Title: InkBiology / Ecology, populations and environmental change
On every note of this deck, unless a note says otherwise:
Footer: Continue on InkBiology · © 2026 InkBiology. All rights reserved.

Front: CHECK YOURSELF
A chalk grassland reserve has been grazed by sheep for two centuries and holds 38 plant species per square metre. Grazing stopped in 1998. By 2010 the reserve held 21 species per square metre and hawthorn scrub covered a third of it; by 2024 the scrub covered most of it and there were 12 species per square metre. Explain the changes, and evaluate the claim that the loss of species was caused by the end of grazing.
Back: Grazing was deflecting the succession. Sheep eat the seedlings of hawthorn and other woody plants, so the sere was held at a grassland stage, a plagioclimax, and the many small light-demanding herbs of chalk grassland could persist because nothing shaded them.
Once grazing stopped, the woody seedlings survived and grew. Hawthorn is taller than the herbs, so it shades them; a shaded herb photosynthesises below its compensation point, cannot make enough organic matter to grow or set seed, and is outcompeted. That is why richness fell as scrub cover rose, and why it fell further as the scrub closed over.
The succession will continue if nothing is done: scrub gives way to woodland, whose ground flora is quite different from and poorer than chalk grassland. Restoring the reserve means clearing the scrub and reintroducing grazing, and the herbs will only return if a seed bank survives in the soil.
On the evaluation: the evidence is good but it is not by itself conclusive, because it is a single site with no control. The change is correlated in time with the end of grazing, and there is a stated mechanism, shading by scrub, with an observable intermediate step, which is much stronger than bare correlation. But other things changed between 1998 and 2024, including the climate and probably the nitrogen deposited from the air, and either could reduce richness on chalk grassland independently.
What would settle it is a comparison: a similar area of the same grassland where grazing continued. If richness held up there and collapsed here, the end of grazing is established as the cause. An evaluation answer is marked on naming the evidence that would discriminate, not on hedging.

Front: DEFINE
Define: Succession.
Back: The change over time in the species occupying an area, in which each community alters the environment and makes it more suitable for the community that follows.

Front: DEFINE
Define: Biotic factor.
Back: A living influence: competitors, predators, prey, parasites, pathogens, mates.

Front: DEFINE
Define: Population.
Back: All the organisms of one species living in the same place at the same time, able to breed with one another.

Front: DEFINE
Define: Plagioclimax.
Back: A community held at a stage short of its climax by a repeated interference, usually human or from grazing animals.

Front: CHECK YOURSELF
Red and grey squirrels both eat tree seeds and both nest in holes in trees. Where greys have been introduced to a British woodland, reds usually disappear within about fifteen years, although greys rarely attack them. Reds still persist in some large conifer forests where greys are present. Explain these observations using the ideas in this lesson.
Back: Reds and greys have very similar niches: the same food and the same nest sites in the same habitat, so interspecific competition between them is heavy. Greys are the better competitors, being larger, storing more fat, and able to digest unripe acorns that reds cannot, so they take a greater share of the shared resource.
Less food and fewer nest sites per red squirrel means a lower birth rate and a higher death rate, so their population falls. Because the niches overlap almost completely the competitive exclusion principle applies and the weaker competitor is eliminated rather than merely reduced, with no direct attack required, which is why greys drive reds out without fighting.
The conifer forests are the informative part. There the main food is small conifer seed, which reds handle better than greys, so the overlap between the niches is smaller: the species partition the resource rather than competing for all of it, and coexistence becomes possible. A full answer adds squirrelpox, a virus greys survive and reds usually do not, as a density-dependent factor acting alongside the competition.

Front: DEFINE
Define: Abiotic factor.
Back: A non-living influence: temperature, light, pH, water, oxygen, wind, soil depth.

Front: DEFINE
Define: Ecosystem.
Back: A community together with the non-living surroundings it interacts with, treated as one working unit.

Front: DEFINE
Define: Competitive exclusion principle.
Back: Two species cannot occupy the same niche in the same place indefinitely; the better competitor eliminates the other.

