Habitats, Resources & Population Change · Grade 7 Science
Chapter: Ecosystems, Matter & EnergyHabitats, Resources & Population Change
Learning goals
- I can explain habitats, resources & population change.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
In 1944, twenty-nine reindeer were put ashore on St Matthew Island in the Bering Sea, where there were no predators and the ground was deep in lichen. By 1963 there were six thousand. Three years later there were forty-two. The lichen that had taken centuries to accumulate was gone, a severe winter finished what overgrazing had started, and the population collapsed almost completely. Nothing about the reindeer changed. What changed was the relationship between how many there were and what the island could supply.
Some definitions make that story analysable. A population is all the individuals of one species in a defined area at a defined time — the definition needs both, since a count without a boundary and a date means nothing. A community is all the populations of different species in that area together. The factors acting on them divide in two: a biotic factor is a living one, such as food, predators, disease or competitors, and an abiotic factor is a non-living one, such as temperature, water, light, soil or space.
Populations change through exactly four pathways, and listing them is the foundation of this unit. Births and immigration add individuals; deaths and emigration remove them. Any change in a count is the net result of those four and nothing else. Carrying capacity is the idea that an environment can support only so many of a species given its resources, and that as a population approaches that number, growth slows — because food runs short, waste accumulates, disease spreads more readily, or space runs out. The reindeer overshot theirs, which is possible when a resource can be consumed faster than it regrows, and the correction was brutal.
Build your understanding
Where this fits
This chapter has followed energy and matter through an ecosystem. The last unit turns to the organisms themselves: how a population changes, what limits it, and why counting the same place twice tells you less than it appears to. It is also the method the sampling unit in the next chapter builds on.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Defining the population before counting it
A population needs a species, an area and a time. "The rabbit population" is not a measurable thing; "the rabbits in this twelve-hectare field in June 2026" is. Without a boundary there is no way to say whether an individual counts, and without a date there is nothing to compare a later count against.
The boundary also determines what counts as movement. A rabbit crossing the field edge is emigration from your population even though it is alive and well a metre away. That sounds like an artefact of the definition, and it is — which is exactly why the definition has to be stated, so that anyone reading your figures knows what they include.
This matters in practice because populations rarely sit neatly inside a chosen boundary. Birds move between study areas, seeds blow in, and a species' range does not respect a fence. Stating the boundary does not solve that; it makes the limitation visible, which is what allows someone else to judge the figures.
Pause and explain
Why is "the deer population" not yet a measurable quantity?
Check your reasoning after answering
Because it names no area and no time. A population is all the individuals of one species in a defined area at a defined time, and without a boundary there is no way to decide which individuals count, nor any basis for comparing one count with a later one. Stating the boundary also defines what counts as movement in or out.
Part 2 · Four pathways, and only four
Individuals enter a population by being born in it or by moving into it, and leave by dying or by moving out. Births, immigration, deaths, emigration — every change in a count is the net effect of these four. Writing them as a sum makes any population change calculable: ending count equals starting count, plus births, plus immigration, minus deaths, minus emigration.
Run the model. A population starts at 50. Births add 12 and immigration adds 5, giving 17 added. Deaths remove 8 and emigration removes 4, giving 12 removed. The ending count is 50 + 17 − 12 = 55, a net increase of five.
The arithmetic is easy and the interpretive point is not. A net increase of five is consistent with many different combinations: 12 births and 8 deaths, or 30 births and 26 deaths, or no births at all and heavy immigration. The net change tells you nothing about which pathways were active, and populations with the same net change can be in completely different conditions.
Pause and explain
A population starts at 50; births add 12, immigration adds 5, deaths remove 8, emigration removes 4. What is the ending count?
Check your reasoning after answering
Fifty-five. Additions total 12 + 5 = 17 and removals total 8 + 4 = 12, so the net change is +5 and the population ends at 50 + 17 − 12 = 55. The same net increase of five could arise from many other combinations of the four pathways, which is why the net alone says little about what is happening.
Part 3 · What limits a population
Resources limit numbers, and which resource bites first depends on the species and the place. Food, water, space to nest or root, light for plants. Abiotic conditions limit too — a population can be held well below what the food would support by temperature or by a lack of suitable soil. The limiting factor is whichever runs short first, and it can change with the season.
As numbers rise towards the carrying capacity, several things usually happen at once. Competition for the limiting resource intensifies, so individuals get less; disease spreads more easily at higher density; and predators may concentrate where prey is abundant. Each of these tends to reduce births or increase deaths, which is a negative feedback of the kind the homeostasis unit described, operating on a population instead of a body.
Overshoot happens when a population grows past what the environment can sustain, which is possible when the limiting resource can be consumed faster than it is replaced. The St Matthew reindeer ate lichen that regrows over decades, so the damage outlasted the population that did it. A carrying capacity is therefore not a fixed ceiling a population bumps against — it is a property of the environment that the population itself can change.
Pause and explain
How can a population exceed its carrying capacity if the environment cannot support it?
Check your reasoning after answering
Because a resource can be consumed faster than it is replaced. A population can live for a time on an accumulated store — lichen built up over centuries, for example — growing past what the environment can sustain in the long run. The correction comes later and can be severe, and the damage to the resource may outlast the population.
