Organisms and their Environment: Energy Flow, Cycles, and Populations
Ecosystem Explorer: Follow the Energy 🌍🌱
Introduction
1. Introduction
Okay, let's talk ecology. This is where everything connects up: photosynthesis, respiration and nutrition all show up here as the way energy and nutrients move through nature. Think of an ecosystem like a giant game economy, energy is the currency, and it's constantly getting spent.
Here's the whole chapter in two lines: energy flows one way and keeps getting lost (so the Sun has to keep topping it up), while nutrients like carbon and nitrogen go round in circles and get reused for ever. Energy = a straight line. Nutrients = a loop. Lock that in and let's go 🌍
Here's the whole chapter in two lines: energy flows one way and keeps getting lost (so the Sun has to keep topping it up), while nutrients like carbon and nitrogen go round in circles and get reused for ever. Energy = a straight line. Nutrients = a loop. Lock that in and let's go 🌍
2. Energy Flow: The Sun Runs Everything
Almost every living thing on Earth is running on sunlight. Green plants (the producers) catch a little of the Sun's light energy and turn it into chemical energy stored in glucose, that's photosynthesis. From there the energy gets passed along when things eat each other: plant → herbivore → carnivore. But here's the catch: at every single step, most of the energy leaks away as heat (from respiration and moving around), so it shrinks fast.
That's why it's a one-way street. Carbon atoms get recycled, but energy doesn't, once it's heat, it's gone from the ecosystem for good. 🔥
That's why it's a one-way street. Carbon atoms get recycled, but energy doesn't, once it's heat, it's gone from the ecosystem for good. 🔥
Key idea🔑 Key idea: the Sun is the principal energy source. Producers turn light energy into chemical energy by photosynthesis, and energy is lost as heat at every step.
Worked example
Worked Example: Trace the Energy
Worked Example: Follow the Currency 💸
Which sequence correctly shows how energy from the Sun is transferred into an organism in an ecosystem?
- 1Start at the source: it's light energy from the Sun. Nothing living made that energy, it arrived as sunlight.
- 2A producer catches the light and converts it into chemical energy stored in food, using photosynthesis.
- 3That chemical energy then passes to a consumer when it eats the plant. So the order is light → producer → chemical energy → consumer. Any answer where a consumer soaks up sunlight directly is wrong, only producers can do that.
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3. Producers vs Consumers: Know Your Roles
Every organism has a role, and the exam wants the exact word. A producer makes its own food using sunlight (plants, algae). A consumer eats other organisms. A herbivore eats plants; a carnivore eats animals; a decomposer (bacteria and fungi) feeds on dead and waste material. Consumers get numbered by their spot: the primary consumer eats the plant, the secondary consumer eats the primary consumer, and so on.
The producer always comes first, because it's the only one that can bring new energy into the chain from outside (sunlight). Everyone else lives on second-hand energy. 🌱
The producer always comes first, because it's the only one that can bring new energy into the chain from outside (sunlight). Everyone else lives on second-hand energy. 🌱
Key idea🔑 Key idea: producer (makes food by photosynthesis), consumer (eats others), herbivore (eats plants), carnivore (eats animals), decomposer (eats dead/waste). A primary consumer is always a herbivore.
Worked example
Worked Example: Where Energy Gets In
Worked Example: The Entry Point 🚪
In a food chain, energy from the Sun first enters the living part of an ecosystem through one type of organism. Which type is it?
- 1Ask which organism can actually catch sunlight. Only the one that photosynthesises, the producer (a green plant or alga).
- 2Herbivores, carnivores and decomposers all get their energy by eating, so they're all second in line. None of them touches sunlight directly.
- 3So energy enters through the producer. That's exactly why every food chain is drawn starting with a plant.
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4. Food Chains and Trophic Levels
A food chain shows energy being passed along, and the arrows matter: each arrow points from the thing being eaten to the thing eating it. Read it as "is eaten by". So "grass → grasshopper → vole → hawk" means grass is eaten by the grasshopper, which is eaten by the vole, and so on.
Each position has a trophic level number: level 1 is the producer, level 2 is the primary consumer (a herbivore), level 3 is the secondary consumer, level 4 the tertiary consumer. "Herbivore", "primary consumer" and "trophic level 2" are all the same spot, just different names for it.
Each position has a trophic level number: level 1 is the producer, level 2 is the primary consumer (a herbivore), level 3 is the secondary consumer, level 4 the tertiary consumer. "Herbivore", "primary consumer" and "trophic level 2" are all the same spot, just different names for it.
Key idea🔑 Key idea: arrows point towards the eater (energy flow direction). Trophic level 1 = producer, level 2 = herbivore, level 3 = carnivore, and so on.
