Predation and predator–prey cycles – Interdependence Among Organisms Within Their Environment (Senior 5 Biology) | Ganzaa

Interdependence Among Organisms Within Their Environment

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Senior 5 · Biology

Interdependence Among Organisms Within Their Environment

4 lessons

Lesson 2 of 4

Predation and predator–prey cycles

25 min

What happens when the hunters come back?

Lions had been missing from Rwanda for years when African Parks and the Rwanda Development Board brought seven of them to Akagera National Park in 2015. Park managers had to think like ecologists. Would the lions eat the impala and buffalo down to nothing? Would the prey become more watchful? Would the number of lions settle at a level the park could support? Questions like these are about predation, an interaction that can control the size of two populations at once.

Definition

Predation: an interaction in which one animal, the predator, kills and eats another animal, the prey. Effect: + for the predator, − for the prey.

Herbivory (grazing and browsing): an animal eats parts of a plant, usually without killing it. Its effect on the plant is also −, so ecologists study it alongside predation.

Camouflage (crypsis): colour, pattern or shape that makes an organism hard to see against its background.

Adaptations of predators

Predator	Adaptation	How it helps
Lion	Forward-facing eyes with a large overlap (binocular vision)	Judges distance accurately during the final rush
Lion	Hunts in groups; tawny coat	Can bring down prey as large as a buffalo; stays hidden while stalking
Leopard	Retractable claws, long canine teeth, strong neck	Grips and kills prey, then drags it into a tree away from lions and hyenas
Chameleon	Eyes that move independently; a fast, sticky tongue longer than its body	Scans in two directions at once and catches insects at a distance
Spider	Silk web; venom	Traps flying insects; paralyses them before they escape

Defences of prey

Camouflage: a stick insect looks like a twig; a bushbuck's spotted coat breaks up its outline in dappled forest light.
Warning colouration (aposematism): bright patterns advertise that an animal is poisonous or distasteful. The orange and black African monarch butterfly, Danaus chrysippus, carries bitter chemicals from its milkweed food plants; a bird that tastes one learns to avoid the pattern.
Batesian mimicry: a harmless species copies the warning pattern of a harmful one. The females of the butterfly Hypolimnas misippus look very like the African monarch and are protected by the resemblance.
Müllerian mimicry: two or more harmful species share one warning pattern, so predators learn to avoid all of them faster.
Group living and vigilance: a herd of impala has many eyes and ears; one alarm snort sends the whole herd leaping away.
Speed, armour and weapons: the long legs of an antelope, the shell of a tortoise, the horns of a buffalo.
Plant defences against herbivores: the long thorns of acacia trees, bitter tannins in leaves, and sticky latex in the stems of euphorbias.

Definition

Warning colouration (aposematism): conspicuous colours that signal to predators that an organism is dangerous or distasteful.

Batesian mimicry: a harmless species (the mimic) resembles a harmful species (the model). Müllerian mimicry: two or more harmful species resemble each other.

Predator–prey cycles

In a simple community where a predator depends mainly on one prey species, the two populations often rise and fall in linked cycles. Follow the stages in Figure 1:

Prey increase. Predators are few, so more prey are born than are eaten.
Predators increase. With plenty of food, more predator young survive. Predators breed more slowly than their prey, so their peak comes later: this delay is the lag.
Prey decrease. Many predators now kill prey faster than the prey can reproduce. Food shortage for the prey can add to the fall.
Predators decrease. With few prey left, predators starve or fail to breed. With few predators left, the prey recover, and the cycle starts again.

Two more features of the graph are worth noticing. The predator line is lower than the prey line, because only a small fraction of the energy in the prey (often about 10 %) reaches the next trophic level. And the two populations regulate each other by negative feedback: more prey leads to more predators, which leads to fewer prey.

Evidence and evaluation

The best-known data come from the fur trade in Canada. Ecologists analysed the numbers of pelts of snowshoe hare and Canada lynx bought by the Hudson's Bay Company from the mid-nineteenth century onwards: both rose and fell in a cycle of about ten years, with lynx peaks following hare peaks. These data have limits. They count pelts, not animals, and trapping effort changed with fur prices. Later field experiments in the Yukon (C. J. Krebs and colleagues, 1995) added extra food and kept predators out of fenced plots. The hare population grew most when both were done, so the hare cycle depends on food as well as on lynx.

In the laboratory, C. B. Huffaker (1958) kept a plant-eating mite and a predatory mite on oranges. In a simple tray the predators ate every prey mite and then starved. When he made the habitat more complex, with barriers and with posts from which the prey mites could disperse, the two species cycled together for months. Hiding places and dispersal let prey survive in some patches, so the predator and prey can coexist.

Worked example 1: measuring the lag

Illustrative survey counts from a grassland:

Year	1	2	3	4	5	6	7
Prey (hundreds)	20	45	60	35	15	25	50
Predators	12	15	24	30	20	11	13

(a) State the lag between the prey peak and the predator peak. (b) Calculate the ratio of prey to predators in year 3. (c) Explain the fall in predators between years 4 and 6.

