Ecosystems and Biodiversity

Two loops, running at once

This topic looks like a pile of unrelated diagrams. A pyramid. A food web. A carbon cycle. A nitrogen cycle. A list of things humans have damaged. Most students meet them one at a time and never find out what holds them together. ⚠️ THERE ARE ONLY TWO THINGS MOVING THROUGH AN ECOSYSTEM, AND THEY BEHAVE IN OPPOSITE WAYS. ⭐ ENERGY FLOWS THROUGH ONCE AND LEAVES. It arrives from the Sun, passes along a food chain, and at every single step most of it escapes as heat. That heat does not come back. It is gone from the living world for good. ⭐ MATTER GOES ROUND AND IS USED AGAIN. The carbon, nitrogen and water in your body were in other living things before you, and will be in others after. No new atoms arrive. The same ones are passed round and rebuilt. ⚠️ THAT IS WHY THE TWO DIAGRAMS ARE DIFFERENT SHAPES, AND THE SHAPES ARE NOT A CONVENTION. A pyramid TAPERS because energy is leaking away at every level and there is less left to support the next one. A cycle CLOSES because the atoms have nowhere else to go. ⭐ THE SHAPE IS RECORDING THE BEHAVIOUR. And it explains something students often find odd. Why must an ecosystem be resupplied with energy every single day, when nobody delivers fresh atoms to it? Because energy is spent and lost, while matter is only ever borrowed. ⚠️ WHICH GIVES YOU THE MOST USEFUL SINGLE QUESTION IN THIS TOPIC, AND IT TAKES ONE SECOND TO ASK: AM I BEING ASKED ABOUT ENERGY, OR ABOUT MATTER? Get that right and the rest of the answer almost writes itself, because the two loops behave in opposite ways and you already know which way you are in.

The two loops, side by side

The whole topic as two columns, plus the question that tells you which one you are in.

Tap what the two loops actually claim

Tap the TWO statements that follow from energy flowing through and matter going round.

• Energy has to be resupplied to an ecosystem constantly, but its atoms do not
• A pyramid tapers because energy leaves at every level and does not come back
• Because matter is recycled, an ecosystem can never run short of any nutrient
• Organisms that break down dead material sit at the top of the energy pyramid because they feed on everything

Why the shapes differ

A student says the carbon cycle is drawn as a circle and the energy pyramid as a triangle simply because those are the conventions people happen to use. How would you correct that?

• Each shape records how the thing actually behaves: matter returns to where it started, so it closes, while energy is lost at every transfer, so it tapers
• They are conventions, but very old ones, so they should be followed anyway
• A triangle fits more labels on a page than a circle does
• Circles are easier to draw, so cycles were drawn that way first

Five words this topic runs on

Five terms, each defined by what it is. Which loop each one belongs to is the work of the steps that follow, and is not given here.

Match each one to what follows from it

Two that follow from the two loops

Select the TWO statements that follow from energy flowing through an ecosystem and matter cycling within it.

• Organisms that break down dead material matter far more to the matter loop than to the energy loop
• Food chains are short because there is not enough energy left after a few transfers to support another level
• Energy is recycled through an ecosystem in the same way that carbon is
• Because atoms are reused, the supply of any particular nutrient is effectively unlimited

Planning the long answer on this topic

You have already met the longer extended-writing question, the QER question, and the thing that makes it different: it is marked on how clearly you communicate as well as on the science. ⚠️ THAT HAS NOT CHANGED HERE. WHAT CHANGES IS THAT THIS TOPIC HANDS YOU A PLAN FOR FREE. ⭐ SAY WHICH LOOP YOU ARE IN BEFORE YOU SAY WHAT HAPPENS IN IT. Almost every long question on this topic is really asking about one loop or the other, and naming it in your first line does three jobs at once. It tells the marker you have understood the question, it stops you drifting into the other loop halfway through, and it gives your answer an obvious order to follow. A plan that works on nearly any question here: FIRST, NAME THE LOOP. Is this about energy passing through, or about matter being reused? THEN SAY WHAT DRIVES IT. Sunlight captured by producers, or organisms breaking down what has died. THEN FOLLOW IT ONE STEP AT A TIME, using the proper terms rather than talking around them. AND FINISH WITH THE CONSEQUENCE. Why the chain is short, why the nutrient runs out, why the water went bad. ⚠️ TWO HABITS THAT COST MARKS ON THIS TOPIC IN PARTICULAR. ⚠️ THE FIRST IS WRITING THAT ENERGY IS RECYCLED. It is the error the whole module exists to prevent, and a marker reading it will discount a good deal of what surrounds it. ⚠️ THE SECOND IS ANSWERING "WHAT" WHEN THE QUESTION ASKED "WHY". Saying that the fish died is a description. Saying that the water was enriched, so tiny organisms multiplied, so the oxygen was used up, so the fish could not breathe is an explanation, and on a question like this the explanation is the thing being paid for.

