The African elephant is the largest land animal alive today, found across dozens of countries in sub-Saharan Africa, from open grassland to dense rainforest. For most of the 20th century, biologists treated every African elephant as one single species. Then, in 2021, that changed.
Field researchers had long noticed two physically distinct groups of African elephants living in very different habitats. Look at the two groups below before you decide anything: judge from the evidence, not from names.
Group 1
- Larger, curved outward tusks
- Large, roughly triangular ears
- Lives in open grassland and savanna
Group 2
- Smaller, straighter, downward-pointing tusks
- Smaller, rounder ears
- Lives in dense Central African rainforest
Same species, or two different species?
Two species. Group 1 is the African savanna elephant (Loxodonta africana); Group 2 is the African forest elephant (Loxodonta cyclotis). In 2021, the IUCN formally recognized them as separate species: not simply because they look different, but because genetic analysis showed their ancestors have been almost completely isolated from each other for around 500,000 years, roughly as different from one another as living Asian elephants are from extinct woolly mammoths. Looking different was a clue; genetic isolation is what actually settled the question. That's the biological species concept in action, and it's the vocabulary this page builds.
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African savanna elephant (Loxodonta africana): the largest living land animal; bulls stand up to ~3.3 m at the shoulder and weigh 5.2–6.9 tonnes. Population roughly 350,000, down over 60% in the last 50 years; IUCN status: Endangered.
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African forest elephant (Loxodonta cyclotis): smaller and straighter-tusked, shoulder height around 2.16 m and weight 1.8–5.4 tonnes, roughly half the mass of a savanna elephant. Population roughly 95,000, down more than 86% over 31 years; IUCN status: Critically Endangered.
- A third species, the Asian elephant (Elephas maximus), is a separate genus entirely and lives outside Africa.
- Adult African elephants eat roughly 150–300 kg of vegetation and drink up to ~150–200 liters of water per day.
- You'll meet this case again in the Communities & Ecosystems tab, including as a keystone species affecting community structure.
Citable in exam answers as a real-world example. Full sources in the page footer.
The biosphere as a nested system
The biosphere, the parts of Earth where life exists, is organized as a nested ecological system: individuals sit inside populations, populations inside communities, communities inside ecosystems, all inside the biosphere. Click each level below and watch the same real photo change to show it, including what actually makes a group of animals one species in the first place.
Individual
One elephant, on its own: the smallest unit in the hierarchy.
Quick check. "All the elephants, zebras, lions and acacia trees in the Serengeti" describes which level?
The species test: can they actually interbreed?
You just saw it above: a species is a group that can interbreed and produce fertile offspring. But "can mate" and "can produce fertile offspring" aren't the same test, and the gap between them matters.
Quick check. A horse and a donkey can mate and produce a mule, but mules are almost always infertile. Are horses and donkeys the same species?
Takeaway: "looks different" is a clue, not proof, and "looks the same" isn't proof of one species either. Interbreeding potential is the defining test, which is why genetic evidence, not just appearance, settled the elephant question.
Classifying life
With millions of species, classification lets us identify organisms efficiently and predict their characteristics from their relatives. Every species gets a two-part (binomial) scientific name: genus first (capitalized), species second (lower case), both italicized, Loxodonta africana, not "loxodonta Africana" or "LOXODONTA AFRICANA."
Sort each name below by whether it follows the binomial naming rules correctly.
Tools for identifying a species
Taxonomists use several tools to work out which species an unfamiliar organism belongs to: dichotomous keys (a series of either/or questions), comparison with reference collections held by taxonomists, and DNA surveys, the genetic evidence that actually resolved the elephant question above. Try a dichotomous key yourself in the Skills tab.
Know how to use dichotomous keys, applications and databases for the identification of species. Full practice: see the Skills tab.
What makes a population?
A population is a group of organisms of the same species living in the same area at the same time, and which are capable of interbreeding. A population is an interbreeding unit: one species can consist of anywhere from one to many separate populations.
Quick check. A savanna elephant herd in Kenya's Amboseli National Park and a completely separate savanna elephant herd in South Africa's Kruger National Park never meet or interbreed in practice, but both are Loxodonta africana. Are they the same population?
Everything about how populations grow, what limits them, and how ecologists actually count them (carrying capacity, growth curves, and sampling techniques) has its own tab: Populations.
Niche: an organism's job description
A niche is the particular set of abiotic and biotic conditions and resources an organism or population depends on: its full role in the ecosystem, not just where it lives. A niche has several dimensions working together:
- Spatial: the physical space and range it uses (a small fixed territory vs. roaming tens of kilometers a day)
- Trophic: what it eats, how it feeds, and what eats it
- Temporal: when it's active (day/night, wet/dry season)
- Interaction: how it affects and is affected by other species (competition, predation, mutualism, and the rest of 2.1.9)
Compare two very different East African herbivores across all four dimensions below.
Pick a species above
A niche is not the same as a habitat. Habitat is where an organism lives (its "address"); niche is its full ecological role: food, water, behavior, interactions (its "job"). Two very different species can share a habitat while occupying different niches, which is exactly why the elephant and the dik-dik above can coexist on the same savanna.
How populations interact
Populations interact through six recognized relationships: herbivory, predation, parasitism, mutualism, disease and competition. Each has ecological, behavioral and evolutionary consequences.
These six interaction types aren't unique to elephants, here are two more real examples from completely different organisms.
Mutualism: ants and aphids
Ants protect aphids from predators and parasites; in return, the aphids secrete honeydew (a sugary waste product) that the ants feed on. Both species benefit.
Photo: LadyDragonflyCC, CC BY 2.0
Parasitism: braconid wasps and a caterpillar
A parasitic wasp lays its eggs inside a live caterpillar; the larvae feed on the host from within and pupate in the small cocoons seen here, usually killing the caterpillar. The wasp benefits; the caterpillar is harmed.
Photo: Michael Scholz, CC BY-SA 4.0
Every scenario below is a real interaction involving elephants: classify each one.
Use models that demonstrate feeding relationships, such as predator–prey. Full practice: see the Skills tab.
Putting it together
Classification, niche and population interactions all answer the same underlying question in different ways: what exactly is this organism, and what is its role? Naming and classifying an organism tells you what it is. Its niche tells you what it does. Its interactions tell you how it affects, and is affected by, everything around it. All three come from the same nested hierarchy: an individual, doing a niche's worth of work, inside a population, inside a community.
This vocabulary reappears constantly: population growth and carrying capacity in the Populations tab; community, habitat and ecosystem definitions, plus keystone species, in the Communities & Ecosystems tab, where the elephant case pays off in full; classification and niche get their HL extension right here (clades, fundamental vs. realized niche).
Apply your knowledge: new organisms
Challenge 1
Transfer test. A clownfish lives among the stinging tentacles of a sea anemone, protected from predators by a mucus layer that stops it triggering the anemone's sting; the anemone gains food scraps and improved water circulation from the clownfish's movement. Answer using the vocabulary from this page.
Which of the six interaction types is this?
Name one abiotic and one biotic factor that could be part of the clownfish's niche
- Interaction type: mutualism, both the clownfish and the anemone benefit (protection and food/circulation), unlike parasitism where only one side benefits.
