating caterpillars, lions eating zebras. They occupy the third trophic level.
Top predators that eat secondary consumers. Examples: eagles, sharks, polar bears. They're at the fourth trophic level and often have no natural predators when adult.
Here's where it gets really interesting - and a bit wasteful! When energy moves from one trophic level to the next, only about 10% actually gets transferred. The other 90% is lost through:
This is why food chains rarely have more than 4-5 trophic levels - there simply isn't enough energy left to support another level!
Let's follow energy through an African savanna: Grass (producer) → Zebra (primary consumer) → Lion (secondary consumer) → Decomposers. If the grass captures 10,000 units of energy, zebras only get 1,000 units, lions get just 100 units and decomposers get the final 10 units. This explains why there are millions of grass plants, thousands of zebras, but only dozens of lions in the same area.
Not all organisms fit neatly into one trophic level. Nature is more complex and interesting than that!
These flexible feeders eat both plants and animals, so they can occupy multiple trophic levels. Humans are perfect examples - we eat vegetables (making us primary consumers) and meat (making us secondary or tertiary consumers). Bears, pigs and many birds are also omnivores.
Decomposers deserve special mention because they're absolutely crucial but often forgotten. These include bacteria, fungi and some insects that break down dead organisms and waste products.
They don't fit into the traditional trophic pyramid because they operate at every level, breaking down dead producers, consumers and even other decomposers. Without them, dead material would pile up everywhere and nutrients would never return to the soil for plants to use again.
Microscopic decomposers that break down organic matter into simple chemicals. They work incredibly fast and are found everywhere.
Including mushrooms and moulds, fungi release enzymes that break down dead material externally before absorbing the nutrients.
Animals like earthworms, woodlice and some beetles that eat dead organic matter and help break it down physically.
While food chains are useful for understanding energy flow, real ecosystems are much more complex. Most animals eat multiple types of food and most can be eaten by several different predators. This creates a food web - an interconnected network of feeding relationships.
Food webs show us how removing one species can affect many others. If rabbits disappeared from a grassland, foxes might switch to eating more birds, which could lead to an increase in the insects that birds normally eat. This is called a cascade effect.
Understanding these connections helps us predict what might happen when:
When wolves were reintroduced to Yellowstone National Park in 1995, they didn't just affect deer populations. Fewer deer meant more vegetation growth, which supported more birds and beavers. Beaver dams created wetlands that supported fish, amphibians and waterfowl. Even rivers changed course because of increased vegetation along banks. This shows how top predators can influence entire ecosystems!
Scientists measure energy flow in ecosystems using several methods. The most common unit is kilojoules per square metre per year (kJ/m²/year). This helps us compare different ecosystems and understand their productivity.
Shows the number of organisms at each trophic level. Usually pyramid-shaped, but can be inverted in some ecosystems (like when one tree supports thousands of insects).
Shows the total mass of organisms at each level. Always pyramid-shaped because energy loss means less biomass can be supported at higher levels.
Humans have a massive impact on energy flow in ecosystems. We often act as top predators, but we also:
Understanding trophic levels helps us make better decisions about conservation and sustainable use of natural resources. For example, protecting primary producers (like forests) is often more effective than trying to save individual top predators.
It takes about 10kg of grass to produce 1kg of beef! This is because of energy loss between trophic levels. That's why eating lower on the food chain (more plants, less meat) is more energy-efficient and environmentally friendly.