The gas that is everywhere and still runs out

Nearly four fifths of the air is nitrogen. Every living thing needs it, because proteins cannot be built without it. And plants growing in that air routinely run short of it. ⚠️ THAT SOUNDS LIKE A CONTRADICTION, AND RESOLVING IT IS THE WHOLE POINT. Nitrogen gas is remarkably unreactive. The two atoms in it are held together so tightly that a plant root cannot do anything with them at all. ⭐ BEING SURROUNDED BY IT IS NO USE WHATEVER, BECAUSE IT IS NOT IN A FORM ANYTHING CAN TAKE UP. What changes that is bacteria. Certain bacteria in the soil, and others living in swellings on the roots of plants such as peas, beans and clover, can break that gas apart and turn it into compounds dissolved in the soil water. ⭐ ONLY AT THAT POINT DOES THE NITROGEN ENTER THE LIVING WORLD AT ALL. Then the loop runs. Roots take those compounds up and build them into proteins. An animal eats the plant and rebuilds those proteins into its own. When either dies, other organisms break the remains down and the compounds return to the soil. And a further group of bacteria converts some of them back into nitrogen gas, which returns to the air. ⚠️ NOTHING WAS CREATED AND NOTHING WAS DESTROYED. THE SAME ATOMS WENT ROUND. Now watch what happens when people speed one part of it up. A farmer spreads fertiliser to put usable nitrogen compounds straight into the soil, skipping the bacteria. The crop takes up what it can, and rain washes the rest into a stream and then into a lake. ⚠️ THE LOOP IS NOT BROKEN. IT IS OVERLOADED. Tiny organisms in the water, which were always limited by how little nitrogen they could get, now multiply enormously and cover the surface. The plants beneath them are shaded and die. Organisms that break down all this dead material multiply in turn, and use up the oxygen dissolved in the water as they work. ⚠️ AND THE FISH, WHICH NEED THAT OXYGEN, SUFFOCATE. ⭐ NOTICE WHAT ACTUALLY KILLED THEM. NOT A POISON. A NUTRIENT, DELIVERED FASTER THAN THE LOOP COULD CARRY IT AWAY. The range of species the lake can support collapses, and that is far harder to rebuild than it was to lose.

Put one nitrogen atom back in order

Starting in the air, put the journey of a single nitrogen atom into the order it happens, ending where it began.

• The atom sits in the air as part of an unreactive gas that no plant can use
• Bacteria break that gas apart and turn it into compounds dissolved in the soil
• A plant root takes those compounds up and builds them into proteins
• An animal eats the plant and rebuilds those proteins into its own body
• The animal dies and its remains are broken down, returning the compounds to the soil
• Other bacteria convert some of those compounds back into the gas, which returns to the air

Work out how much was left behind

A farmer applies fertiliser containing 120 kg of nitrogen to a field. The crop takes up 78 kg of it. Work out the nitrogen left in the soil as a percentage of the amount applied, since that is the part at risk of being washed into the stream. Give your answer to the nearest whole number.

The two loops run

Five questions on telling the energy loop from the matter loop. Three lives.

Complete the ecosystems and biodiversity facts

An organism that makes its own food from simple substances rather than eating others is a _____. The conversion of nitrogen gas into compounds living things are able to use is _____. The variety of different species present in an area is its _____. The enrichment of a body of water with nutrients is _____.

Three answers to put right

Three student answers on this topic. In each case the mark is in the reasoning, not the verdict.

• An answer explains a short food chain by saying that energy is recycled but gets used up a bit each time round. What is wrong with it?
• A student writes that the fish in a fertiliser-polluted lake were poisoned by the fertiliser. What should they say instead?
• Asked how human activity damages ecosystems, an answer lists pollution, deforestation and overfishing with no further comment. How would you strengthen it?

Explain what happened to the lake

A lake beside farmland turns green over a summer, and by autumn its fish have died. Explain how this happened, and what it shows about the way matter moves through an ecosystem.

• Open by naming which of the two loops this question is about, and say how you can tell
• Explain how nitrogen normally enters the living world, and why plants cannot use the gas in the air directly
• Explain what the farmer changed about that loop, and why the loop is overloaded rather than broken
• Follow the chain from the nutrient arriving to the fish dying, one step at a time, using the proper terms
• Explain why the fish died of something other than the substance that was added
• Finish by saying what the loss of species means for the lake, and why it is harder to rebuild than it was to lose