- Abiotic factor: water temperature or salinity, since anemones (and the coral reefs many live on) tolerate only a narrow range of both.
- Biotic factor: the presence of the specific host anemone species itself, since the clownfish's niche depends directly on that relationship for survival.
Challenge 2 · Extension
Transfer test. Five species of warbler (small insect-eating songbirds) all live in the same spruce trees in North American forests, and all eat insects. But biologist Robert MacArthur found each species forages in a different part of the tree: one mostly near the trunk, another mostly at the tips of high outer branches, another in the mid-canopy, and each moves through the tree in a slightly different pattern.
Using two dimensions of niche from this page, explain why these five species do not have identical niches, even though they share the same tree and food type
Predict what would likely happen if two of these warbler species began foraging in the exact same part of the tree at the same time
- Different niches: even though all five species share the same spatial location (the tree) and trophic category (insect-eaters), their niches differ in the specific spatial zone used within the tree (trunk vs. outer branches vs. mid-canopy) and in their temporal/behavioral foraging pattern: niche is defined by the full combination of dimensions, not just habitat or food type alone.
- Prediction: foraging in the same part of the tree at the same time would push their niches to overlap directly, increasing competition for the same limited insect prey in that zone, by evolving to partition the tree spatially, the five species largely avoid this competition, which is part of why they can coexist.
- IUCN Red List (2021), "African elephant species now Endangered and Critically Endangered"
- IUCN SSC African Elephant Specialist Group, 2024–2025 status report
- Science / AAAS, "Researchers Split African Elephants Into Two Species"
- Wikipedia, "African bush elephant" and "African forest elephant" (population and body-size figures)
- MacArthur, R.H. (1958). Population ecology of some warblers of northeastern coniferous forests. Ecology, 39(4).
Photographs:
- African savanna elephant, Amboseli National Park: Diego Delso (delso.photo), CC BY-SA 4.0
- African forest elephant: Wich'yanan L, CC BY 4.0
- Roan antelope, buffalo, zebra and eland at a waterhole: Bernard DUPONT, CC BY-SA 2.0
- Ants tending aphids (mutualism): LadyDragonflyCC, CC BY 2.0
- Parasitized caterpillar with braconid wasp cocoons (parasitism): Michael Scholz, CC BY-SA 4.0
- Kirk's dik-dik, Serengeti National Park: © Giles Laurent, gileslaurent.com, CC BY-SA 4.0
From 1967 to 1994, South Africa's Kruger National Park did something drastic: every year, rangers culled elephants to hold the population at almost exactly 7,000, roughly one elephant per square mile of the park. For 27 years, the number barely moved.
In 1994, international pressure ended the culling program. No more artificial cap. The population was free to grow on its own.
What happened to Kruger's elephant population after 1994?
It grew well beyond 7,000. By the 2021 aerial census, Kruger's elephant population had reached almost 28,000: roughly four times the culling-era cap.
That raises the real question this tab answers: was 7,000 ever the park's true carrying capacity, or just a number a park director picked in 1965? Kruger's own scientists have since said the 7,000 figure "was not related to the sustainable carrying capacity" of the elephants' actual habitat.
- Kruger National Park (South Africa) culled elephants annually from 1967 to 1994 to hold the population near 7,000.
- Culling ended in 1994 after international pressure; the population has grown steadily since.
- 2021 aerial census: approximately 27,998 elephants in Kruger, up from a culling-era low of roughly 7,000.
- You'll use this same case across carrying capacity, population regulation and growth curves below.
Citable in exam answers as a real-world example. Full sources in the page footer.
A family group in Kruger National Park, the population this whole tab tracks from a 7,000-strong managed cap to almost 28,000 today.
Derek Keats, CC BY 2.0
What decides where a population lives?
Every population's distribution is shaped by two categories of factor. Abiotic factors are the non-living physical conditions of an environment; biotic factors are the influences of other living organisms.
Six abiotic factors matter most: temperature, sunlight, pH, salinity, dissolved oxygen and soil texture. Not every factor matters equally to every species: for elephants, water availability (linked to temperature and rainfall) and vegetation cover are the abiotic and biotic factors that matter most; for a coral reef fish, salinity, dissolved oxygen and pH matter far more than soil texture ever could.
Quick check. Elephant calves face heavy predation risk from lions, and adult elephants compete with other large herbivores (buffalo, zebra) for the same waterholes. Are these abiotic or biotic factors?
Use methods for measuring at least three abiotic factors in an aquatic or terrestrial ecosystem, including the use of data logging. Full practice: see the Skills tab.
Carrying capacity
Carrying capacity (K) is the maximum size of a population that a given system can sustain, determined by competition for limited resources. Both abiotic and biotic resources can set the limit: water and food availability (abiotic-linked), and competition, predation or disease (biotic) can all cap how large a population gets.
Kruger's elephants, revisited. For elephants, the resources most likely to set a real carrying capacity are water availability in the dry season and the amount of browse (leaves, bark, branches) the vegetation can regenerate each year without being permanently degraded. Kruger's rangers picked 7,000 as a management target in 1965, but the population's actual ecological carrying capacity, set by real resource limits, may be considerably higher, which is consistent with the population settling well above 7,000 once culling stopped.
Carrying capacity is set by limited resources, not by a fixed number someone decides. Always name the actual resource (water, food, nesting space, browse) that runs out: "the environment can't support more" on its own doesn't explain the mechanism.
What actually regulates population size?
Density-dependent factors have a bigger effect the more crowded a population gets: competition for limited resources, increased predation risk, and faster disease transmission all intensify as density rises. These are what actually regulate a population around its carrying capacity, through negative feedback: as the population grows past K, these pressures increase, pulling growth back down; as it falls below K, the pressures ease, allowing growth again.
Density-independent factors affect a population regardless of how crowded it is: a drought, a wildfire, or an unusually harsh winter can devastate a population of any size. They can strongly influence population size, but they don't regulate it around K the way density-dependent factors do.
Sort each factor below by whether it is density-dependent or density-independent.
J-curves, S-curves, and boom-and-bust
Population growth follows one of two basic shapes, depending on whether limiting factors are operating.
MikaPr65, CC0
Exponential growth. With no limiting factors, growth is exponential: a J-shaped curve. Numbers grow slowly at first, then faster and faster, because a fixed percentage growth rate compounds on an ever-larger population. Nothing caps it.
Boom and bust: reindeer on St Matthew Island. In 1944, the US Coast Guard introduced 29 reindeer to St Matthew Island, Alaska: an isolated island with abundant lichen and no predators. The population followed a textbook J-curve: roughly 1,350 by 1957, then about 6,000 by 1963. But the island's lichen, the reindeer's main winter food, could not regenerate fast enough to support that many animals. Body weights had already dropped sharply by 1963. When an unusually severe winter hit in 1963-64, the population collapsed to just 42 reindeer by 1966: a 99% crash. The population had overshot the island's true carrying capacity, and both an exhausted food supply and a harsh winter (density-dependent and density-independent factors together) brought it down.
Quick check. Before the 1963-64 winter even hit, the reindeer population was already showing falling body weights and reduced fawn survival. What does this suggest?
