48 exam-style questions on Individuals, populations, communities and ecosystems — topic 2.1 of Ecology in the IB Environmental Systems and Societies syllabus — with answers and explanations. Samples below; the full set is free with an account.
Grown on its own, a barnacle species survives across the whole height of a rocky shore. When a second barnacle species is present, the first is found only on the upper shore. The band it occupies when the competitor is present is its
Answer: realised niche
The fundamental niche is everything a species could tolerate; the realised niche is what competition and predation actually leave it. The gap between the two is the measurable effect of the competitor, which is why the transplant experiment is done at all.
Two protist species with identical food and temperature requirements are cultured together in one flask under constant conditions. Within a few weeks one of them has disappeared. This outcome illustrates
Answer: competitive exclusion of one species by the other
Where two species need exactly the same resources, the slightly better competitor takes them all and the other is eliminated; coexistence requires the niches to differ in some dimension. In the field, species usually avoid this by partitioning the resource in time, space or particle size.
Bare ground is exposed by a landslide. Which combination of traits is typical of the species that colonise it first?
Answer: Many offspring, short life, early maturity, wide dispersal
Colonisers of disturbed ground are r-strategists: they reach the site quickly, breed early, produce large numbers of small offspring and accept heavy losses. K-strategists invest in a few well-provisioned offspring and only prosper once conditions are stable.
An earthworm swallows dead leaf litter and digests it inside its gut, while a soil fungus growing on the same litter secretes enzymes onto it and absorbs the products. The earthworm is acting as
Answer: a detritivore
Detritivores ingest dead organic matter and break it down internally; decomposers digest it externally and absorb the soluble products. Both return nutrients to the soil storage, and both are routinely left out of food webs drawn by students.
Sea otters were hunted to near-extinction along parts of the North Pacific coast. Sea urchin numbers then rose sharply and the kelp forests were grazed down to bare rock. The sea otter is best described as
Answer: a keystone species
A keystone species has an effect on community structure far larger than its own abundance would suggest, and removing it restructures the whole system. An indicator species signals conditions rather than controlling them.
Reindeer introduced to an island with deep lichen cover rise to several thousand animals within twenty years and then crash to a few dozen over two winters. The crash is best explained as
Answer: overshoot of the carrying capacity followed by dieback
Lichen grows extremely slowly, so the herd consumed the food storage far faster than it was replaced and pushed the population above what the island could support. Once the storage was gone the carrying capacity itself had fallen, which is why the crash goes well below the original level rather than levelling off.
That was 6 of 48 Individuals, populations, communities and ecosystems questions.
Practise all 48 freeOutlineAO2 · 1–4 marks
A brief account or summary: the main points in two or three sentences, no evaluation.
DistinguishAO2 · 1–4 marks
Make the difference clear for BOTH terms — a definition of one of them is only half an answer.
SketchAO4 · 1–4 marks
The shape and key features (intercepts, asymptotes, turning points) — proportions, not exact values.
48 Individuals, populations, communities and ecosystems questions — free with an account. Spaced repetition, streaks and full exam simulations included.
Practise them freeThis platform is independently developed and is not endorsed by, affiliated with, or sponsored by the International Baccalaureate Organization. “International Baccalaureate” and “IB” are registered trademarks of the IBO.