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: in Britain, grey squirrels from North America compete with native red squirrels for nuts and seeds, and carry squirrelpox, which kills red squirrels.
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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: in Canada's forests, snowshoe hares and lynx rise and fall together about every ten years.
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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: when sand eels around Shetland were overfished in the 1980s, puffin and kittiwake chicks starved: fewer prey lowered the seabirds' carrying capacity.
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
- Yellowstone elk spend less time in open valleys where wolves hunt.
Evolution: a real example
- Myxoma virus killed over 99% of Australia's rabbits in 1950.
- Within ten years, surviving rabbits resisted it and the virus was milder.
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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: in the Serengeti, over a million wildebeest graze the grass short, their dung feeds dung beetles, and lions and hyenas limit their numbers.
How this comes up: Paper 1: state the relationship between two species in a food web [1], then predict what a change does [2]. Paper 2: explain predator-prey links and stability [7].
The figure above shows a simplified North Sea food web. Fishing boats catch large numbers of sand eels to make fishmeal.
Predict how a large fall in the number of sand eels could affect the North Sea food web.
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