Biome Distribution

The blobs are repeats, not neighbours

Open any world biome map and it looks like a revision problem. Six or seven colours, spread across the continents, and a strong feeling that the job is to remember which colour sits where. ⚠️⚠️ THAT IS THE EXPENSIVE WAY TO CARRY THIS TOPIC INTO AN EXAM, AND IT IS NOT WHAT THE MAP IS SHOWING YOU. Look at the colours again and notice something the memorising approach walks straight past. The patches of any one colour are not next to each other. Tropical forest turns up in three widely separated regions of the world. Hot desert turns up on several continents at once. Grassland the same. These are not one region each, spilling outwards from a centre. ⭐⭐ THEY ARE REPEATS. AND WHEN SOMETHING REPEATS IN SEPARATE PLACES THAT HAVE NO CONNECTION TO EACH OTHER, THAT IS A CLUE ABOUT WHAT IS CAUSING IT. ⭐⭐ HERE IS THE IDEA THE WHOLE TOPIC RESTS ON. A BIOME IS NOT A PLACE. IT IS A SET OF CONDITIONS. What repeats is not the location and not the continent. It is how much energy arrives, how much water arrives, and how reliably they arrive through the year. ⭐ WHEREVER THAT COMBINATION REPEATS, THE SAME KIND OF PLACE APPEARS: BUILT BY COMPLETELY DIFFERENT SPECIES, OUT OF THE SAME MATERIALS, TO THE SAME PATTERN. You may already have met why those conditions are arranged in bands across the world, which is a question about the atmosphere. This module is about what the bands produce. ⭐ AND HERE IS WHAT THAT BUYS YOU. IF YOU KNOW WHAT CONDITIONS A BIOME NEEDS, YOU CAN WORK OUT ROUGHLY WHERE IT GOES. That is a much cheaper thing to carry into an exam than six remembered outlines, and it does not fall apart when the question shows you an unfamiliar map. ⭐ IT ALSO JOINS UP THE OTHER HALF OF THIS TOPIC. The parts of an ecosystem depend on each other in an order: the climate decides what the water and the soil are like, those decide what can grow, and what grows decides what can live there. The conditions are the first link in that chain, which is why they settle everything after it. Carry one question through everything that follows: is this a fact about the place, or about the conditions? A fact about conditions travels to every repeat of that biome. A fact about a place stays where it is.

Six biomes, described by conditions

The six biomes this specification names, each given by what the conditions are like rather than by where it is. This is overview depth on purpose: none of them is studied in detail here.

Tap the two about conditions

Tap the TWO statements that are facts about CONDITIONS rather than facts about a particular place.

• Grassland appears where there is enough water for grasses but not reliably enough for forest
• Polar biomes form where so little energy arrives that growth is slight and slow
• One region of tropical forest is larger than the others
• A particular species of tree grows in one region of tropical forest and nowhere else

Why the same biome appears twice

Tropical forest appears in three widely separated regions with almost no species in common. Why do they count as the same biome?

• Because a biome is defined by its conditions rather than by its location or its species, and all three have warmth and water all year, so different species build the same kind of place
• Because the three regions were joined together in the distant past
• Because they do in fact share most of their species, despite appearances
• Because all three are roughly the same size

Five terms for this topic

Five terms, each defined by what it is. The third one names the relationship the rest of this chapter runs on.

Match each biome to the conditions it needs

Two that follow from conditions

Select the TWO statements that follow from treating a biome as a set of conditions rather than a place.

• The map becomes something you can reason about rather than memorise, because knowing what a biome needs tells you roughly where it can be
• A fact about the conditions applies to every repeat of that biome, while a fact about one region stays where it is
• Two areas of the same biome must contain mostly the same species
• Areas of the same biome are usually found close to one another

Writing about a pattern you did not memorise

You may already have studied how the atmosphere arranges the world's conditions into bands. ⚠️ THAT IS A DIFFERENT QUESTION FROM THIS ONE, AND IT IS WORTH KEEPING THEM APART: THAT ONE ASKS WHY THE CONDITIONS ARE WHERE THEY ARE, AND THIS ONE ASKS WHAT LIVES IN THEM. A routine for any question that shows you a distribution and asks you to describe or explain it. ⭐⭐ DESCRIBE THE PATTERN BEFORE YOU EXPLAIN ANYTHING. Say where the thing is, where it is not, and whether it repeats. A great many answers start explaining before they have said what it is they are explaining, and the description marks go unclaimed. ⭐ THEN NAME WHAT THE AREAS HAVE IN COMMON, AND MAKE IT ABOUT CONDITIONS. If the only thing you can say they share is that they are all a long way from here, you have not found the pattern yet. THEN EXPLAIN USING THE TWO QUESTIONS THAT DECIDE EVERYTHING: HOW MUCH ENERGY ARRIVES, AND HOW MUCH WATER, AND HOW RELIABLY. ⭐ NEARLY EVERY EXPLANATION IN THIS TOPIC IS ONE OF THOSE TWO, OR BOTH. AND IF THE QUESTION ASKS ABOUT CHANGE, USE THE ORDER OF THE CHAIN. The climate settles the water and the soil, those settle what can grow, and what grows settles what can live there. ⭐ SO A CHANGE THAT STARTS AT THE CLIMATE END REACHES EVERYTHING, AND A CHANGE THAT STARTS AT THE ANIMAL END REACHES LESS. ⚠️ THREE HABITS THAT COST MARKS HERE. ⚠️ THE FIRST IS LISTING PLACES INSTEAD OF DESCRIBING A PATTERN. Naming three regions is not a description; saying that they sit in the same band and are otherwise unconnected is. ⚠️ THE SECOND IS EXPLAINING WITH DETAIL THE QUESTION DID NOT ASK FOR. ⭐ THIS PART OF THE SPECIFICATION WANTS THE OVERVIEW. AN ANSWER THAT DIVES INTO ONE BIOME'S INNER WORKINGS HAS LEFT THE QUESTION, HOWEVER GOOD THE MATERIAL IS. ⚠️ AND THE THIRD IS TREATING THE CHAIN AS A LOOSE ASSOCIATION. Saying the parts are all connected is worth very little. Saying which way the influence runs, and therefore what a change would do, is worth a great deal. One last thing. Where you are unsure of a number, leave it out. This topic can be explained entirely in terms of more, less, and reliably, and an answer built that way cannot be wrong about a figure it never gave.

