IB Diploma Environmental Systems & Societies SL · Ecosystems and Ecology
Mini-Lesson
Ecosystems and Ecology
This mini-lesson covers the living machinery of ecosystems: species, populations and communities, the interactions between them, population dynamics (S- and J-curves, carrying capacity), energy flow (trophic levels, GPP, NPP and ecological efficiency), the carbon and nitrogen cycles, zonation and succession, and how ecologists measure abiotic and biotic components — including capture–mark–recapture and Simpson’s diversity index.
Systems lens: an ecosystem is an open system — energy flows through it in a one-way stream, while nutrients cycle round and round.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Ecology · levels of organisation
Species, populations, communities and niche
Species — a group of organisms that can interbreed to produce fertile offspring.
Population — all the individuals of one species in an area at one time.
Community — all the populations of different species living and interacting in an area.
Habitat — the environment in which a species normally lives.
Ecological niche — a species’ full role: how it obtains energy, when it is active, and the abiotic and biotic conditions it needs and tolerates.
Competitive exclusion: no two species can occupy exactly the same niche in the same place for long — one out-competes the other.
Quick check
Habitat or niche?
?A biologist records exactly how a warbler feeds, when it is active, which part of the tree it uses, and the temperatures it tolerates. This complete description of its role is the species’...
Ecology · interactions
Interactions between species
Predation — one organism (predator) kills and eats another (prey).
Herbivory — an animal eats a plant.
Competition — organisms use the same limited resource (intraspecific = same species; interspecific = different species).
Mutualism — both species benefit (e.g. mycorrhizal fungi and plant roots).
Parasitism — one benefits, the host is harmed.
Disease — pathogens spread more easily in dense populations.
Keystone species have a disproportionately large effect on community structure relative to their abundance (e.g. sea otters controlling urchins).
Ecology · population dynamics
Population growth and carrying capacity
Plotted over time, populations follow characteristic curves:
J-curve — exponential growth when resources are unlimited; ends in a sudden crash (dieback).
S-curve (sigmoidal) — rapid growth slows and levels off at the carrying capacity (K), the maximum population the environment can support sustainably.
Limiting factors (food, water, space, predation, disease) increase environmental resistance as density rises, flattening the curve. r-strategists (many small offspring, little care) exploit new habitats fast; K-strategists (few large offspring, much care) dominate stable habitats near K.
Quick check
Why does the S-curve level off?
?A yeast population grows quickly then its numbers plateau. Which best explains the plateau of the S-curve?
Calculate
Your turn — capture–mark–recapture
1Ecologists catch and mark 40 woodlice, release them, and later catch a second sample of 50, of which 10 are marked. Use the Lincoln index N = (M × n) ÷ m to estimate the total population.
individuals
Hint: N = (40 × 50) ÷ 10.
Ecology · energy flow
Trophic levels, GPP and NPP
Energy enters through producers and flows one way along trophic levels (producer → primary consumer → secondary consumer → ...), losing energy as heat at each step.
NPP = GPP − RNPP = net primary productivity · GPP = gross primary productivity · R = respiratory loss
GPP — total energy fixed by producers in photosynthesis.
NPP — energy left as new biomass after producers respire; this is what is available to consumers.
For consumers, secondary productivity is the gain in consumer biomass after respiration and losses in faeces.
Calculate
Your turn — net primary productivity
2A grassland fixes 12 000 kJ m⁻² yr⁻¹ of energy as GPP. The plants lose 4000 kJ m⁻² yr⁻¹ in respiration. Calculate the net primary productivity (NPP).
kJ m⁻² yr⁻¹
Hint: NPP = GPP − R = 12 000 − 4000.
Ecology · efficiency and pyramids
Ecological efficiency and ecological pyramids
Only about 10% of the energy in one trophic level is passed to the next; the rest is lost as heat (respiration), in faeces, and to decomposers. This is why food chains rarely exceed 4–5 links.
efficiency = (energy at next level ÷ energy at previous level) × 100
Ecological pyramids show this:
Pyramid of numbers — counts of organisms; can be inverted (one tree feeds many insects).
Pyramid of biomass — dry mass at each level; usually upright but a snapshot.
Pyramid of energy — energy flow per unit area per year; always upright because of the second law.
Calculate
Your turn — ecological efficiency
3Producers hold 20 000 kJ m⁻² yr⁻¹; the primary consumers that feed on them hold 1800 kJ m⁻² yr⁻¹. Calculate the ecological efficiency of this energy transfer.
%
Hint: efficiency = (1800 ÷ 20 000) × 100.
Quick check
Why so few links?
?Why does a food chain rarely have more than four or five trophic levels?
Ecology · nutrient cycles
The carbon and nitrogen cycles
Unlike energy, nutrients are recycled. Two key biogeochemical cycles:
Carbon cycle — photosynthesis fixes CO₂ into biomass; respiration, decomposition and combustion return it. Long-term stores: fossil fuels, limestone, the deep ocean.
Nitrogen cycle — nitrogen fixation (bacteria, lightning) turns N₂ into ammonia; nitrification makes nitrates; plants absorb them; denitrification returns N₂ to the air.
Human impact: burning fossil fuels and clearing forests add CO₂ faster than sinks remove it; the Haber process and fertilisers flood ecosystems with reactive nitrogen.
Quick check
Turning air into nutrients
?Plants cannot use nitrogen gas (N₂) directly. Which process converts atmospheric N₂ into ammonia that living things can use?
Ecology · change over space and time
Zonation and succession
Zonation — the arrangement of communities in bands across space, driven by a changing abiotic gradient (e.g. altitude up a mountain, or up a rocky shore).
Succession — the change of a community over time. Primary succession starts on bare rock with pioneer species (lichens); secondary succession follows a disturbance where soil already exists.
As succession proceeds, biomass, soil depth, biodiversity and complexity generally rise until a climax community in equilibrium with the climate is reached.
Quick check
First on the rock
?A volcanic island cools to leave bare, lifeless rock. Which organisms are the typical pioneer species that begin primary succession?
Ecology · measuring diversity
Measuring biotic components and diversity
To sample communities we use quadrats (for abundance/percentage cover of non-motile organisms) and transects (to record change along a gradient — good for zonation). Motile animals are estimated by capture–mark–recapture.
Simpson’s D = N(N−1) ÷ Σ n(n−1)N = total organisms of all species · n = number of each species
A higher Simpson’s D means greater diversity (more species and more even abundance). Diversity tends to be higher in stable, undisturbed ecosystems.
Calculate
Your turn — Simpson’s diversity index
4A quadrat holds three species: A = 6, B = 3, C = 1 (so N = 10). Using D = N(N−1) ÷ Σ n(n−1), calculate Simpson’s diversity index.