Ecosystems, Habitats & Biomes — Print · Grade 6 Science

Chapter: Ecosystems, Watersheds & Conservation

Ecosystems, Habitats & Biomes

Learning goals

I can distinguish a habitat, a population, a community and an ecosystem.
I can explain how temperature and rainfall determine which biome occurs in a place.
I can explain why similar-looking species evolve in separate continents with the same climate.

Readings for this unit

A Clearer Stream Is Still a Question

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

A habitat is where an organism lives. A population is all the individuals of one species in an area. A community is all the populations living there together. An ecosystem is that community plus the non-living conditions, the soil, water, light and temperature they interact with.

Those non-living conditions do most of the deciding. Plot the world's major vegetation types against annual temperature and annual rainfall and they sort themselves almost completely: tropical rainforest where it is hot and wet, desert where rainfall is under about 250 mm a year, tundra where the growing season is too short and cold for trees.

A biome is one of those large types, defined by climate and vegetation structure rather than by which species are present. That distinction matters, because the deserts of Australia, Africa and North America share a structure and almost no species at all.

Build your understanding

Where this fits

This chapter is about living things and the systems they are part of, and it draws directly on the water cycle and climate from chapter 6. The next three lessons follow energy and matter through these systems, and the last applies all of it to a watershed you could actually sample.

Use three flexible 40-45-minute sessions: the levels from habitat to ecosystem; climate and the distribution of biomes; why similar forms appear in separate places, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · Four words that are not interchangeable

The terms are often used loosely and the distinctions are worth keeping because they tell you what question is being asked. If you ask where a particular newt lives, you are asking about its habitat: a pond with shallow weedy margins. If you ask how many newts there are and whether the number is falling, you are studying a population. If you ask what else lives in that pond and which eats which, you are describing a community. If you ask why the pond dries in August, how much nitrogen runs in from the field beside it, and how that affects the newts, you have moved to the ecosystem.

The last step is the one that matters most for this chapter, because it brings in the non-living conditions, which ecologists call abiotic factors. Light, temperature, water availability, soil type, oxygen concentration and pH all set limits on what can live somewhere, and a change in any of them can remove a species far more effectively than a predator can. A pond that warms by a few degrees Celsius holds measurably less dissolved oxygen, which is enough to kill fish that are otherwise perfectly healthy, and nothing about that is a biological interaction at all. Scale is a choice here, exactly as it was when chapter 1 asked where to draw a system boundary. An ecosystem can be a rotting log, a pond, a woodland or the entire Amazon basin, and all four are legitimate; what changes is which transfers cross the boundary and which stay inside it. Stating the scale you have chosen is the first line of any ecological description, and leaving it unstated is the commonest reason two people end up arguing past each other.

Two numbers account for most of the pattern, which is a satisfying result at this level. The note points at the striking part: similar conditions produce similar-looking plants in places that share no history.

Pause and explain

What does studying an ecosystem include that studying a community does not?

Check your reasoning after answering

The non-living conditions and their interactions with the organisms. A community is all the populations living together and the relationships between them, while an ecosystem adds the soil, water, light, temperature and chemistry those populations depend on. Many of the strongest effects on living things, such as a pond warming and losing dissolved oxygen, come from that non-living side.

Part 2 · Temperature and rainfall do most of the sorting

Two numbers predict the vegetation of a place remarkably well. Where average temperature is high and rainfall exceeds about 2000 mm a year, you get tropical rainforest. Where rainfall falls below roughly 250 mm, you get desert whatever the temperature, which is why both the Sahara and parts of Antarctica qualify as deserts. Between those extremes, grassland occupies the band with 250 to 750 mm, enough to support grasses but too little and too seasonal for forest, and temperate forest occupies the wetter, cooler middle.

Temperature sets a second limit through the growing season rather than through heat alone. Trees need a certain number of days above about 10 degrees Celsius each year to replace their leaves and add wood, and north of the treeline that requirement is not met, so tundra supports only low plants that can complete their growth in a few weeks. The same boundary appears going up a mountain rather than north, which is why a single alpine slope can pass through forest, then stunted trees, then tundra within a few hundred metres of height, compressing into a short walk what would otherwise take a journey of thousands of kilometres. Rainfall timing can matter as much as the total, which is why two places with identical annual figures support different vegetation. A Mediterranean climate delivers most of its 600 mm in winter, when plants are least able to use it, and almost none through the hot summer, so its plants are small, tough-leaved and drought-resistant. The same 600 mm spread evenly through a temperate year supports full deciduous forest.