Front: DEFINE
Define: Community.
Back: All the populations of all the species living in the same place at the same time.

Front: DEFINE
Define: Habitat.
Back: The place where an organism lives: a rock pool, a hedgerow, the gut of a cow.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Predators control the numbers of their prey: that is what keeps a prey population in check.
Back: The graph offered as proof argues the other way: the predator peak arrives after the prey peak, which is what you would expect if the prey were driving the predator. The two drive each other, with the prey's own food supply usually setting the ceiling.

Front: DEFINE
Define: Niche.
Back: The role a species plays in its community: all the conditions it can tolerate and all the resources it uses, including where it lives, what it eats, what eats it and when it is active.

Front: DEFINE
Define: Pioneer species.
Back: The first species to colonise a bare surface, adapted to survive there when almost nothing else can.

Front: CHECK YOURSELF
A river is sampled at four points below a farm. Point 1 is upstream of the farm; point 2 is immediately below a field drain; point 3 is two kilometres downstream; point 4 is ten kilometres downstream. Nitrate concentration is 2, 34, 30 and 11 mg per litre at the four points, and dissolved oxygen is 9.8, 9.4, 3.1 and 7.6 mg per litre. Explain the two patterns and explain why the lowest oxygen is not found at the point with the most nitrate.
Back: Nitrate is low upstream because the water has not yet passed the farm. It jumps at point 2 because nitrate has leached from the fertilised field, since nitrate ions are very soluble and are not held by negatively charged soil particles, so rain washes them through into the drain, and falls downstream as the river is diluted and as algae and plants absorb it.
Dissolved oxygen is barely affected at point 2 even though the nitrate is at its highest there. That is the key observation: nitrate does not remove oxygen, and nor do the algae, which photosynthesise and release it. Nothing has yet happened that consumes oxygen.
By point 3 the enriched water has had time for the sequence to run. The algal bloom that the nitrate allowed has multiplied, matted over the surface and blocked light from the plants below, which have died; the bloom itself has begun to die back. Saprobiotic bacteria are decomposing all of that dead material and have multiplied enormously, and their aerobic respiration is taking dissolved oxygen out of the water faster than it can dissolve in. Hence the minimum at 3.1 mg per litre.
By point 4 most of the dead material has been decomposed, the bacterial population has fallen back and the oxygen demand with it, so oxygen dissolves in again from the air and the concentration recovers. The distance between cause and effect is the point of the question: each step takes time, so the damage appears downstream of where the nutrient entered, and the farm responsible for a fish kill is often well upstream of the dead fish.

Front: DEFINE
Define: Exponential growth.
Back: Growth in which the population multiplies by a constant factor per unit time, so the number added per unit time keeps increasing.

Front: DEFINE
Define: Deflected succession.
Back: Succession that has been diverted from its natural course by an ongoing external factor.

Front: DEFINE
Define: Mycorrhiza.
Back: A mutualistic association between a fungus and a plant root, in which the fungus increases the surface area for absorbing water and mineral ions and receives organic compounds in return.

Front: DEFINE
Define: Carrying capacity.
Back: The population size that a particular environment can support over a sustained period, given the resources available in it.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Woodland is the natural state of Britain, so the best thing conservation can do for a piece of land is leave it alone.
Back: Chalk downland, lowland heath, hay meadow and coppiced woodland are among the richest habitats in Britain, and each exists because succession has been interrupted for centuries by grazing, cutting or burning. Leaving them alone turns them into scrub and then woodland.

Front: DEFINE
Define: Climax community.
Back: The final, relatively stable community, which persists until something disturbs it.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Eutrophication kills fish because the algae are toxic.
Back: The fish suffocate. The algae die, bacteria decompose them, and those bacteria respire aerobically in enormous numbers, stripping the dissolved oxygen from the water. The killer is several steps past the bloom, and each step is a marking point.

Front: DEFINE
Define: Seral stage.
Back: One identifiable community in the sequence; the whole sequence is a sere.

Front: DEFINE
Define: Saprobiont.
Back: An organism that obtains its energy from dead material by secreting enzymes onto it and absorbing the soluble products.