Biology › Ecology, populations and environmental change › Populations and their limits: what stops the numbers rising
Populations and their limits: what stops the numbers rising
Nothing breeds for ever. A population put into a new place grows slowly, then explosively, then hardly at all, and the bend in that curve is where the biology is. Work out which factor bent it and you can answer almost anything this topic asks — including why a predator peak always arrives after the prey peak it is supposed to have caused.
Before this Energy transfer between trophic levels · Random sampling with quadrats · Natural selection and adaptation
Before you start
Predators control the numbers of their prey: that is what keeps a prey population in check. It is the wrong way round at least half of the time, and the graph students are shown to prove it actually argues against it. Look at any pair of predator-prey curves and the predator peak arrives after the prey peak — which is what you would expect if the prey were driving the predator, not the other way about. The honest answer is that the two drive each other, with the prey's own food supply usually setting the ceiling, and the predator following it round.
What you should be able to do
- Use population, community, habitat and ecosystem as four different words with four different meanings.
- Describe each phase of a sigmoid growth curve in terms of birth rate and death rate.
- Sort a limiting factor into density-dependent or density-independent, and justify the choice.
- Distinguish intraspecific from interspecific competition and predict what each does to a population.
- State the competitive exclusion principle and use the ecological niche to explain it.
- Explain why the predator peak lags the prey peak, and why that undermines the usual story about predators.
- Estimate a population by mark, release and recapture, and say which assumption a given problem breaks.
Four words that are not synonyms
Ecology questions are marked on vocabulary more than most topics, and four terms get used interchangeably in ordinary speech in a way that costs marks. Fix them first and the rest of the lesson has somewhere to sit.
- Population
- All the organisms of one species living in the same place at the same time, able to breed with one another.
- Community
- All the populations of all the species living in the same place at the same time.
- Habitat
- The place where an organism lives — a rock pool, a hedgerow, the gut of a cow.
- Ecosystem
- A community together with the non-living surroundings it interacts with, treated as one working unit.
- Abiotic factor
- A non-living influence: temperature, light, pH, water, oxygen, wind, soil depth.
- Biotic factor
- A living influence: competitors, predators, prey, parasites, pathogens, mates.
One species, one population. All the species together, a community. Add the abiotic surroundings and you have an ecosystem. A question that says 'the population of the pond' is either badly written or means one species, and an ecosystem is dynamic by definition: stable means the changes stay within a range, not that there are none.
The growth curve, and the four phases
Put a small number of organisms into a place that suits them and keep supplying them, and the number rises in a shape that turns up again and again — in a flask of yeast, in bacteria on an agar plate, in reindeer landed on an island, in a species arriving somewhere it has no history.
The lag phase. Numbers barely move, because there are few individuals to reproduce in the first place, so even a healthy birth rate produces very few offspring in absolute terms. Organisms are also acclimatising — bacteria synthesise the enzymes for a new food source before they can grow on it. That acclimatisation is why a real lag is longer than a mathematical curve predicts.
The log phase. Resources are plentiful and little is in the way, so birth rate greatly exceeds death rate. Growth here is exponential: the population multiplies by a constant factor in each equal interval rather than adding a constant number, and twenty doublings takes one bacterium past a million.
The slowing phase. Food per individual falls, waste accumulates, disease spreads more easily between crowded hosts, and nesting or rooting space runs short. Birth rate falls, death rate rises, and the gap between them narrows.
The stationary phase. Birth rate and death rate match. The population fluctuates about a level rather than sitting exactly on it, because the factors setting that level themselves wobble with the weather and the season.
- Carrying capacity
- The population size that a particular environment can support over a sustained period, given the resources available in it.
- Exponential growth
- Growth in which the population multiplies by a constant factor per unit time, so the number added per unit time keeps increasing.
Two things about carrying capacity get tested. It is not a fixed property of a place — a good summer raises it and a drought lowers it, so the population tracks a moving ceiling. And a population can overshoot it, because the damage done by too many individuals takes time to feed back; an overshoot is usually followed by a crash to below the original level.
In a closed system such as a sealed flask a fifth phase follows, with numbers falling as food runs out and toxic products accumulate. That death phase belongs to the culture, not to the wild.
Reading a growth curve you have never seen before
Rabbits are released onto an island of grassland with no predators. Numbers rise slowly for two years, then rapidly for four, overshoot to 11 000 in the seventh year, crash to 4 000 in the eighth and settle between 5 000 and 6 000. Account for each part of the pattern.
The slow start is the lag phase: the founding group is small, so even a high birth rate produces few offspring, and the rabbits are establishing burrows. The rapid rise is the log phase — grass abundant, no predators, no established diseases, so birth rate far exceeds death rate.
