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Populations and their limits: what stops the numbers rising questions
Population, community and habitat as separate terms; the sigmoid growth curve and its lag, log, slowing and stationary phases; carrying capacity; density-dependent against density-independent limiting factors; intraspecific and interspecific competition; the competitive exclusion principle and the ecological niche; predator-prey cycles and the lag between the two peaks; and estimating the size of a mobile population by mark, release and recapture with the Lincoln index.
6 original questions · 21 marks · the populations and their limits: what stops the numbers rising notes · Ecology, populations and environmental change
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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
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
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
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.
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.
State what is meant by the carrying capacity of a habitat, and state why a population can rise above it for a time.
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