Two forests that never met

Take two areas of warm wet forest on separate continents, far enough apart that nothing has crossed between them for an extremely long time. Neither is named here, because the point does not depend on which two they are. Stand in one and then the other and the impression is almost identical. Tall trees with their crowns pressed together overhead, so that very little light reaches the ground. Trunks that go up a long way before the first branch. Plants growing on other plants rather than in the soil. Something in flower somewhere at any time of year. A great deal of noise, most of it from things you cannot see. ⚠️⚠️ NOW LOOK AT WHAT IS ACTUALLY GROWING, AND ALMOST NOTHING IS SHARED. DIFFERENT TREES. DIFFERENT BIRDS. DIFFERENT INSECTS. THE LISTS BARELY OVERLAP. Which ought to be strange. Two places built out of entirely different components, and they come out the same shape. ⭐⭐ THE REASON IS THAT THE SHAPE IS NOT A PROPERTY OF THE COMPONENTS. IT IS A SOLUTION TO A PROBLEM, AND BOTH PLACES WERE SET THE SAME PROBLEM. Warmth all year and water all year mean that nothing ever has to stop. So growth continues, and where growth continues without a pause, the competition is for light. ⭐ AND ONCE LIGHT IS THE PRIZE, THE ANSWERS ARE LIMITED. GO UP. GET YOUR LEAVES ABOVE SOMEBODY ELSE'S. DO NOT WASTE MATERIAL ON LOW BRANCHES THAT WILL BE SHADED ANYWAY. AND IF YOU CANNOT WIN ON HEIGHT, START HIGHER UP BY GROWING ON SOMETHING THAT ALREADY DID. Every feature in that first paragraph is one of those answers. Different species arrived at them separately, because the problem was the same. ⭐⭐ WHICH IS THE GENERAL POINT, AND IT IS WORTH MORE THAN THE EXAMPLE: WHEN TWO UNCONNECTED PLACES LOOK ALIKE, LOOK FOR THE PROBLEM THEY BOTH HAD TO SOLVE. ⚠️ AND NOTICE THE OPPOSITE CASE, BECAUSE IT IS THE SAME IDEA. TWO PLACES ON THE SAME CONTINENT, WITH THE SAME HISTORY AND PLENTY OF SPECIES IN COMMON, CAN LOOK COMPLETELY UNALIKE IF ONE GETS RELIABLE WATER AND THE OTHER DOES NOT. Nearness explains far less here than the conditions do.

Order the chain of dependence

Put these six links into the order that influence runs in, from the first thing that is settled to the last.

• How much energy and water arrive, and how reliably through the year
• The climate of the area is settled by that
• The water available in the ground and the kind of soil that forms follow from the climate
• What can grow there is decided by the water and the soil
• What can live there is decided by what grows
• So a change near the start of the chain reaches everything after it

Build the sentence about the pattern

This is the sentence a good answer about biome distribution is built around. Assemble it.

Which way the influence runs

A student writes that in an ecosystem everything is connected to everything else. Why does that earn very little?

• Because it names no direction: saying which way the influence runs is what lets you predict what a change would do, and that prediction is what is being marked
• Because it is too short to be a full answer
• Because it is not actually true of ecosystems
• Because it does not use enough subject terminology

The distribution run

Five questions on conditions, repeats and the chain. Three lives.

Complete the ecosystem facts

A large type of natural environment defined by its conditions, which can occur in several separate parts of the world, is a _____. The living things in an area together with the non-living surroundings they depend on make up an _____. The way each part relies on the others, so that a change in one reaches the rest, is _____. Where something is found across an area, and the pattern its locations make, is its _____.

Three answers about the map

Three students answer questions on biome distribution. Nothing they say is false; it is what they have done with it that needs work.

• Asked to describe the distribution of hot desert, a student names three regions where it occurs and stops. What is missing?
• Asked about the distribution overview, a student writes two detailed paragraphs on how one biome recycles its nutrients. Is that a good use of the material?
• A student argues that because everything in an ecosystem is connected, the effect of any change is impossible to predict. Is that the strongest version?

Explain the pattern on the map

A friend has been given a world biome map and is trying to memorise which biome goes where. Explain why that is the hard way to do it, and what to understand instead. Keep to the overview: do not go into detail on any single biome.

• Describe what the pattern on the map actually looks like, including that areas of one biome are usually far apart
• Explain what a biome is defined by, and why that makes the separate areas repeats rather than neighbours
• Give the two questions that decide which biome forms somewhere, and say why reliability matters as well as amount
• Use one biome briefly as an example of conditions producing a result, without going into its inner workings
• Explain why two areas of the same biome can share almost no species and still look alike
• Explain the order the chain of dependence runs in, and what that lets you predict about a change
• Finish by saying what your friend should learn instead of six outlines, and avoid any figure you are not sure of