The overshoot to 11 000 happens because the feedback is delayed. Rabbits go on breeding while they graze the grass down faster than it regrows, and the shortage only shows up as deaths and failed litters some months later. The crash to 4 000 is that feedback arriving, and it undershoots because the grassland itself has been damaged.
Settling between 5 000 and 6 000 is the stationary phase, birth rate matching death rate, the fluctuation caused by the carrying capacity itself moving from year to year. The 11 000 was never the carrying capacity — it was a number on the way past it.
Two kinds of limiting factor
Every factor that holds a population down falls into one of two camps, and the test is simple: does its effect get stronger as the population gets denser?
| Density-dependent | Density-independent | |
|---|---|---|
| What it is | Its effect intensifies as the population becomes more crowded | Its effect is the same whatever the density |
| Usually | Biotic | Abiotic |
| Examples | Competition for food, light, space and mates; predation; parasites; infectious disease; accumulated waste | Extremes of temperature; drought; flood; fire; storms; a pesticide sprayed over a field |
| Effect | Regulates the population around the carrying capacity | Knocks it up or down regardless of where it was |
Only density-dependent factors can hold a population at a carrying capacity, because only they act as negative feedback: more individuals means less food each, so more deaths and fewer births, so fewer individuals. A cold winter does not care how many voles there were, so it cannot regulate them, only reset them. Infectious disease belongs firmly in the density-dependent column, which is why an epidemic tears through a dense seabird colony or a stocked fish farm and fizzles out in a sparse population.
Competition, inside a species and between them
Intraspecific competition is between members of the same species. It is the most intense competition there is, for a reason worth stating in an answer: individuals of one species need exactly the same resources in exactly the same way, so the overlap is total. As the population rises, the share each individual gets falls, breeding success drops, deaths rise, and the population is pushed back down. Intraspecific competition is the engine that produces a carrying capacity.
Interspecific competition is between different species that need some of the same resource. It reduces the population sizes of both, and it restricts the range of conditions in which each can persist. Where the overlap is severe, one species is driven out altogether.
- Niche
- The role a species plays in its community: all the conditions it can tolerate and all the resources it uses, including where it lives, what it eats, what eats it and when it is active.
- Competitive exclusion principle
- Two species cannot occupy the same niche in the same place indefinitely; the better competitor eliminates the other.
A niche is not a place. That single sentence is worth more marks in this topic than almost anything else, because 'niche' in ordinary English means a nook in a wall and students write it as a synonym for habitat. A habitat is where something lives; a niche is everything about how it makes a living there. Two species can share a habitat quite happily, and share a niche only briefly.
Gause put the principle on a bench in 1934. Grown separately, Paramecium aurelia and Paramecium caudatum each reached a steady population in a tube of the same bacterial culture. Grown together, P. aurelia took over and P. caudatum declined to nothing. Neither killed the other; one simply collected the food slightly faster, and slightly faster was enough.
In the wild, complete overlap is rare precisely because exclusion has already happened. On a rocky shore, Connell showed that young barnacles of one species settle right down the shore but survive only high up, because a faster-growing species undercuts and smothers them lower down; remove the competitor and they persist much lower. The conditions a species could tolerate are wider than the conditions it is found in, and competition trims one to the other. Some specifications call those two the fundamental and the realised niche; check your own, because a niche question is nearly always marked on the definition.
The same trimming explains coexistence. Where two similar species do live together they usually differ in some dimension — feeding at different heights in one tree, hunting at different times of day, taking different sizes of seed. That is resource partitioning, and it is the alternative to exclusion rather than an exception to it.
Predators and prey, and why the peak comes late
Draw the numbers of a predator and its prey on the same axes over several decades and you get two curves that rise and fall together with the same period, the predator curve smaller and shifted to the right.
Follow one cycle round. Prey are abundant, so predators are well fed and rear more young that survive — but eggs take time to hatch and cubs take time to grow, so the predator population only rises months or years later. By the time it has risen, the prey are already being eaten faster than they breed, and their numbers fall. Now there are many predators and little food, so predators starve and their numbers fall in turn. With predation pressure relieved, the prey recover, and the whole thing begins again.
The lag has a cause you can name in one sentence: a predator population can only respond to a food supply by reproducing, and reproduction takes time. Any answer that gets that sentence down has the mark.
The classic illustration is the Canadian lynx and the snowshoe hare, cycling roughly every ten years in the fur-trade records kept from the 1840s, and it is also the classic warning. Hares cycle where lynx are absent, which they should not do if lynx were the cause, and the Yukon field experiments settled it: adding food raised hare density roughly threefold, fencing out predators roughly doubled it, and doing both together raised it about elevenfold. Predation and food supply act together.
So be careful with the wording. Each population affects the other; predators do not 'control' prey numbers in any one-way sense. Say that the two oscillate, that the predator response is delayed, and that the prey's own food supply usually sets the ceiling they oscillate below.