Pause and explain

Why does climbing a mountain pass through the same sequence of vegetation as travelling north?

Check your reasoning after answering

Because both reduce temperature and shorten the growing season. Temperature falls about 6.5 degrees Celsius per kilometre of height, so rising a few hundred metres has the same effect on the number of days warm enough for growth as travelling a long distance towards the pole. Trees fail at the same limit in both cases, giving the same sequence.

Part 3 · Same shape, unrelated animals

Because climate rather than ancestry decides what can survive somewhere, similar conditions on separate continents produce organisms of similar form from completely different ancestors. Desert plants in the Americas, the cacti, and desert plants in southern Africa, the euphorbias, both have thick water-storing stems, spines instead of leaves and waxy surfaces, and are not closely related at all; the resemblance is so strong that they are regularly mistaken for one another.

The same happens with animals. A dolphin, a shark and an extinct ichthyosaur all have streamlined bodies and fins, and are a mammal, a fish and a reptile respectively, because water imposes the same requirements on anything that must swim fast through it. This process, called convergent evolution, is good evidence that environments do the shaping: if ancestry were the main factor, Australia's deserts would hold animals resembling Asian ones because the continents were once joined, and instead they hold marsupials that look like unrelated placental mammals in similar habitats elsewhere. The list of Australian examples is long and striking. There was a marsupial wolf, the thylacine, hunted to extinction by 1936, which resembled a dog closely enough to be mistaken for one in photographs; there are marsupial moles that burrow like true moles, and gliding possums that resemble flying squirrels. None is closely related to the animal it resembles, and in each case the shared way of life produced the shared shape.

Pause and explain

What does the resemblance between American cacti and African euphorbias show?

Check your reasoning after answering

That environment shapes form more than shared ancestry does. The two groups are not closely related and evolved separately on different continents, yet both developed thick water-storing stems, spines and waxy surfaces, because those are the features that allow survival where rainfall is under about 250 mm a year. Similar conditions produced similar solutions independently.

Part 4 · Limits, tolerance and why ranges shift

Every species has a range of conditions it can tolerate, and it is usually one factor at the edge of that range, rather than all of them, that sets where the species stops. A plant might tolerate a wide temperature range and be limited entirely by soil pH; a fish might be limited entirely by dissolved oxygen. Finding the limiting factor is the central practical problem in ecology, because changing it is the only intervention that will work. Adding fertiliser to a crop limited by water does nothing at all.

Because the limits are set by conditions, a change in climate moves the ranges. Species whose northern boundary is set by winter cold shift north as winters warm, and species on mountains shift upward, both of which are now measured in hundreds of long-term studies. The difficulty is that different species move at different speeds: a bird can shift its range in a year while an oak woodland takes centuries, so communities do not move as units. They come apart and reassemble in new combinations, which is why predicting the effect of a changing climate on a particular ecosystem is so much harder than predicting the temperature itself. There is a second obstacle that has no natural equivalent. A species shifting its range needs somewhere to shift to and a route to get there, and roads, cities and farmland now break the landscape into fragments. A plant whose suitable climate has moved 50 km north is of no use if the intervening land is built on, which is why conservation increasingly focuses on connecting habitats rather than only on protecting individual sites.

Pause and explain

Why do ecological communities not simply move as units when the climate changes?

Check your reasoning after answering

Because the species in them move at very different speeds. A bird or a flying insect can shift its range within a year, while a woodland dominated by slow-growing trees may take centuries to advance. The community therefore comes apart, with some members gone and new ones arriving, producing combinations of species that did not previously occur together.

Apply the whole unit · Predict the biome

Four sites. Site 1: average 26 degrees Celsius, 2200 mm rain a year, spread evenly. Site 2: average 24 degrees Celsius, 180 mm rain a year. Site 3: average 9 degrees Celsius, 900 mm rain a year. Site 4: average minus 8 degrees Celsius, 200 mm precipitation, growing season 8 weeks.

Your task

Name the likely biome at each site and identify which factor is limiting in each case.

What your response should show

Names all four biomes correctly from the figures
Identifies a specific limiting factor at each site rather than repeating the climate
Explains the contrast between sites 2 and 4 using evaporation as well as rainfall
Uses the growing season, not temperature alone, for the tundra case

Response guidance — after your attempt

Name the biome at each site using the temperature and rainfall figures.
State the limiting factor at each site.
Explain why sites 2 and 4 have similar precipitation but completely different vegetation.

