Population interactions at Higher Level: The same six interactions as SL. At HL, practise tracing each change through a food web and linking it to evolution.
Practise this as you read
- Trace every knock-on effect one step at a time, with its cause.
- Link interactions to selective pressures: tolerant hosts, milder parasites, wary prey.
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Six ways to interact: Populations that share an ecosystem affect each other in six ways: herbivory, predation, parasitism, mutualism, disease and competition.
The points to remember
- Herbivory: an animal feeds on a plant.
- Predation: a predator kills and eats its prey.
- Parasitism: a parasite lives on or in a host and feeds on it, harming it.
- Mutualism: both species benefit.
- Disease: a pathogen infects a host and harms it.
- Competition: two populations need the same limited resource. Different species: interspecific. Same species: intraspecific.
Remember it as: Plus or minus: four interactions are +/-, mutualism is +/+, competition is -/-.
Name the interaction, in its own word: Write 'herbivory', not 'herbivore' or 'food source'; 'mutualism', not 'symbiosis' alone. Two different species compete interspecifically, never intraspecifically.
Real example: the chytrid fungus has spread to frogs on every continent where they live, and has wiped out some species: disease.
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A model of a feeding relationship: Predator and prey numbers rise and fall in linked cycles. The predator peak comes after the prey peak.
The points to remember
- More prey means more food, so predator numbers rise.
- More predators eat more prey, so prey numbers fall.
- With less food, predators fall; with fewer predators, prey recover.
- The predator peak comes after the prey peak: a time lag, because predators take time to breed.
- Each population regulates the other: this is negative feedback.
- Both gain: the predator gets food; the prey is kept below its food limit and loses its weak and diseased members, so it stays healthy.
Remember it as: Prey up, predators up, prey down, predators down: and round again.
Read the graph: hares peaked in 1903 and 1913; lynx peaked in 1904 and 1915. The gap is the time lag.
Name it, do not describe it: Write 'predation' or 'predator-prey'. 'Big fish eat small fish' describes the relationship but does not name it.
Real example: on Isle Royale, wolves and moose have been counted every winter since 1959, and their numbers rise and fall together.
Change one, change many: A change in one population passes along the food web to the others, one step at a time. It can change the carrying capacity for other species.
The points to remember
- More predators: fewer herbivores, so plants recover and soil is held in place.
- Fewer predators: more herbivores, overgrazing, less plant cover and more soil erosion.
- A competitor that increases leaves less food for the other species, so its numbers fall.
- A change in one species can lower the carrying capacity for another: fewer prey for a predator, more herbivores for a plant, a lost partner in mutualism.
- Give every step: what changes, why, and what that changes next.
Remember it as: Follow the arrows: one step, its cause, then the next step.
Too vague to score: 'The food web will collapse' or 'the ecosystem becomes unstable' says nothing. Give each step and its cause.
Real example: as the Arctic warms, red foxes are moving north onto the tundra, where they compete with Arctic foxes for lemmings and even kill them, so Arctic fox numbers fall.
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More than numbers: Interactions change population sizes, how animals behave and, over many generations, how species evolve.
The points to remember
- Ecological: population sizes rise and fall, and changes pass along the food web.
- Behavioural: prey avoid places where predators hunt; predators switch to the prey easiest to catch that season.
- Evolutionary: predators, parasites and competitors are selective pressures: individuals with helpful traits survive and breed more.
- A new parasite spreads slowly, then fast; hosts die, so the parasite falls; hosts recover; the cycle repeats around a dynamic equilibrium.
- Over time tolerant hosts survive and milder parasites spread best, so the parasite becomes less deadly.
Remember it as: Numbers, behaviour, evolution: three kinds of consequence.
Behaviour: a real example
- Moose calves are born in spring, so wolves kill more calves in early summer.
Evolution: a real example
- Avian malaria kills many Hawaiian honeycreepers.
- Some honeycreepers in the lowlands now survive it: tolerance has evolved.
Checks and balances: Herbivores, predators and decomposers hold each other in check, so the whole ecosystem stays steady.
The points to remember
- Herbivores eat producers, which limits plant numbers; when plants fall, herbivores fall too: negative feedback.
- Herbivores pass energy to carnivores, whose predation limits herbivore numbers.
- Dung, dead animals and dropped leaves feed decomposers, which return nutrients to plants.
- Herbivores spread seeds, and large grazers such as elephants and bison keep grasslands open.
- Many kinds of herbivores and predators make the ecosystem more resilient.
- A predator makes its prey's growth slow sooner, level off lower and rise and fall; by easing competition among the prey it can prevent a crash.
Remember it as: Eat, be eaten, rot, recycle: each link holds another in check.
Answer the question about the curve: Say what the predator does to the prey's growth. The prey's damage to its habitat, or a J-curve with no predator, does not answer it.
Real example: on the North American prairies, bison graze and wallow, opening patches where other plants grow, and wolves once limited their numbers.
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How this comes up: Paper 2, Section B (a): outline host-parasite cycles [4]. Paper 1: name the relationship [1], predict knock-on effects [2].
A parasitic mite arrives on an island and infects the island's only species of lizard.
Outline how the populations of the lizard and the mite may reach an equilibrium over time.
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