Carrying capacity is not a fixed ceiling a population settles at and stays at forever. Because K is set by resource availability, it moves whenever the environment does: St. Matthew Island's lichen supply, and therefore its true K for reindeer, fell as overgrazing degraded it faster than it could regrow, and a harsh winter can lower K again independently of population size. A population that overshoots K is not breaking a rule, it is running ahead of a limit that can itself be shifting.
Why hasn't human population growth slowed the same way?
Every population studied so far is eventually held near its carrying capacity by limiting factors. Human populations are the exception, because humans have systematically removed the factors that would normally limit growth.
Match each limiting factor to how humans have bypassed it.
Total population keeps climbing, but the growth rate tells a different story.
Is the world's population growth rate higher or lower today than it was in 1970?
Lower, by a wide margin. The growth rate has roughly halved since its 1965-1970 peak, even though the population keeps climbing, because a shrinking percentage now applies to a far bigger base.
Annual growth rate at each point, with the approximate total population at the time. World population reached 8 billion in November 2022. Even at 0.84% per year, 2026 is still adding around 69 million people, because the rate now applies to a much larger base.
World population since 1800: the total keeps rising even as the annual growth rate falls, exactly the pattern behind the bar chart above.
Bdm25, CC BY-SA 4.0
Why can't we just calculate humanity's carrying capacity?
Carrying capacity is straightforward to define for a wild population with a fixed niche, but human carrying capacity cannot be easily assessed, for reasons unique to our species:
- Humans have an unusually broad and constantly changing ecological niche: unlike an elephant, we aren't locked into one habitat type or one set of resources.
- Resources are mobile: food, water and materials can be transported across the planet, so local resource limits don't cap a local population the way they would for a wild species.
- The human niche keeps expanding through technology: new farming methods, energy sources and consumption patterns repeatedly shift what "the limit" even means.
The result: any estimate of human carrying capacity can only really describe conditions right now, it changes as fast as technology and consumption patterns do. A widely cited 1995 review of 65 published estimates found figures ranging from under 1 billion to over 1,000 billion, though more than half clustered between 4 and 16 billion once extreme outliers were set aside.
The gap between estimates comes down entirely to which assumptions get plugged in. Compare three:
Under 1 billion to around 4 billion. The most cautious published estimates assume a high-consumption lifestyle for everyone and a firm ceiling on one resource, usually farmland or freshwater. Under these assumptions Earth is already at or past its human carrying capacity.
Quick check. Why is it misleading to say "Earth's carrying capacity for humans is X billion people," as though X were a fixed number?
How do ecologists actually count a population?
Counting every individual in a wild population is almost never possible. Instead, ecologists estimate population abundance using one of three sampling procedures, chosen to fit the situation:
Random sampling
Sample points are chosen using random coordinates, avoiding bias from the surveyor's own judgement. Best when a habitat is broadly uniform.
Systematic sampling
Sample points are taken at fixed, regular intervals across the site. Useful for detecting a known pattern, like distance from a water source.
Transect sampling
Samples are taken along a line laid across an environmental gradient (e.g. from shoreline inland) to capture how abundance changes across that gradient.
Quadrat sampling (percentage cover and percentage frequency, for non-mobile organisms) and capture-mark-release-recapture with the Lincoln index (for mobile organisms) are full hands-on skills in their own right: practice both in the Skills tab.
Putting it together
Every population sits somewhere on the path between exponential growth and its carrying capacity, held there by a running balance of density-dependent and density-independent factors: whether it's a reindeer herd on a remote island, an elephant population recovering from a management decision made in 1965, or the entire human species with a carrying capacity nobody can pin down.
Carrying capacity and growth curves return at HL as r- and K-strategist life cycles (see HL Extension). They also connect directly to keystone species and ecosystem tipping points, where the elephant case pays off again in the Communities & Ecosystems content.
Apply your knowledge: new populations
Challenge 1
Transfer test. In 1859, 24 European rabbits were released in Australia, a continent with no natural rabbit predators and ideal breeding conditions. Within decades, the population had exploded to an estimated several hundred million. In 1950, the myxomatosis virus was deliberately introduced and wiped out an estimated 90%+ of the population almost immediately. Answer using the vocabulary from this tab.
What shape would the rabbit population's growth curve have taken between 1859 and the 1940s, and why?
Is myxomatosis a density-dependent or density-independent limiting factor? Justify your answer.
- Growth curve shape: a J-curve (exponential growth), with no predators and abundant food, the rabbit population's growth rate wasn't held back by limiting factors, so numbers grew slowly at first then increasingly rapidly.
- Myxomatosis: density-dependent, the virus spreads through direct contact and shared burrows, so transmission (and therefore its population impact) scales with how densely packed the rabbit population is, just like disease transmission in the Kruger elephant/St Matthew Island examples.
Challenge 2 · Extension
Calculation transfer test. Ecologists studying a fish population in a lake capture and mark 60 fish, then release them back into the lake. A week later, they capture a second sample of 45 fish, of which 9 are found to be marked.
Use the Lincoln index to estimate the total population size, showing your working
- Population estimate = (M × N) ÷ R, where M = 60 (marked initially), N = 45 (total recaptured), R = 9 (marked individuals recaptured).
- Estimate = (60 × 45) ÷ 9 = 2,700 ÷ 9 = 300 fish.
- Whyte, I.J., van Aarde, R.J. & Pimm, S.L. (2003). Kruger's elephant population: its size and consequences for ecosystem heterogeneity. In: The Kruger Experience.
- SANParks Scientific Services, "Big Trunks, Bigger Questions: Rethinking Elephant Numbers in Kruger", 2021 census figure (~27,998).
- Klein, D.R. (1968). The introduction, increase, and crash of reindeer on St Matthew Island. Journal of Wildlife Management, 32(2).
- United Nations, World Population Prospects (2022, 2024 revisions): 8 billion (Nov 2022) and growth-rate figures.
- World Bank / UN historical population growth-rate data: approximately 2.1% (1965-70), 1.48% (1990), 1.17% (2010), 0.84% (2026).
- Cohen, J.E. (1995). How Many People Can the Earth Support? W.W. Norton. Synthesis of 65 published carrying-capacity estimates.
- Austin, T. (1859 release) and Fenner, F. & Ratcliffe, F.N. (1965), Myxomatosis: European rabbit population history in Australia.
Photographs:
- African savanna elephant family, Kruger National Park: Derek Keats, CC BY 2.0
- Reindeer on tundra: MikaPr65, CC0 1.0
- World population since 1800 (chart): Bdm25, CC BY-SA 4.0
In 1963, ecologist Robert Paine pried purple sea stars off the rocks of Makah Bay, Washington, and threw them back into the ocean, out of reach. He wanted to know what would happen to the rest of the tide-pool community if this one predator simply vanished.
Fifteen other species shared those rocks with the sea stars: mussels, barnacles, limpets, snails, algae. Paine kept the sea stars out for years and watched.
What happened to the community once the sea stars were gone?
One species took over. Freed from their main predator, mussels multiplied and crowded out almost everything else. Within a few years, a rock face that had held 15 species was dominated by a single mussel monoculture, and richness collapsed to around 8 species.
