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 group chat, energy is the message getting passed along, and it fades a little every time someone forwards it.
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 carbon goes round in a circle and gets reused forever. Energy = a straight line. Carbon = a loop. Lock that in and let's go 🌍
Energy Flow: The Sun Runs EverythingC19.1
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, some of that energy leaks away as heat, mostly from respiration.
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: the Sun is the principal energy source. Producers turn light energy into chemical energy by photosynthesis, and energy is lost, mainly as heat from respiration, at every step.
Worked Example: Trace the EnergyC19.1
Problem What is the principal source of energy input to biological systems?
Step 1 Start at any animal and trace backwards: what did it eat, and what did that eat?
Step 2 Keep going and you always land on a plant, a producer that photosynthesises. $$\text{Sun} \rightarrow \text{producer (photosynthesis)} \rightarrow \text{chemical energy in food}$$
Step 3 So the original source, however many steps the chain has, is always the Sun. Not "photosynthesis" (that's the process), not "the soil" (that supplies raw materials, not energy).
Producers vs Consumers: Know Your RolesC19.2
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: primary, secondary, tertiary, and if the chain is long enough, quaternary.
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: 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: Both Herbivore and CarnivoreC19.2
Problem In a rainforest food web, a bearded pig eats a jackfruit tree (a plant) and also eats a stick insect (an animal). Is it a herbivore, a carnivore, or both?
Step 1 Check each meal separately: eating the jackfruit tree is herbivore behaviour.
Step 2 Eating the stick insect is carnivore behaviour.
Step 3 Since the bearded pig genuinely does both, it counts as both a herbivore and a carnivore, depending which meal you're looking at.
Food Chains and Trophic LevelsC19.2
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 → shrew → owl" means grass is eaten by the grasshopper, which is eaten by the shrew, and so on.<br><br>Each position has a trophic level number: level 1 is the producer, level 2 is the primary consumer (a herbivore), level 3 the secondary consumer, level 4 the tertiary consumer, level 5 the quaternary consumer if the chain goes that far.
> 🔑 Key idea: arrows point towards the eater (energy flow direction). Trophic level 1 = producer, level 2 = primary/herbivore, level 3 = secondary, level 4 = tertiary, level 5 = quaternary.
Worked Example: Count the LevelsC19.2
Problem A food chain reads phytoplankton to zooplankton to herring to seal to killer whale. How many trophic levels does this contain?
Step 1 Count one level per organism: phytoplankton = producer (level 1), zooplankton = primary (level 2), herring = secondary (level 3).
Step 2 Keep going: seal = tertiary (level 4), killer whale = quaternary (level 5).
Step 3 So the chain has five trophic levels, and the killer whale is a quaternary consumer, the fourth consumer in the chain.
Food Webs and Human ImpactC19.2
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 changes, what happens to that?" You answer it by following the arrows one step at a time, and this works whether the change is natural or caused by people, pollution, pesticides, overfishing and hunting all disturb a food web in exactly the same traceable way.
> 🔑 Key idea: a food web is many food chains joined up. To predict a knock-on effect, including human impact, follow the arrows one link at a time.
Worked Example: When the Decomposers DisappearC19.2
Problem A factory releases a chemical that kills most of the fungi and bacteria that normally break down dead leaves and waste in a community. What's the long-term effect?
Step 1 Decomposers normally break dead material down and put nutrients back in the soil.
Step 2 Kill the decomposers, and dead material just piles up instead of breaking down.
Step 3 With fewer nutrients returned to the soil, the producers (crops) grow worse, so the long-term effect is dead material building up and fewer nutrients reaching producers, not an improvement.
Ecological Pyramids: Draw the LevelsC19.2
A pyramid shows each trophic level as a bar, producer at the bottom. A pyramid of numbers counts organisms, but it can go upside-down (one huge tree feeds thousands of caterpillars, so the bottom bar is tiny). A pyramid of biomass measures the mass instead, which usually fixes the shape into a normal pyramid, because it accounts for how big the organisms actually are.
> 🔑 Key idea: pyramid of numbers (can be inverted by a size mismatch), pyramid of biomass (accounts for size, usually a true pyramid and more useful).
Worked Example: The Upside-Down PyramidC19.2
Problem One giant water lily is eaten by many water lily beetles, which are eaten by frogs, which are eaten by a few herons. What shape is the pyramid of numbers, and why would biomass look different?