Part 4 · Why before-and-after counts tell you less than they seem to
Count a population in April and again in September and you have two numbers and a difference. What you do not have is any information about which of the four pathways produced it. A fall of twenty could be twenty deaths, or twenty animals moving to the next valley, or fifty births and seventy deaths. Each would call for a completely different explanation and response.
Separating them requires different evidence. Marking individuals lets you distinguish movement from death, since a marked animal found elsewhere emigrated rather than died. Counting nests or young gives a birth rate directly. Surveying a wider area catches animals that merely moved. Each pathway needs its own measurement; none can be inferred from the net.
That is the honest conclusion to this chapter. A number is a starting point, and the question to ask of any population figure is what it was measured over, what boundary it used, and which of the four pathways it can distinguish. The sampling unit in the next chapter takes up the further problem that most of these counts are estimates in the first place.
Pause and explain
A field's rabbit count falls from 60 to 40. Name two different explanations and the evidence that would distinguish them.
Check your reasoning after answering
Twenty rabbits could have died, or twenty could have moved to neighbouring land, or births and deaths could both have been high with deaths exceeding births. Marking individuals would distinguish movement from death, since a marked rabbit found elsewhere emigrated; counting young or nests would establish the birth rate; surveying surrounding land would find emigrants.
Apply the whole unit · Balance a population model
Illustrative interval, not surveyed data: a population begins at 50; births add 12; immigration adds 5; deaths remove 8; emigration removes 4. No other pathways are stipulated.
Your task
Calculate the ending count, and explain why a before-and-after count on its own could not have identified the pathways involved.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Total the additions: 12 births + 5 immigrants = 17 individuals added.
Total the removals: 8 deaths + 4 emigrants = 12 individuals removed.
Calculate the ending count: 50 + 17 − 12 = 55, a net increase of five.
Explain the limitation: the same net increase of five is consistent with many combinations of the four pathways, so the net cannot identify which were active.
The population ends at 55. Additions are 12 births and 5 immigrants, totalling 17; removals are 8 deaths and 4 emigrants, totalling 12; so the ending count is 50 + 17 − 12 = 55, a net increase of five over the interval. The reason a before-and-after count could not have produced this account is that it would have given only the net figure. A rise from 50 to 55 is equally consistent with 5 births and no other activity, with 40 births and 35 deaths, or with no births at all and heavy immigration from neighbouring land — and those describe completely different situations. A population with high births and high deaths is under very different pressure from one that is simply drifting upward, and only the first would be worth investigating for disease or resource shortage. Distinguishing the pathways needs evidence aimed at each one. Marking individuals separates movement from death, since a marked animal recorded elsewhere emigrated rather than died. Counting nests, eggs or young gives a birth rate directly. Surveying the surrounding area finds emigrants and identifies where immigrants came from. Each pathway needs its own measurement, and none can be inferred from the net change. Two further limits apply to this model: it stipulates that no other pathways exist, where a real study would have to argue for that, and it gives no interval length, without which none of these figures can be turned into a rate.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
A population includes individuals of the same species in a defined area and interval. A community includes populations of different species. A habitat is the setting supplying conditions and resources for an organism. Biotic factors involve organisms; abiotic factors include physical conditions such as water, light, and temperature.
Worked examples
Example 1 · Work backwards to a missing pathway
- Calculate the net change: 74 − 80 = −6 individuals over the interval.
- Total the known additions and the known removal: +10 births, +2 immigrants, −9 deaths gives +3 so far.
- Find the remaining removal: the net is −6, so emigration must account for 3 − (−6) = 9 individuals.
A balance can find a missing pathway, but only if you have genuinely listed all the others.
Example 2 · Judge a conclusion about a decline
- Identify what the survey measured: a difference between two counts, which is a net change.
- List the pathways that could produce it: deaths, emigration, or reduced births alongside ordinary deaths — any combination netting to −30.
- Ask what evidence would support the specific claim: bodies found, a measured pollutant, comparison with similar unaffected ponds, and a check of neighbouring water for emigrants.
A net change names no pathway and no cause. Both need their own evidence.
Avoid these mistakes
- Mistake: a population is all the members of a species. It is those in a defined area at a defined time; without a boundary and a date the count cannot be compared with anything.
- Mistake: a falling count means deaths. Emigration and reduced births produce the same net change, and distinguishing them needs marking, nest counts or a wider survey.
- Mistake: carrying capacity is a fixed ceiling. A population can overshoot by consuming a resource faster than it regrows, as the St Matthew reindeer did, and damage the capacity itself.
- Mistake: the net change describes what happened. A rise of five could be 5 births or 40 births and 35 deaths; those are very different situations with the same net.
Check your understanding
Why should a count include area and effort?
Discuss the answer
Without them, equal or different counts might reflect different survey scope rather than biological change.
Does an increased count prove more births?
Discuss the answer
No. Immigration or detection differences can also increase the recorded number.
Words to know
- population
- All the individuals of one species in a defined area at a defined time; both the boundary and the date are part of the definition.
- community
- All the populations of different species living in the same area, interacting with each other.
- biotic factor
- A living influence on a population — food, predators, disease, competitors.
- abiotic factor
- A non-living influence — temperature, water, light, soil, space — which can hold a population well below what its food supply would allow.
- carrying capacity
- The number an environment can sustain given its resources; not a fixed ceiling, since a population can overshoot it and damage the resource.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Why might a lower observed count not mean the same number died?
Check your explanation after answering
Movement or lower detectability can affect counts; births and deaths are not the only pathways.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.