Paine named a species whose impact is wildly out of proportion to its numbers a keystone species: the same idea behind an architectural keystone, the wedge-shaped stone that holds an entire arch together. This tab covers that idea, and the bigger picture it sits inside: communities, ecosystems, and what happens when human activity pushes them past the point where they can hold themselves together.
- Robert Paine's removal experiment, Makah Bay, Washington state, USA (1963-1969); published in The American Naturalist, 1966.
- Tide-pool species richness fell from 15 species to around 8 within a few years of sea star removal, as mussels formed a monoculture.
- The same collapse happened again for real, unplanned, starting in 2013 (see the Keystone species stage below).
Citable in exam answers as a real-world example. Full sources in the tab footer.
Community and habitat
A community is a collection of interacting populations within an ecosystem: every mussel, sea star, barnacle and alga on a rock face, interacting through predation, competition and space. A habitat is different: the location in which a community, species, population or organism lives, including the type of ecosystem it needs to survive.
Sort each description below into the term it matches.
Ecosystems are open systems
An ecosystem is a community together with the physical environment it interacts with. Ecosystems are open systems: both energy and matter can cross their boundary, entering and leaving.
Tap a flow, then tap the zone it belongs to. One flow does not cross the boundary at all.
The falling-leaves flow is the trap. Nutrients cycling between fallen leaves and the trees that reabsorb them never cross the ecosystem's boundary, so nothing about that cycle makes the system open. A system counts as open because energy and matter can cross the boundary at all, not because everything inside it is constantly leaving.
Sustainability, and reversing biosphere loss
Sustainability is a natural property of ecosystems. In a steady-state ecosystem, inputs are balanced by outputs, and some ecosystems have persisted this way for millions of years.
Real example: the Daintree Rainforest, Queensland, Australia. Often cited as the oldest continuously surviving tropical rainforest ecosystem on Earth, with a history stretching back roughly 180 million years, tens of millions of years older than the Amazon. Its persistence is a real-world illustration of a steady-state ecosystem: inputs such as sunlight, rainfall and nutrient cycling have stayed balanced with outputs on a true deep-time scale.
That natural sustainability is not guaranteed. Once a system is pushed past a tipping point (next stage), sustainability needs to be actively reversed, not just protected. Protecting ecosystem integrity matters because it preserves the specific niche requirements that species depend on for survival: lose the niche, and the species that depends on it has nowhere left to go.
The subtopic's own engagement suggestions: take part in an ecological investigation of a local ecosystem, raise awareness of biodiversity loss, or contribute to a citizen science project collecting species distribution or abundance data.
Human activity can push ecosystems past a tipping point
A tipping point is reached when a disturbance is severe enough that the original ecosystem collapses and a new, different equilibrium develops in its place.
Real case: deforestation and the Amazon rainforest. Roughly 17-18% of the Amazon has already been deforested, and in the hardest-hit southeastern region forest loss reaches about 28%. Research from 2024-2026 estimates the tipping point could be crossed at around 20-25% total deforestation combined with continued warming, meaning parts of the Amazon could reach it as soon as the 2040s, with up to 47% of the biome facing critical tipping points by 2050.
Click the six steps below in the correct order to build the feedback loop.
Your order:
This loop is positive feedback: each step amplifies the original disturbance rather than correcting it. Contrast this with the negative feedback covered in the Populations tab (2.1.11), where density-dependent factors pull a population back toward carrying capacity instead of away from it.
Keystone species: a disproportionate role
A keystone species has a disproportionate impact on community structure relative to its abundance. Remove it, and the risk of ecosystem collapse is far higher than its small numbers would suggest.
Keystone status has nothing to do with being the biggest, most numerous, or top-of-the-food-chain species in a community. Purple sea stars were never abundant in Paine's tide pools, they were a minor part of the biomass. What makes a species keystone is the size of its effect relative to how common it is, not its raw numbers or size. A dominant species that makes up most of a community's biomass is not automatically keystone; a rare one can be.
Purple sea stars and mussels
Starting in 2013, sea star wasting disease killed an estimated 59-90% of ochre sea stars, by density and biomass, at monitored sites along the west coast of North America within about a year. Mussel populations grew larger and spread into lower tidal zones once their main predator was gone: the same collapse Paine predicted, playing out as an unplanned real-world event.
Sea otters and kelp forests
Sea otters prey on sea urchins, keeping their grazing in check. Where otter numbers have collapsed, urchins have multiplied unchecked and grazed kelp forests down to bare rock, called an urchin barren: parts of northern California have lost up to 90% of their kelp forest in recent years as a result. Where otter populations recover, kelp tends to recover with them.
Do not say a keystone species is simply "important." Say its removal causes a disproportionate change in community structure relative to its abundance, and name the mechanism.
Sort each scenario below: is the species described a keystone species, or not?
The planetary boundaries model
The planetary boundaries model identifies nine processes that regulate the stability of the Earth system, each with a proposed safe limit. Biosphere integrity is one of the nine, and changes to it have already passed a critical threshold: a proposed safe boundary of around 10 extinctions per million species-years, against a current rate estimated well over 100.
Potsdam Institute for Climate Impact Research (PIK), 2024, CC BY 4.0
Biosphere integrity is not transgressed alone. Assessments have tracked more boundaries being crossed over time: six of the nine by 2023, seven of nine by the most recent 2025 assessment, including climate change and biosphere integrity. This framework is regularly reassessed, so treat any specific count as a snapshot rather than a fixed number.
Putting it together
This tab climbed the hierarchy one more level: individual organisms form populations, populations form communities, and communities plus their physical environment form ecosystems. Ecosystems are naturally sustainable open systems until a disturbance pushes them past a tipping point into a new, degraded equilibrium. Keystone species and biosphere integrity are two lenses on the same risk: lose the right piece, or lose too many pieces at once, and the whole structure can come down.
This tab's ideas, niche requirements, tipping points, and how quickly a species can respond to disturbance, come back together in the HL Extension's synthesis section (2.1.30), which uses a real climate-driven mismatch between a bird and its food supply to show classification, niche and life cycle working as one toolkit.
Apply your knowledge: a new ecosystem
Challenge 1
Transfer test. Sea otters prey on sea urchins along the Pacific coast of North America. Where otters have been hunted to local extinction, urchin populations have exploded and grazed kelp forests down to bare rock, a degraded state ecologists call an "urchin barren." Where otters remain, kelp forests stay dense and support far more species overall. Answer using the vocabulary from this tab.
Is the sea otter a keystone species here? Justify your answer.
Using the idea of a tipping point, explain why an urchin barren might not recover into a kelp forest on its own, even after otters return.
- Keystone species: yes. Sea otters have a disproportionate impact on community structure relative to their abundance. By preying on urchins, they control grazing pressure on kelp, and their removal triggers a collapse from a diverse kelp forest to a much simpler urchin barren, the same disproportionate-impact pattern that defines a keystone species.
- Tipping point and recovery: once an urchin barren forms, it can become a self-sustaining alternate state. Urchins survive on scraps of algae and drift kelp even at very low kelp density, continuing to graze down any new kelp that tries to establish, so the system does not automatically flip back once otters return. This matches the tipping-point idea directly: the original ecosystem has collapsed and a new equilibrium has developed in its place, which may need active intervention to reverse.
