Stimulus, response and the external environment
What counts as the external environment
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The external environment is everything outside the organism that can affect it.
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The standard splits it into two kinds of factor, and exam questions almost always involve both.
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Abiotic factors are the non-living parts of the environment.
- Light intensity, day length, temperature, water availability, salinity, pH, wind, tide, soil nutrients, gravity.
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Biotic factors are the living parts of the environment.
- Predators, prey, parasites, competitors, mates, offspring, food plants, pollinators.
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The distinction matters because the type of factor decides which group of responses is relevant.
- An abiotic factor such as day length triggers orientation in time.
- A biotic factor such as a competitor triggers an interspecific or intraspecific response.
Stimulus and response
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A stimulus is a detectable change in the environment.
- It must be detectable by that organism. Light is not a stimulus for an organism with no photoreceptor.
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A response is the change in the organism caused by that stimulus.
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A receptor detects the stimulus; an effector carries out the response.
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The chain is always the same, and writing it out is the fastest way into a Merit answer:
- In animals the transmission step is usually nervous (fast) or hormonal (slower, longer-lasting).
- In plants there are no nerves, so transmission is always chemical — a hormone such as auxin moving through tissue.
Ecological niche
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An organism's ecological niche is its full role in its community.
- Where it lives — its habitat and the physical conditions it tolerates.
- What it consumes — its food, water and space requirements.
- When it is active — the time of day, tide or season it uses.
- How it interacts — what it competes with, eats, is eaten by, and cooperates with.
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The niche is not the same as the habitat. The habitat is the address; the niche is the entire job description.
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Two species can share a habitat and have different niches — a tūī and a korimako (bellbird) both feed in the same kōwhai tree but take nectar at different heights and times, so they do not compete directly.
Selective advantage
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A selective advantage is any feature of a response that makes an organism more likely to survive and reproduce than one without it.
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This is the idea the whole standard is built on. It links a response back to natural selection.
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The logic runs in four steps:
- Individuals vary in how they respond, and much of that variation is heritable.
- A response that improves survival or reproduction means those individuals leave more offspring.
- Those offspring inherit the alleles for that response.
- Over generations the response becomes more common in the population.
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So a selective advantage always has to end in one of two currencies:
- more offspring produced, or
- more offspring surviving to reproduce themselves.
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Saying a response "helps the organism" is worth nothing. Saying it "increases the energy available for egg production, so more offspring are produced" is worth the mark.
Why responses evolve at all
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Responses cost energy, so a response only persists if its benefit outweighs its cost.
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This trade-off explains behaviour that otherwise looks irrational, and Excellence questions are frequently built on it.
- A migrating kuaka (bar-tailed godwit) spends enormous energy flying to Alaska — but gains a summer of near-continuous daylight feeding and far fewer predators than it would face breeding here.
- A plant that grows tall to reach light spends resources on stem rather than seed — worth it under a closed canopy, wasteful in the open.
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Two organisms in the same place can therefore have opposite responses and both be correct, because their niches differ.
Worked Example
Worked Example
A researcher studies two plants growing on the same coastal dune at Muriwai:
- Species A grows prostrate (flat along the ground), reaching 8 cm tall.
- Species B grows upright, reaching 60 cm tall.
- The dune is exposed to strong salt-laden onshore wind.
- Species A occupies the seaward face of the dune; species B occupies the sheltered landward face.
Explain why each growth response provides a selective advantage in relation to the plant's ecological niche.
Answer:
To explain a selective advantage we must connect the response to survival and reproduction in the conditions each plant actually experiences — so we deal with the two niches separately.
Species A — the seaward face.
The stimulus is strong, salt-laden wind, an abiotic factor. The response is prostrate growth.
Growing flat keeps the plant inside the boundary layer of still air at the ground surface. This means:
- less physical damage, because wind force increases with height above the surface,
- less salt deposition on leaves, so less osmotic water loss from leaf cells,
- lower transpiration rate, conserving water in a substrate that drains almost instantly.
Because the plant loses less water and suffers less tissue damage, more of its fixed carbon goes into growth and seed production rather than repair. It therefore produces more surviving offspring than an upright individual on the same face would — which is the selective advantage.
Species B — the landward face.
Here wind is not the limiting factor; the sheltered face has denser vegetation, so light is. The response is upright growth.
Growing tall lifts the leaves above neighbouring plants, intercepting light that would otherwise be absorbed by competitors. More light means a higher rate of photosynthesis, more glucose, and so more resources for flower and seed production.
Linking the two. The same trait — height — is advantageous in one niche and disadvantageous in the other. That is the key idea: a response has no selective advantage in the abstract, only in relation to a particular set of environmental factors. Species A on the landward face would be shaded out; species B on the seaward face would be stripped by salt and wind.