Worked example
Worked Example: Build the Chain
Worked Example: Assemble the Squad 🔗
A student has these meadow organisms: hawk, grass, vole and grasshopper. Arrange them into one food chain with the arrows pointing correctly.
- 1Find the producer, it always goes first. Only the grass makes its own food, so grass starts the chain.
- 2Now work out who eats whom: grasshopper eats grass, vole eats grasshopper, hawk eats vole.
- 3Double-check the arrows all point towards the eater. "hawk → vole" would mean the vole eats the hawk, which is nonsense, so keep them pointing the right way.
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5. Food Webs: Everything's Connected
In real life nothing eats just one thing. A food web links loads of food chains together to show all the feeding at once. The classic exam move is: "if this animal disappears, what happens to that one?" You answer it by following the arrows one step at a time.
Remember the ripple effect: remove a predator and its prey usually increase; remove a prey and its predators drop (or switch to other food). A change can spread to species that never even ate the one that changed. 🌊
Key idea🔑 Key idea: a food web is many food chains joined up. To predict a knock-on effect, follow the arrows one link at a time.
Worked example
Worked Example: Knock On Wood
Worked Example: The Domino Effect 🎳
In a coastal food web, kelp is eaten by sea urchins; sea urchins are eaten by sea otters. Sea otters are hunted and their numbers crash. What happens to the kelp?
- 1Otters eat sea urchins, so fewer otters means fewer urchins get eaten, and the urchin population rises.
- 2Sea urchins eat kelp, so more urchins means more kelp gets eaten.
- 3So the kelp population falls, even though otters never touch kelp directly. It's a two-step chain reaction: otter down → urchins up → kelp down.
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6. The 10% Rule: Energy Leaks Everywhere
Here's the big number: only about 10% of the energy at one trophic level makes it to the next, and about 90% is lost. Where does it go? Mostly heat from respiration and movement, plus energy in undigested waste (faeces) and excretion.
This one fact explains two big things. First, food chains are short (usually 4-5 levels max), because after a few steps there's just too little energy left to feed anyone higher up. Second, it's why eating plants is more efficient than eating meat: eat crops directly and you get ~10% of their energy; feed the crops to cattle first and you lose another 90% in the animal, so way less energy reaches you.
This one fact explains two big things. First, food chains are short (usually 4-5 levels max), because after a few steps there's just too little energy left to feed anyone higher up. Second, it's why eating plants is more efficient than eating meat: eat crops directly and you get ~10% of their energy; feed the crops to cattle first and you lose another 90% in the animal, so way less energy reaches you.
Key idea🔑 Key formula: about 10% of energy passes to the next trophic level; about 90% is lost (respiration/heat, movement, waste). Shorter chain = less energy wasted.
Worked example
Worked Example: Do the 10% Math
Worked Example: Energy Budget 🧮
A grassland ecosystem has 20 000 kJ of energy at the producer level in one year. Using the 10% rule, how much energy reaches the secondary consumers?
- 1Go up one level (producers → primary consumers), take 10%:
- 2Go up another level (primary → secondary consumers), take 10% again:
- 3So the secondary consumers get about 200 kJ. The trick: the secondary consumer is two steps up, so you take 10% twice, don't stop after one.
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7. Ecological Pyramids: Draw the Levels
A pyramid shows each trophic level as a bar, producer at the bottom. Three types, and Extended wants you to compare them. A pyramid of numbers counts organisms, but it can go upside-down (one oak tree feeds thousands of aphids, so the bottom bar is tiny). A pyramid of biomass measures the dry mass instead, which usually fixes the shape. A pyramid of energy measures energy per level, and because energy always drops going up, it's never upside-down and it's the most accurate.
Key idea🔑 Key idea: pyramid of numbers (can be inverted), biomass (usually a true pyramid), energy (never inverted, most accurate). Energy pyramids win because energy always decreases up the chain.
Worked example
Worked Example: The Upside-Down Pyramid
Worked Example: Flip It 🙃
Zebras eat grass, many tiny blood-sucking insects live on the zebras, and birds eat the insects. What shape is the pyramid of numbers, and why would a pyramid of biomass look different?
- 1Count the organisms: a few big zebras, then a huge number of tiny insects living on them, then fewer birds.
- 2So the pyramid of numbers is inverted in the middle, a narrow zebra bar under a super-wide insect bar. It looks weird because numbers ignore that a zebra is massive compared to an insect.
- 3A pyramid of biomass uses total mass, and the few big zebras weigh way more than all the insects combined, so it goes back to a normal pyramid shape. That's biomass fixing what numbers got wrong.