Challenge 2 · Extension
Discuss the extent to which the planetary boundaries model is a useful tool for guiding conservation policy.
Tick each point you're confident your answer actually included (max 3 credited from each side):
Points in favor, max 3
Points against, max 3
A conclusion weighing which side is more persuasive is good practice, though not a separate marking point on its own.
Self-marked score: 0 / 6
- Paine, R.T. (1966). Food web complexity and species diversity. The American Naturalist, 100(910).
- Harvell, C.D. et al. and multiple follow-up studies (2014-2019): sea star wasting disease density and biomass decline estimates, Pisaster ochraceus, west coast North America.
- Whyte, I.J., van Aarde, R.J. and Pimm, S.L. (2003). Kruger's elephant population: its size and consequences for ecosystem heterogeneity. In: The Kruger Experience.
- Estes, J.A. and Palmisano, J.F. (1974). Sea otters: their role in structuring nearshore communities. Science, 185(4156). Recent kelp-loss estimates: UC Santa Cruz / NOAA surveys of northern California kelp forests, 2014-2021.
- Daintree Rainforest, Queensland: widely cited as the world's oldest continuously surviving tropical rainforest ecosystem, roughly 180 million years; UNESCO World Heritage listed 1988.
- Boulton, C.A., Lenton, T.M. and Boers, N. (2022), and subsequent 2024-2026 studies: Amazon rainforest deforestation, transpiration and rainfall feedback, tipping-point threshold estimates.
- Richardson, K. et al. (2023) and the Stockholm Resilience Centre's Planetary Health Check (2024-2025 assessments): planetary boundaries framework, biosphere integrity boundary and extinction-rate thresholds.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.1, statements 2.1.18-2.1.25.
Photographs:
- Sea star and other intertidal organisms, Prince William Sound: Dr. Terry McTigue, NOAA Photo Library, public domain
- Mussel bed at Worm's Head: Alan Hughes, CC BY-SA 2.0, via Geograph
- Sea otter with sea urchin: Matt Knoth, CC BY 2.0
- Planetary boundaries wheel diagram: Potsdam Institute for Climate Impact Research (PIK), 2024, CC BY 4.0
Read a dichotomous key
The real exam skill is reading a key like this one, not guessing your way through it. Start at couplet 1, compare the specimen to each pair of statements, and follow the instruction until you reach a name.
aHas tusksgo to 2
bNo tusksgo to 3
aHas a trunkAfrican elephant
bNo trunkCommon warthog
aHas a maneLion (male)
bNo manePlains zebra
Worked example: tracing an unknown specimen through the key
Unknown specimen
Couplet 1: Has tusks?
Yes: visible tusks curve from the sides of the mouth.
→ go to couplet 2Couplet 2: Has a trunk?
No.
African elephantCommon warthog
Lion (male)
Plains zebraPhotographs: African elephant: Diego Delso, delso.photo, CC BY-SA 4.0. Warthog, lion and zebra: Charles J. Sharp, sharpphotography.co.uk, CC BY-SA 4.0.
Challenge: African vultures
The elephant key above only gets easier with practice: real keys are harder. All four of these vulture species share the same savanna skies, and three are Critically Endangered. Read the key, then use it to identify each unlabeled photo yourself before revealing the answer.
aBare head with pink/reddish fleshy folds of skin ("lappets") on the sides, and a massive billLappet-faced vulture
bNo lappets on the headgo to 2
aSmall, thin bill, bare pale-pink face topped by a fluffy whitish "hood" of downHooded vulture
bLarger, head and neck covered in sparse whitish-gray down rather than bare pink skingo to 3
aBill pale/whitish; plumage strongly scaly, with rows of pale-tipped wing feathersRüppell's vulture
bBill black; plumage more evenly pale brown, without strong scalingWhite-backed vulture
Specimen A
Specimen B
Specimen C
Specimen D
Photographs: Hooded vulture: Quejaytee, CC BY-SA 4.0. Lappet-faced vulture: Bernard DUPONT, CC BY-SA 2.0. White-backed vulture: Alexf, CC BY-SA 3.0. Rüppell's vulture: CC BY-SA.
Read a predator–prey graph
Snowshoe hare and Canada lynx populations, based on the classic Hudson's Bay Company fur-trapping records (approximate, illustrative shape): one of the best-known real predator–prey datasets in ecology.
Read the graph. When the hare population peaks, what happens to the lynx population next?
Read the graph. Around year 30, the hare population is near its lowest point. What would you expect the lynx population to do over the following few years?
Describe this pattern as a time lag, and say which direction it runs: the predator population rises and falls after the prey population, because predator numbers depend on prey abundance rather than the other way round. "The graphs are similar" or "they follow each other" is too vague to earn marks; name the lag and its cause.
Measuring abiotic factors
Six abiotic factors turn up repeatedly across ESS: temperature, sunlight, pH, salinity, dissolved oxygen, and soil texture. Each has a standard instrument.
Temperature & light
Thermometer or temperature probe; light meter (lux meter) for sunlight intensity.
pH & salinity
pH meter or pH probe for acidity; refractometer or conductivity probe for salinity.
Dissolved O₂ & soil
Dissolved oxygen probe/meter for aquatic sites; soil texture assessed by hand-texturing or a sieve series.
Pick the tool.
A single manual reading only captures one instant. A data logger is a probe connected to a device that automatically records a chosen abiotic factor at set intervals (e.g. temperature every 10 minutes for a full week) without anyone present. This reveals patterns a one-off reading would completely miss, like daily temperature cycles or a brief pollution spike overnight.
Quick check. An ecologist wants to know whether a stream's dissolved oxygen level drops dangerously low overnight, when nobody is there to take a manual reading. What's the best tool?
Quadrat sampling
Quadrat sampling estimates abundance for non-mobile organisms (plants, or slow-moving/sessile animals) using a fixed-size sampling frame.
A 10×10 grid quadrat (100 squares total) laid over a patch of savanna grass. The shaded squares are where the grass species is present.
Choose a sampling method to place a new quadrat, then work out the percentage frequency yourself.
Your turn. Count the shaded squares in the quadrat above, then calculate the percentage frequency of the species.
Mark-release-recapture: the Lincoln index
For mobile organisms, ecologists estimate population size with capture-mark-release-recapture, then calculate the Lincoln index:
Population estimate = (M × N) ÷ R
M = number marked and released · N = total number recaptured · R = number recaptured that were already marked
Try it yourself. At a waterhole in Kenya, ecologists capture 50 impala, fit each with a small ear tag, and release them back into the herd. A week later, they return and capture a second sample of 40 impala. Of these, 8 already have ear tags from the first capture.
Pull the numbers you need out of the story above, apply the Lincoln index formula yourself, and type your estimate of the total impala population below.
The Lincoln index only gives an accurate estimate if: marked individuals redistribute randomly and mix fully back into the population; the population is closed (no significant births, deaths, immigration or emigration between samples); marking doesn't harm the animal or affect its recapture chance; and enough time passes for mixing, but not so much that the population itself changes substantially.
- Predator–prey graph shape is illustrative, based on the well-documented Hudson's Bay Company snowshoe hare/lynx fur-trapping records.