(a) Prey peak in year 3 (6 000); predator peak in year 4 (30). Lag = 1 year.
(b) Prey in year 3 = $60 \times 100 = 6\,000$ . Ratio $= \frac{6\,000}{24} = 250$ , so 250 prey to 1 predator.
(c) Prey fell from 6 000 to 1 500 by year 5. With less food, fewer predator young survive and adults starve, so predators fall from 30 to 11, one year behind the prey.

Worked example 2: how many prey does a pride need?

Illustrative figures: a pride of 6 lions eats on average 7 kg of meat per lion per day, and an impala provides about 25 kg of edible meat. Estimate the number of impala the pride would need in a year if it ate only impala.

Meat per day: $6 \times 7 = 42\,\mathrm{kg}$ .
Meat per year: $42 \times 365 = 15\,330\,\mathrm{kg}$ .
Impala per year: $\frac{15\,330}{25} = 613.2 \approx 610$ .

Interpretation: a few predators need a large, productive prey population. This is why predators are always far fewer than their prey, and why managers watch prey numbers before adding more lions to a park.

Effects of predation on the community

It regulates prey numbers and so protects the vegetation from overgrazing.
It removes mainly the young, old, sick and slow, so the fittest prey survive to breed: predation is a force of natural selection.
Some predators are keystone species: when R. T. Paine (1966) removed the starfish Pisaster from shore rocks in North America, the mussels it ate took over the rock and the number of species on it fell sharply.
Farmers use predators for biological control of pests: ladybird beetles and their larvae eat large numbers of aphids.

Khan Academy: "Predator-prey population cycles". Watch for why the predator peaks come after the prey peaks, and how the lynx and hare data are interpreted.

Warning

Common mistakes

"The predator and prey peaks happen at the same time." The predator peak comes later, because predators need time to breed on the extra food.
"Predators cause the whole prey cycle." Food supply, disease and weather can also drive prey numbers, as the Yukon experiments showed. Say "predation is one cause".
Confusing Batesian (harmless mimic) with Müllerian (all harmful) mimicry.

Tip

Describing a cycle graph: use the four-step story (prey up, predators up, prey down, predators down), quote the time of each peak from the axis, and state the lag with its unit.

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Practice: predation

Define predation and give its effect on each partner.
Describe three adaptations of a named predator for catching prey.
Distinguish between Batesian and Müllerian mimicry, with an example of either one.
Explain why the predator population in a predator–prey cycle is usually smaller than the prey population.
In a survey, the prey population peaked in 2018 and 2022, and the predator population peaked in 2019 and 2023. State the period of the cycle and the lag.
A cheetah eats about 3 kg of meat a day (illustrative). How many Thomson's gazelles, each giving 15 kg of meat, would it need in a year?
Evaluate the claim that "the hare population cycles only because of lynx predation".
Huffaker's predatory mites wiped out their prey in a simple tray, but cycled with them in a complex one. Explain why.

Show answersHide answers

A predator kills and eats another animal, its prey: + for the predator, − for the prey.
For example, a lion: binocular vision to judge distance; group hunting to kill large prey; a tawny coat for camouflage while stalking (also strong canines and claws).
Batesian: a harmless mimic looks like a harmful model (female Hypolimnas misippus resembles the distasteful African monarch). Müllerian: several harmful species share one warning pattern.
Energy is lost at each trophic level (often about 90 %), so a given amount of prey supports far fewer predators; predators also breed more slowly.
Period = 2022 − 2018 = 4 years; lag = 1 year.
$3 \times 365 = 1\,095\,\mathrm{kg}$ ; $\frac{1\,095}{15} = 73$ gazelles.
Only partly true. Pelt data show linked cycles but count pelts, not animals. Field experiments showed that adding food and excluding predators together raised hare numbers most, so food and predation both drive the cycle.
In the simple tray, prey had nowhere to hide and could not disperse, so every prey mite was found and eaten, and then the predators starved. Barriers and dispersal posts let some prey escape to new patches, so the prey survived somewhere and the cycle continued.

Key points

Predators are adapted to find, catch and kill prey; prey are adapted to avoid detection, warn, mimic, flee or defend.
Predator–prey cycles: the predator peak lags behind the prey peak, and the predator population is smaller.
Predation regulates prey numbers, acts as natural selection and can keep a community diverse (keystone predators).
Food supply and habitat complexity also shape the cycles (Yukon hares, Huffaker's mites).

Sources and credits

Written by Ganzaa. The lion reintroduction is reported by African Parks, which manages Akagera National Park with the Rwanda Development Board. The experiments described follow C. B. Huffaker, Hilgardia 27 (1958); C. J. Krebs and colleagues, Science 269 (1995); and R. T. Paine, The American Naturalist 100 (1966). No text was copied from them.
Photo: Danaus chrysippus female, Jeevan Jose, CC BY-SA 4.0, via Wikimedia Commons.
Video: Predator-prey population cycles, Khan Academy, embedded from YouTube.

Figure 1, the tables, the worked examples and the practice questions are written by Ganzaa. The data in the worked examples and practice questions are illustrative.

1What happens when the hunters come back?

2Adaptations of predators

3Defences of prey

4Predator–prey cycles