Site 1 is tropical rainforest: high temperature with over 2000 mm of evenly spread rain means neither water nor warmth limits growth, and the limiting factor is likely to be light at ground level, since the canopy intercepts most of it. Site 2 is hot desert, with 180 mm of rain falling well below the roughly 250 mm threshold; water is clearly limiting. Site 3 is temperate forest, with enough rain and a moderate temperature; the limiting factor is likely to be the length of the growing season, since growth stops through the cold months. Site 4 is tundra, and although its 200 mm of precipitation is similar to site 2, the limiting factor is completely different: the growing season of 8 weeks is far too short for trees, which need many more days above about 10 degrees Celsius to replace leaves and add wood. Sites 2 and 4 show that the same shortage of water produces different results depending on temperature, because a cold site loses far less water to evaporation, so 200 mm goes much further there than it would in a desert at 24 degrees Celsius.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

An ecosystem is every living thing in an area together with the non-living conditions they depend on, and the two cannot be separated. Which organisms live where is decided mostly by temperature and rainfall, which is why the same kind of biome appears on continents that have never been connected.

Worked examples

Example 1 · Compare two sampled habitats

Compare richness and abundance, then explain one limit to a habitat-quality judgment.

Keep number of species separate from number of individuals.
Compare like sampled areas and effort.
State an unmeasured factor or time limit.

Answer: The shaded patch has greater recorded species richness, while the sunny patch has more recorded individuals. Neither measure alone proves a universally better habitat. Species identity, seasonal changes, detection, and the needs of a particular organism matter. These are illustrative patch records, not a survey of every forest.

Example 2 · Transfer to a different case

A bird uses nesting cover in one patch and feeds in another. Must its habitat be one small fixed box?

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: No. Habitat use can span several features and locations. The study boundary should match the question rather than assume the organism never moves.

Example 3 · Distinguish habitat from biome · a focused example

Can a biome name replace a local habitat observation?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: No. It supplies broad context, while local conditions need their own evidence.

Avoid these mistakes

Misleading claim: More individuals always means more species.
Misleading claim: A biome label establishes identical conditions at every site.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

Why should nonliving conditions be included?

Discuss the answer

They influence resources and processes, so the organism list alone cannot explain the ecosystem’s relationships.

Can a biome name replace a local habitat observation?

Discuss the answer

No. It supplies broad context, while local conditions need their own evidence.

What additional comparison would improve the inference?

Discuss the answer

Use comparable soil and sampling conditions, repeated observations, and moisture records while evaluating other differences.

Why does the boundary matter?

Discuss the answer

It determines which conditions, interactions, and movements the study includes and limits the population-level conclusion.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · Ecosystems, Habitats & Biomes · Model and explain

Assigned reading and preparation

Read A Clearer Stream Is Still a Question and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Ecosystems, Habitats & Biomes, study Parts 3–4 and attempt the worked case: Compare richness and abundance, then explain one limit to a habitat-quality judgment. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

The shaded patch has greater recorded species richness, while the sunny patch has more recorded individuals. Neither measure alone proves a universally better habitat. Species identity, seasonal changes, detection, and the needs of a particular organism matter. These are illustrative patch records, not a survey of every forest.

Activity 2 · Ecosystems, Habitats & Biomes · Practice, revise and exit

Assigned reading and preparation

Read A Clearer Stream Is Still a Question and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Ecosystems, Habitats & Biomes, complete practice questions 1–6 and both choice checks, then answer independently: A bird uses nesting cover in one patch and feeds in another. Must its habitat be one small fixed box? Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

No. Habitat use can span several features and locations. The study boundary should match the question rather than assume the organism never moves.

Words to know

habitat: The setting and conditions an organism uses to live and obtain resources.
species richness: The number of different species recorded under a stated method.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

Wildlife managers estimate the carrying capacity of parks and forests to decide how many deer, fish, or bison an area can support — and when a population needs protection or has grown too large for its habitat.

Exit check

A bird uses nesting cover in one patch and feeds in another. Must its habitat be one small fixed box?

Check your explanation after answering

No. Habitat use can span several features and locations. The study boundary should match the question rather than assume the organism never moves.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

Problem 1

Why should nonliving conditions be included?

Answer and explanation

They influence resources and processes, so the organism list alone cannot explain the ecosystem’s relationships.