- IUCN Red List conservation status: Hooded, Rüppell's and White-backed vultures are Critically Endangered; Lappet-faced vulture is Endangered.
Photographs:
- African savanna elephant, Amboseli National Park: Diego Delso (delso.photo), CC BY-SA 4.0
- Common warthog, Burchell's zebra and lion (dichotomous key photos): Charles J. Sharp, sharpphotography.co.uk, CC BY-SA 4.0
- Hooded vulture: Quejaytee, CC BY-SA 4.0
- Lappet-faced vulture, Kruger National Park: Bernard DUPONT, CC BY-SA 2.0
- White-backed vulture, Miami Metrozoo: Alexf, CC BY-SA 3.0
- Rüppell's vulture: CC BY-SA
Every key term from 2.1, in one place.
- Biosphere
- The parts of the Earth where life exists; the highest level of the nested ecological hierarchy.
- Species (biological species concept)
- A group of organisms that can interbreed and produce fertile offspring.
- Classification
- Organizing organisms for efficient identification and prediction of characteristics; uses binomial (genus + species) naming.
- Dichotomous key
- A tool for identifying an organism through a series of either/or questions about its characteristics.
- Population
- A group of organisms of the same species living in the same area at the same time, capable of interbreeding.
- Community
- A collection of interacting populations within an ecosystem.
- Niche
- The particular set of abiotic and biotic conditions and resources an organism or population depends on: its role in the ecosystem.
- Habitat
- The location in which a community, species, population or organism lives: not the same as niche.
- Herbivory
- An animal feeding directly on living plant material.
- Predation
- One organism (the predator) killing and consuming another (the prey).
- Parasitism
- One organism benefits at the expense of another (the host), typically without killing it outright.
- Mutualism
- Both interacting organisms benefit from the relationship.
- Competition
- Organisms contend for the same limited resource, to the detriment of at least one.
- Abiotic factor
- A non-living physical factor that influences organisms, e.g. temperature, sunlight, pH, salinity, dissolved oxygen, soil texture.
- Biotic factor
- A living component of an ecosystem that influences organisms, e.g. predation, competition, disease.
- Carrying capacity (K)
- The maximum population size a system can sustain, determined by competition for limited resources.
- Limiting factor
- A resource or condition in short enough supply to restrict population growth.
- Density-dependent factor
- A factor whose effect on a population intensifies as population density rises, e.g. competition, predation risk, disease transmission: regulates a population around K via negative feedback.
- Density-independent factor
- A factor that affects a population regardless of its density, e.g. drought, wildfire, extreme weather.
- Negative feedback
- A mechanism that returns a system toward equilibrium: here, density-dependent factors intensifying as a population exceeds K, pulling it back down.
- Exponential growth (J-curve)
- Growth with no limiting factors, producing an accelerating, J-shaped curve: numbers grow by a fixed percentage of an ever-larger total.
- Logistic growth (S-curve)
- Growth that slows as density-dependent limiting factors intensify near carrying capacity, producing an S-shaped curve.
- Random sampling
- Sample points chosen using random coordinates, avoiding surveyor bias; suits a broadly uniform habitat.
- Systematic sampling
- Sample points taken at fixed, regular intervals across a site.
- Transect sampling
- Samples taken along a line laid across an environmental gradient, to capture how abundance changes across it.
- Quadrat
- A fixed-size sampling frame used to estimate abundance of non-mobile organisms.
- Percentage cover
- An estimate of the area within a quadrat covered by the species in question.
- Percentage frequency
- The number of occurrences of a species divided by the number of possible occurrences (e.g. grid squares) within a quadrat, as a percentage.
- Capture-mark-release-recapture
- A sampling method for mobile organisms: a sample is captured, marked and released, then a second sample is later taken to see what proportion is already marked.
- Lincoln index
- A formula estimating population size from mark-release-recapture data: (M × N) ÷ R, where M = marked and released, N = total recaptured, R = marked individuals recaptured.
- Ecosystem
- A community together with the physical environment it interacts with.
- Open system
- A system across whose boundary both energy and matter can enter and exit: the norm for real ecosystems.
- Sustainability (ecological)
- A natural property of ecosystems in which inputs are balanced by outputs, allowing the system to persist in a steady state.
- Tipping point
- The point at which a disturbance is severe enough that the original ecosystem collapses and a new, different equilibrium develops in its place.
- Positive feedback
- A feedback mechanism that amplifies an initial change, driving a system further away from its original state rather than back toward it: contrast with negative feedback.
- Keystone species
- A species with a disproportionate impact on community structure relative to its abundance; its removal carries a high risk of ecosystem collapse.
- Planetary boundaries
- A framework identifying nine Earth-system processes, each with a proposed safe upper limit ("boundary"), collectively defining a safe operating space for humanity.
- Biosphere integrity
- One of the nine planetary boundaries, assessed using species extinction rate and the proportion of nature's energy humans extract; currently assessed as strongly transgressed.
- CladeHL
- A group consisting of a common ancestor and all of that ancestor's descendants, with nothing excluded.
- MonophyleticHL
- A group containing a common ancestor and every one of its descendants: a valid clade.
- ParaphyleticHL
- A group containing a common ancestor and some, but not all, of its descendants, e.g. traditional "Reptiles," which excludes birds.
- PolyphyleticHL
- A group whose members are combined by a shared trait that evolved independently more than once, rather than by descent from a single recent common ancestor.
- Fundamental nicheHL
- The full range of conditions and resources a species could occupy in the absence of limiting factors such as competition.
- Realized nicheHL
- The narrower range of conditions and resources a species actually occupies once limiting factors, such as competition, are accounted for.
- r-strategistHL
- A species adapted to colonizing new or unstable habitats: many offspring per reproductive event, little or no parental care, short lifespan.
- K-strategistHL
- A species adapted to a stable environment near carrying capacity: few offspring per reproductive event, high parental investment, longer lifespan.
The questions and markschemes below attempt to mimic IB wording, phrasing and expectations, but are not IB-written questions.
Score: 0 / 11
Type your answer, then compare it to the markscheme and tick off what you actually included, that's your real score, marked by you.
Written questions: 0 / 25 (self-marked against the markscheme)
Distinguish between a species and a population.
Do not accept the terms as interchangeable:
Self-marked score: 0 / 2
Identify the type of population interaction shown in each example.
- Ticks attach to an elephant and feed on its blood.
- Elephants and impala use the same limited dry-season grazing.
- An elephant eats fruit and disperses viable seeds in dung.
Only mark the first response for each example if more than one interaction is given. Do not accept herbivory for item 3, as the example requires recognizing the seed-dispersal benefit to the plant:
Self-marked score: 0 / 3
Explain why Kruger National Park's elephant population increased after culling ended in 1994.
Do not accept only "culling stopped so numbers increased" for full marks without linking to reduced removal and resource availability/carrying capacity:
Self-marked score: 0 / 3
Explain how density-dependent factors can regulate a population around carrying capacity.
Do not accept only a list of density-dependent factors without explaining regulation/negative feedback:
Self-marked score: 0 / 4
Identify the most suitable sampling method for estimating plant abundance along a gradient from a shoreline inland.