Counting what will not stand still
You can count plants in a quadrat because plants stay put. Woodlice, beetles and field voles have to be estimated instead, by marking some, putting them back, and seeing what fraction of a later sample carries a mark.
estimated population = (number in the first sample × number in the second sample) ÷ number in the second sample that were markedThe Lincoln index. Both samples are counts of individuals, so the estimate has no units — it is a number of organisms.
Working an estimate, and then breaking it
A student collects woodlice from a 90 m² patch of leaf litter, marks 96 of them with a dot of non-toxic paint on the underside, and releases them. Two days later she collects again: 112 woodlice, of which 15 carry a mark. Estimate the population and its density. Then work out what her estimate would have been had three of the marks washed off.
Put the numbers straight into the index: (96 × 112) ÷ 15 = 10 752 ÷ 15 = 716.8, so about 720 woodlice. Quoting it to one woodlouse claims a precision the method has not got.
Density is the estimate divided by the area: 717 ÷ 90 = 8.0 woodlice per square metre. Give the unit; a bare number is not a density.
If three marks had washed off she would have recorded 12 instead of 15, and the estimate becomes (96 × 112) ÷ 12 = 896 — a quarter too high. The recaptured marks sit on the bottom of the fraction, so anything that reduces them inflates the answer. Fewer marks found, bigger estimate; more marks found, smaller one.
The method assumes a great deal, and questions almost always probe one assumption rather than asking for the list.
| The assumption | What breaks it | Effect on the estimate |
|---|---|---|
| The marked individuals mix back in randomly | Sampling again too soon, or in the same spot | Too many marks found, so too low |
| The mark stays on and stays visible | Paint that washes off, or a mark lost at a moult | Too few marks found, so too high |
| The mark neither harms nor advertises the animal | A bright mark that attracts predators, or a toxic paint | Marked animals removed, so too high |
| The population is closed between the samples | Births, deaths, immigration or emigration in the interval | Unreliable in whichever direction the numbers moved |
| Every individual is equally likely to be caught | Animals that learn to avoid the trap, or to raid it for bait | Trap-shy, too high; trap-happy, too low |
Practical detail earns marks here too. Mark on a surface the animal does not use for camouflage, use a paint that is neither toxic nor heavy, leave long enough for mixing but not long enough for a generation to turn over, and sample the second time by the same method over the same area. Cambridge names the Lincoln index explicitly in its A-level ecology content; AQA and OCR expect the same calculation whether or not they use the name.
TRY IT — Which assumption has failed?
A student estimates the ground beetles in a walled garden using pitfall traps. He marks 60 beetles with correction fluid and releases them beside the traps. The next morning he catches 75, of which 30 are marked, giving an estimate of 150 — far too low for a garden of that size. Suggest two reasons why, and one change to his method for each.
Check your answer
He released the beetles beside the traps and sampled again after a single night, so the marked beetles have not had time or distance to mix back in. They are over-represented around the traps, too many marks turn up, and a large denominator gives a small estimate. He should release them across the whole garden and leave several days.
Pitfall traps that are baited or offer shelter make beetles trap-happy: one caught before and unharmed is more likely to be caught again. Recaptures are inflated and the estimate deflated once more. He should use unbaited traps and reposition them for the second sample.
Each reason ends with the same chain: more marks recaptured, larger denominator, smaller estimate. Examiners want the direction argued rather than asserted.
In the exam
- A niche is a role, not a place. If your answer would still make sense with the word 'habitat' substituted, you have defined the wrong thing.
- Say what happens to birth rate and death rate in every phase of a growth curve. 'There is more food' is half an answer; 'more food, so birth rate exceeds death rate' is the whole one.
- For density-dependent or density-independent, justify with the test: does the effect strengthen as the population gets denser?
- Carrying capacity is the level a population can be sustained at, not the highest number on the graph.
- In a predator-prey answer, put the delay in explicitly: predators can only respond by breeding, and breeding takes time. That sentence is the explanation the mark scheme is looking for.
- In mark-release-recapture, work out which way the estimate moves before you write. Fewer marks recaptured always means a larger estimate, and saying so turns a vague answer into a reasoned one.
- Quote a population estimate to a sensible precision and a density with its unit. 716.8 woodlice is arithmetic; about 720 woodlice, or 8.0 per square metre, is an answer.
Check yourself
Red and grey squirrels both eat tree seeds and both nest in holes in trees. Where greys have been introduced to a British woodland, reds usually disappear within about fifteen years, although greys rarely attack them. Reds still persist in some large conifer forests where greys are present. Explain these observations using the ideas in this lesson.
Answer
Reds and greys have very similar niches: the same food and the same nest sites in the same habitat, so interspecific competition between them is heavy. Greys are the better competitors — larger, storing more fat, and able to digest unripe acorns that reds cannot — so they take a greater share of the shared resource.