Step 1 Count the organisms: just 1 water lily, but many, many beetles.
Step 2 So the pyramid of numbers is inverted: a narrow water-lily bar under a wide beetle bar.
Step 3 A pyramid of biomass measures mass instead, and the single water lily has a huge mass (way more than all the beetles put together), so biomass gives a normal pyramid shape.
The Carbon Cycle: Round and RoundC19.3
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.<br><br>The reason CO₂ levels are climbing is that we're burning fossil fuels way faster than photosynthesis can soak the carbon back up.
> 🔑 Key idea: photosynthesis removes CO₂; respiration, decomposition and combustion return it. Burning fossil fuels is why atmospheric CO₂ is rising.
Worked Example: In and Out of the AirC19.3
Problem A power station burns coal to generate electricity. Which carbon-cycle process is this, and what does it do to atmospheric CO₂?
Step 1 Burning a fossil fuel is combustion.
Step 2 Coal formed from long-dead organisms, so the carbon in it has been locked away for millions of years.
Step 3 Combustion releases that stored carbon, so it increases the amount of CO₂ in the atmosphere.
Populations, Communities and EcosystemsC19.4
Quick vocab: a population is one species in one area (all the dandelions in a field), a community is all the different species together, and an ecosystem is the community plus its non-living surroundings (water, soil, gases, light). Population growth depends on food, competition, predators, disease and human activity.
> 🔑 Key idea: population = one species; community = all species; ecosystem = community + environment.
Worked Example: One Species OnlyC19.4
Problem A field has 200 dandelion plants, 50 daisy plants and 30 rabbits. Which of these groups is a single population?
Step 1 A population is organisms of one species only.
Step 2 Plants + rabbits together mixes species, so that's community-level, not a population.
Step 3 Only the 200 dandelion plants are all one species, so that's the single population.
Factors that Speed Up or Slow Down a PopulationC19.4
More food, more space = faster growth. More competition, more predators, more disease, more human disturbance (overfishing, pollution, hunting) = slower growth or a decline. Every population question comes down to asking: does this factor add a resource, or take individuals away?
> 🔑 Key idea: food supply and space speed growth up; competition, predation, disease and human activity slow it down.
Worked Example: Two Reasons for a DeclineC19.4
Problem The population of chambo fish in a lake has decreased over twenty years. Give two possible reasons.
Step 1 A decline needs factors that remove fish or remove their resources.
Step 2 Overfishing removes fish directly.
Step 3 A fall in the fish's food supply (fewer of the small fish it eats) means slower growth too, so both reasons fit.
The Sigmoid Growth CurveC19.4
When a population grows with limited resources it makes an S-shaped (sigmoid) curve: a slow lag phase (few individuals, just getting established), a rocketing exponential (log) phase while resources are plentiful, then a stationary phase where it levels off at the carrying capacity because resources run short and growth stops.
> 🔑 Key idea: the sigmoid curve goes lag → exponential/log → stationary (levels off at the carrying capacity).
Worked Example: Name That PhaseC19.4
Problem What is the correct sequence for the stages in a sigmoid population growth curve?
Step 1 Match the shape to a name: a flat, slow start is the lag phase.
Step 2 A steep, rapid rise is the exponential (log) phase.
Step 3 A flat plateau at the top is the stationary phase. So the order is: lag phase, then log phase, then stationary phase.
Key Points to Remember
- The Sun is the principal energy source; producers capture light as chemical energy by photosynthesis; energy flows one way and is lost, mainly as heat from respiration, at every step.
- Food chains start with a producer; arrows point from the organism eaten to the eater. Trophic levels: 1 = producer, 2 = primary, 3 = secondary, 4 = tertiary, 5 = quaternary.
- Food webs link food chains together; follow the arrows to predict knock-on effects, including human impact like pollution or overfishing.
- Pyramids: numbers (can be inverted by a size mismatch), biomass (usually a true pyramid, more useful).
- Carbon cycle: photosynthesis removes CO₂; respiration, decomposition and combustion return it.
- Population = one species; community = all species; ecosystem = community + environment. Growth depends on food, competition, predators, disease, human activity.
- Sigmoid curve: lag → exponential/log → stationary (levels off at the carrying capacity).