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8. The Carbon Cycle: Round and Round
Unlike energy, carbon gets recycled forever. Photosynthesis pulls carbon dioxide out of the air and into plants. Respiration (in plants, animals AND decomposers) puts it back. Feeding passes carbon along food chains. Decomposition returns it when decomposers break down dead stuff and respire. And combustion (burning fossil fuels like coal, oil and gas) dumps loads of long-buried carbon straight back into the air.
The reason CO₂ levels are climbing? We're burning fossil fuels way faster than photosynthesis can soak the carbon back up.
The reason CO₂ levels are climbing? We're burning fossil fuels way faster than photosynthesis can soak the carbon back up.
Key idea🔑 Key idea: photosynthesis removes CO₂; respiration, decomposition and combustion return it. Burning fossil fuels is why atmospheric CO₂ is rising.
Worked example
Worked Example: In and Out of the Air
Worked Example: CO₂ Traffic 🚦
In the carbon cycle, which process removes carbon dioxide from the atmosphere, and which returns it from plants, animals and microorganisms alike?
- 1Only one process pulls CO₂ out of the air on a big scale: photosynthesis, done by plants using light.
- 2The process that puts CO₂ back, and is done by plants, animals and microbes without exception, is respiration. Every living thing respires.
- 3So: removed by photosynthesis, returned by respiration. (Combustion also adds CO₂, but that's burning, not something living organisms do.)
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9. The Nitrogen Cycle: Bacteria Run the Show
Living things need nitrogen for proteins, and the air is 78% nitrogen gas, but plants and animals can't use it directly. Bacteria save the day. Nitrogen fixation (by nitrogen-fixing bacteria in soil and bean/pea roots, plus lightning) turns nitrogen gas into usable nitrates. Plants absorb those nitrates and build proteins. When things die, decomposition breaks proteins into ammonium ions. Nitrification (nitrifying bacteria, needs oxygen) turns ammonium into nitrates. And denitrification (denitrifying bacteria, in waterlogged soil) turns nitrates back into nitrogen gas, which is bad news for farmers.
Key idea🔑 Key idea: the four bacteria are nitrogen-fixing (N₂ → nitrates), decomposers (proteins → ammonium), nitrifying (ammonium → nitrates), and denitrifying (nitrates → N₂). Fixation brings nitrogen in; denitrification sends it out.
Worked example
Worked Example: Lightning and Bacteria
Worked Example: Two Ways In ⚡
Which process in the nitrogen cycle can be caused by both lightning and bacteria?
- 1Look for the step that turns nitrogen gas from the air into usable nitrogen compounds, that's the one both can do.
- 2Nitrogen-fixing bacteria do it biologically, and lightning does it too (its energy makes nitrogen and oxygen react in the air). Both give the same result: N₂ → usable compounds.
- 3So the answer is nitrogen fixation. Nitrification and denitrification are bacteria-only, lightning can't do those.
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10. Populations and Growth Curves
Quick vocab: a population is one species in an area (all the perch in a lake), a community is all the species together, and an ecosystem is the community plus its non-living surroundings (water, soil, gases). Population growth depends on food, competition, predation and disease.
When a population grows with limited resources it makes an S-shaped (sigmoid) curve: a slow lag phase, a rocketing exponential (log) phase while resources are plentiful, then a stationary phase where it levels off at the carrying capacity because food runs short, competition and disease rise, and the birth rate drops to equal the death rate.
When a population grows with limited resources it makes an S-shaped (sigmoid) curve: a slow lag phase, a rocketing exponential (log) phase while resources are plentiful, then a stationary phase where it levels off at the carrying capacity because food runs short, competition and disease rise, and the birth rate drops to equal the death rate.
Key idea🔑 Key idea: population = one species; community = all species; ecosystem = community + environment. The sigmoid curve goes lag → exponential/log → stationary (carrying capacity, birth rate = death rate).
Worked example
Worked Example: Read the Curve
Worked Example: Name That Phase 🎯
A population growth curve has region 1 (slow start), region 2 (steeply rising) and region 3 (levels off). Name each phase and say why regions 2 and 3 look like they do.
- 1Region 1 (slow start) is the lag phase: only a few individuals, still getting established, so numbers rise slowly.
- 2Region 2 (steep) is the exponential/log phase: loads of food and space, little competition, so almost every offspring survives and numbers shoot up.
- 3Region 3 (flat) is the stationary phase: resources run short, competition and disease rise, so the birth rate drops to match the death rate and the population holds steady at the carrying capacity. (It's still breeding, births and deaths are just equal.)
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