Less food and fewer nest sites per red squirrel means a lower birth rate and a higher death rate, so their population falls. Because the niches overlap almost completely the competitive exclusion principle applies and the weaker competitor is eliminated rather than merely reduced — no direct attack required, which is why greys drive reds out without fighting.
The conifer forests are the informative part. There the main food is small conifer seed, which reds handle better than greys, so the overlap between the niches is smaller: the species partition the resource rather than competing for all of it, and coexistence becomes possible. A full answer adds squirrelpox — greys carry a virus they survive and reds usually do not — as a density-dependent factor acting alongside the competition.
Questions
Question 14 marks
Describe the four phases of a sigmoid population growth curve in terms of birth rate and death rate.
Mark scheme
- B1 lag phase: numbers barely move because there are few individuals to reproduce, so even a birth rate above the death rate produces very few offspring, and the organisms are still acclimatising
- B1 log phase: resources are plentiful and little is in the way, so birth rate greatly exceeds death rate and the population multiplies by a constant factor in each equal interval
- B1 slowing phase: food per individual falls, waste accumulates and disease spreads more easily between crowded hosts, so birth rate falls, death rate rises and the gap between them narrows
- B1 stationary phase: birth rate and death rate match, and the population fluctuates about a level rather than sitting on it, because the factors setting that level vary with the weather and the season
Question 24 marks
Explain why the peak in a predator population arrives after the peak in the population of its prey, and explain what that lag shows about the claim that predators control prey numbers.
Mark scheme
- B1 when prey are abundant the predators are well fed and rear more young that survive
- B1 a predator population can only respond to a food supply by reproducing, and reproduction takes time, so the rise in predator numbers appears months or years after the rise in prey
- B1 by the time predator numbers have risen the prey are being eaten faster than they breed and fall, so predators then starve and fall in turn, and with predation relieved the prey recover and the cycle repeats
- B1 if predators set prey numbers the predator peak should come first, so the lag argues that the prey drive the predators rather than the reverse; the Yukon experiments on snowshoe hares raised density about threefold by adding food, about twofold by excluding predators and about elevenfold by doing both, so the two act together
Question 34 marks
A researcher catches 150 field voles in a wood of area 3.0 hectares, marks them and releases them. A week later she catches 120 voles, of which 18 carry a mark. Calculate the estimated size of the population and calculate its density.
Mark scheme
- M1 estimated population = (number in the first sample × number in the second sample) ÷ number in the second sample that were marked
- M1 (150 × 120) ÷ 18 = 18 000 ÷ 18
- A1 an estimated population of 1000 voles
- A1 density = 1000 ÷ 3.0 = about 330 voles per hectare, which needs its unit to be a density at all
Question 44 marks
Larvae of two barnacle species settle across the whole of a rocky shore, but adults of one species are found only high up. When the other species is removed, the first survives much lower down. Explain these observations in terms of niche and competition.
Mark scheme
- B1 a niche is the role a species plays in its community — all the conditions it can tolerate and all the resources it uses — and it is not the same thing as a habitat
- B1 the range of conditions the first species can tolerate is wider than the range in which it is found, because its larvae settle and survive low on the shore when the other species is absent
- B1 lower on the shore the second species grows faster and undercuts or smothers the first, so interspecific competition removes it from that part of the shore without any direct attack
- B1 competition therefore trims the conditions a species could occupy down to the conditions it does occupy, and where two similar species coexist they usually differ in some dimension of the niche instead
Question 53 marks
A student estimates the number of woodlice in a patch of leaf litter, but between her two samples several marked animals moult and lose their marks. Suggest how this affects her estimate, and suggest two changes to her method that would reduce the problem.
Mark scheme
- B1 fewer marked animals are found in the second sample, and that number is the denominator of the index, so a smaller denominator makes the estimated population too high
- B1 she should leave a shorter interval between the samples — long enough for the marked animals to mix back in, but not long enough for many of them to moult
- B1 she should use a mark that survives handling and is placed where it is not rubbed off or hidden, using a light, non-toxic paint that neither harms the animal nor makes it conspicuous to predators
Question 62 marks
State what is meant by the carrying capacity of a habitat, and state why a population can rise above it for a time.
Mark scheme
- B1 the population size that a particular environment can support over a sustained period, given the resources available in it
- B1 the damage done by too many individuals takes time to feed back as deaths and failed breeding, so the population overshoots and is usually followed by a crash to below the original level
Worth remembering
- Population is one species; community is all of them; ecosystem adds the abiotic surroundings.
- A growth curve bends because birth rate falls and death rate rises, not because growth simply 'stops'.
- Density-dependent factors regulate a population; density-independent ones only reset it.
- Intraspecific competition is the fiercest kind, because the niches are identical.
- The predator peak always follows the prey peak, because predators respond by breeding.
- In the Lincoln index, fewer marks recaptured always means a larger estimate.