Do not accept random sampling unless no gradient is involved:
Self-marked score: 0 / 1
Use the Lincoln index to estimate the population size if 50 impala are marked and released, 40 are captured in a second sample, and 8 of these are marked.
Award 1 mark for correct substitution into the Lincoln index, and 1 mark for the correct final answer:
Self-marked score: 0 / 2
Describe how a steady-state ecosystem can be considered naturally sustainable.
Do not accept only "it lasts a long time" without linking to balanced inputs/outputs or stability:
Self-marked score: 0 / 2
Explain how removal of purple sea stars could reduce species richness in a rocky shore community.
Do not accept only "sea stars are keystone species" without explaining the mechanism:
Self-marked score: 0 / 4
Explain how deforestation could push the Amazon rainforest toward a tipping point.
Do not accept only "deforestation causes climate change" without linking to transpiration/rainfall/feedback/tipping point:
Self-marked score: 0 / 4
To what extent can protecting keystone species reverse biodiversity loss in ecosystems?
Essays like this are marked holistically against markbands, not ticked off point by point. Read the sections below, then rate your own answer against the bands.
Answers may demonstrate
- understanding of concepts and terminology such as keystone species, biodiversity, species richness, community structure, trophic cascade, predation, competition, ecosystem stability, resilience, tipping point, habitat, niche, biosphere integrity and conservation;
- breadth in addressing and linking a range of mechanisms by which keystone species affect biodiversity, including predator-prey control, competition reduction, habitat maintenance, food-web structure, grazing control and prevention of ecosystem state shifts;
- examples of keystone species, such as purple sea stars controlling mussels in rocky shores, sea otters controlling sea urchins and allowing kelp forests to persist, parrotfish grazing algae on coral reefs, or fig trees providing food during scarce seasons;
- balanced analysis of why protecting keystone species may be effective, such as preventing trophic cascades, maintaining community structure, supporting many indirectly dependent species and reducing the risk of ecosystem collapse;
- balanced analysis of limitations, such as biodiversity loss also being driven by habitat destruction, climate change, pollution, invasive species, overharvesting, disease or nutrient pollution, which may not be solved by protecting one species;
- evaluation of the idea that protecting keystone species is most effective when combined with habitat protection, pollution control, climate action, restoration, legal protection and sustainable resource management;
- a conclusion that is consistent with, and supported by, the analysis and examples given, for example: protecting keystone species can be highly effective where biodiversity loss is driven by trophic imbalance or loss of community regulation, but it cannot reverse biodiversity loss alone if wider causes such as habitat loss, climate change or pollution continue.
Indicative content
- Protecting keystone species can maintain community structure because keystone species have a disproportionate impact relative to their abundance.
- Purple sea stars control mussel populations; their removal can allow mussels to dominate rocky shore space and reduce species richness.
- Sea otters control sea urchins; without otters, urchins may overgraze kelp forests, reducing habitat and biodiversity.
- Parrotfish may maintain coral reef resilience by grazing algae that could otherwise smother coral.
- Some keystone species maintain food availability, habitat structure, seed dispersal, pollination or nutrient cycling.
- Protecting keystone species may reduce the risk of trophic cascades and ecosystem tipping points.
- However, biodiversity loss may be caused by multiple pressures, not only the loss of a keystone species.
- Habitat destruction, fragmentation, pollution, invasive species, climate change, overharvesting and disease may continue even if a keystone species is protected.
- If the wider habitat is degraded, protecting or reintroducing a keystone species may not be enough for recovery.
- Protecting keystone species may be most effective when the main cause of biodiversity loss is trophic imbalance or loss of community regulation.
- A wider conservation strategy may be required, including habitat protection, restoration, pollution control, climate mitigation, sustainable harvesting and legal protection.
Markbands
The response shows limited understanding of keystone species or biodiversity loss. The answer may state that keystone species are "important" without explaining their disproportionate impact. Examples may be absent, vague or inaccurate. The response may be one-sided, with little or no consideration of limitations. There is little or no supported judgement on "to what extent."
The response shows sound understanding of keystone species and their role in community structure. There is some explanation of how protecting keystone species may maintain or restore biodiversity. At least one relevant example is used. There is some consideration of limitations, such as other drivers of biodiversity loss. The response includes some judgement, although it may be uneven or only partly supported.
The response gives a balanced and well-developed evaluation of protecting keystone species as a conservation strategy. It clearly explains mechanisms such as trophic cascades, predator-prey regulation, competition control or habitat maintenance. Relevant examples are used effectively. It evaluates limitations and recognizes that biodiversity loss is often caused by multiple interacting pressures. It considers the need for wider conservation strategies beyond protecting a single species. There is a clear and supported conclusion that directly answers "to what extent."
Self-assessed band: not yet rated
Beyond the traditional hierarchy
This tab covers five new HL-only ideas that extend everything in the Learn and Populations tabs: a better way to classify organisms by evolution, the limits of the traditional classification hierarchy, a sharper two-part definition of niche, the r/K life-cycle spectrum, and a synthesis of how classification, niche and life cycle together predict a species' vulnerability to human impact.
Classifying by evolutionary relationship: clades
A clade illustrates evolutionary relationships in which every member of a taxonomic group has evolved from a single common ancestor. Unlike the traditional Linnaean hierarchy (kingdom → phylum → class → order → family → genus → species), which groups organisms mainly by shared physical traits, clade-based classification groups them by shared ancestry, which can reveal relationships that appearance alone would miss or mislead.
The running case from the Learn tab, revisited: these two elephants look broadly similar, but ~500,000 years of near-total genetic isolation is what actually placed them in separate clades, appearance alone wasn't the deciding factor.
Monophyletic. Circling "Apes" and "Humans" together captures the shared common ancestor and every one of its descendants: nothing is left out. That completeness is what makes a group monophyletic; this Apes+Humans clade is sometimes called Hominoidea.
Biologymama53, public domain (CC0)
Quick check. Click through the toggle above. Why does circling "Apes" alone: the traditional, everyday use of the term, which excludes humans, create a paraphyletic group rather than a valid clade?
Monophyletic, paraphyletic and polyphyletic groups
Clades are judged by how completely they capture a common ancestor's descendants. Traditional "Reptiles" groups lizards, snakes and crocodilians together but leaves birds out, even though crocodilians actually share a more recent common ancestor with birds than with lizards and snakes: that makes traditional "Reptiles" paraphyletic. Add birds back in and the group becomes monophyletic: every descendant of the shared ancestor, with nothing left out.
Monophyletic
A common ancestor and all of its descendants. A valid clade. E.g. Apes + Humans above; lizards + snakes + crocodilians + birds together.
Paraphyletic
A common ancestor and some, not all, of its descendants. E.g. traditional "Reptiles" (excludes birds); traditional "Apes" (excludes Humans).
Polyphyletic
Members grouped by a shared trait that doesn't come from their most recent common ancestor. E.g. grouping whales with fish by streamlined body shape, ignoring that whales' closest relatives are land mammals.
Paraphyletic and polyphyletic are not the same mistake. A paraphyletic group leaves some descendants out of an otherwise-real ancestor group: it's an incomplete clade. A polyphyletic group combines species around a shared trait (like body shape or habitat) that evolved independently more than once: convergent evolution, so the group doesn't even share the single recent ancestor its name implies.
Quick check. Whales were once grouped with fish based on body shape and aquatic lifestyle. Why is this classification polyphyletic rather than paraphyletic?
Classify each grouping below.
Refining niche: fundamental vs. realized
Section 2.1.8 introduced niche as an organism's role within its community. At HL, this idea splits into two more precise terms. A species' fundamental niche is the full range of conditions and resources it could occupy in the absence of limiting factors such as competition. Its realized niche is what it actually occupies once those limiting factors are accounted for: usually narrower than the fundamental niche.
Legend: gold cylinders = Chthamalus, white cylinders = Balanus. Panel I shows both species' realized niches when competing together: Chthamalus confined to the upper shore, Balanus to the lower shore. Panels II and III isolate each species' fundamental niche. Notice Balanus alone (panel II) still can't reach the high-tide zone: its fundamental niche is capped by desiccation tolerance, not competition. Chthamalus alone (panel III) spans the full shore, top to bottom: it's competition, not physical tolerance, that shrinks its realized niche in panel I.
Jbell16, CC BY-SA 3.0, adapted from Mader, S.S. Biology (10th ed.), 1998
Classic example: Joseph Connell's barnacle study (Scotland, 1961). Two barnacle species, Chthamalus and Balanus, both settle across the full intertidal zone when raised without competitors: their fundamental niches overlap broadly. But when both are present, the faster-growing Balanus outcompetes and excludes Chthamalus from the lower shore, restricting Chthamalus's realized niche to the upper intertidal zone, which it can tolerate but Balanus cannot (it dries out there). Competition, not physical tolerance, is what actually shrinks the realized niche.
Say a species' realized niche is narrower than its fundamental niche due to competition (or another limiting factor): don't just say the niches are "different," name the mechanism that shrinks it.
Quick check. In the toggle diagram above, why doesn't Balanus simply spread into the upper shore once Chthamalus is pushed out of the lower shore?
Life cycles: r-strategists and K-strategists
Species differ enormously in how they invest in reproduction, and those differences are adaptations to how stable their environment is. A K-strategist is adapted to a stable, saturated environment already near carrying capacity: it produces few offspring but invests heavily in each one, giving them a high chance of survival. An r-strategist is adapted to colonizing new or unstable habitats and exploiting short-lived resources: it produces many offspring quickly, with little or no individual investment, accepting that most will not survive.
K-strategist
Few offspring per reproductive event; high parental investment/care; larger body size; longer lifespan; thrives in stable, late-succession communities near K. Example from the running case: African elephants, a single calf after a ~22-month gestation, followed by years of maternal and allomaternal care.
r-strategist
Many offspring per reproductive event; little or no parental care; smaller body size; shorter lifespan; thrives in unstable or newly available habitats, exploiting resources before they run out. Example from the Populations tab: European rabbits in Australia, 24 individuals released in 1859 exploded to hundreds of millions within decades.
This connects directly back to the growth-curve work in the Populations tab: r-strategists' rapid, opportunistic reproduction is what produces the steep, near-exponential rise of a J-curve when a species colonizes somewhere new. K-strategists' slower, more heavily invested reproduction is far more tightly constrained by density-dependent regulation, tracking the S-curve closely as the population settles near K.
Quick check. Green sea turtles lay around 100 eggs per clutch, bury them and leave: providing no further care, and roughly 1 in 1,000 hatchlings survives to adulthood. Are green sea turtles r-strategists or K-strategists?
Classify each species below by its life-history strategy.
Synthesis: classification, niche and life cycles under human impact
Classification (2.1.1-2.1.4, 2.1.26-2.1.27), niche (2.1.8, 2.1.28) and life-history strategy (2.1.29) are not separate topics: together they form a toolkit for predicting how a species will respond to human-driven environmental change. A species' niche breadth tells you how many conditions it depends on; its position on the r/K spectrum tells you how quickly it can adapt across generations if those conditions shift.
Real case: great tits and a shifting caterpillar peak, Wytham Woods, UK. Great tits (Parus major) time their egg-laying so that the peak food demand of their chicks lines up with the annual peak abundance of winter moth caterpillars. Spring temperatures at Wytham Woods have risen by roughly 2°C since 1965, and long-term data (1960-2020) show the caterpillar peak now arrives about 15 days earlier than it did in 1960. Great tits have shifted their laying dates earlier too: a real example of phenotypic plasticity, but researchers project that under continued warming, the mismatch between chick demand and caterpillar peak will widen again in coming decades.
The r/K framework helps explain why this mismatch is a real risk rather than a minor inconvenience. Great tits are comparatively K-selected: a handful of eggs per clutch, heavy parental investment, and one generation per year, so natural selection can only nudge laying date a little each generation. A species with a much shorter generation time and looser niche requirements could track a shifting resource peak far more quickly. The narrower a species' niche and the more K-selected its life cycle, the more exposed it is to tipping points like the ones introduced in ecosystem sustainability.
A synthesis question ("discuss the impact of climate change on a named species") is really asking you to combine content statements. Name the species' niche requirement that is under threat, state whether it is more r- or K-selected and what that implies for its ability to adapt, and only then describe the human impact itself.
Apply it. The pied flycatcher, a long-distance migrant, times its return to Britain using cues from its wintering grounds and migration route, since it cannot see the local caterpillar peak building before it arrives. The resident great tit stays put year-round and can shift its egg-laying date using local temperature cues it experiences directly. Which species is more exposed to a growing phenological mismatch as spring keeps arriving earlier, and why?
HL practice questions
Distinguish between monophyletic and paraphyletic groups.
Do not accept only "one is correct and one is incorrect" without explaining ancestry/descendants:
Self-marked score: 0 / 2
Distinguish between fundamental niche and realized niche, using the barnacle example.
Award 1 mark for fundamental niche, 1 mark for realized niche, and 1 mark for a valid applied example. Do not accept only "they are different niches" without naming the limiting factor:
Self-marked score: 0 / 3
- Connell, J.H. (1961). The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus. Ecology, 42(4).
- Mader, S.S. (1998). Biology (10th ed.). Boston: WCB/McGraw-Hill, p. 843: source for the Chthamalus/Balanus zonation diagram.
- The monophyletic/paraphyletic toggle overlay is drawn on top of the real cladogram photograph credited below; it is not a separate illustration.
- Visser, M.E. et al. (2021). Recent natural variability in global warming weakened phenological mismatch and selection on seasonal timing in great tits (Parus major). Proceedings of the Royal Society B, 288(1963): source for the Wytham Woods caterpillar-peak/great tit timing data.
- Both, C. et al. (2009). Climate change and unequal phenological changes across four trophic levels: constraints or adaptations? Journal of Animal Ecology, 78(1): source for the migrant (pied flycatcher) versus resident (great tit) mismatch contrast.
Photographs and diagrams:
- African savanna elephant, Amboseli National Park: Diego Delso (delso.photo), CC BY-SA 4.0
- African forest elephant: Wich'yanan L, CC BY 4.0
- Cladogram of primates: Biologymama53, public domain (CC0)
- Chthamalus/Balanus zonation diagram: Jbell16, CC BY-SA 3.0