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Unit 3
10–15% of examThis unit is about how populations grow, shrink and run into limits. You'll compare how different species reproduce, read survivorship curves and age structure diagrams, and use simple math like the rule of 70 to explain why human populations grow at different rates in different countries.
Longer videos that cover the whole unit. Good for a first pass or a final review.
A specialist has a narrow niche, like eating only one food or living in one habitat, so it does well when conditions stay the same. A generalist has a broad niche and can live in many places and eat many things, so it handles a changing environment better.
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r-selected species, like insects and many fish, produce lots of small offspring, give them little care and often boom and crash. K-selected species, like elephants and whales, have few offspring, invest heavily in each one and tend to stay near carrying capacity, and many species fall somewhere in between.
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A survivorship curve shows how many members of a cohort (a group born at the same time) are still alive at each age. Type I species, like humans, mostly survive to old age, Type II species die at a steady rate, and Type III species, like many fish, lose most of their young early on.
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Carrying capacity (K) is the largest population an environment can support over the long run. When a population overshoots it, resources run short and the population often suffers a dieback, a sharp drop in numbers.
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When resources are plentiful, a population can grow exponentially, making a J-shaped curve. As food, water and space run short, growth slows and levels off near carrying capacity, making an S-shaped logistic curve.
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An age structure diagram, or population pyramid, shows how many people are in each age group, split into males and females. A wide base means fast growth, fairly straight sides mean slow or stable growth, and a narrow base with a wider top means the population is shrinking.
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Total fertility rate (TFR) is the average number of children a woman has in her lifetime. Replacement-level fertility, about 2.1 in developed countries, keeps a population roughly stable, and TFR tends to fall when girls get more education, women have children later, family planning is available and fewer infants die.
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A human population changes through births, deaths, immigration and emigration. The rate of natural increase is the crude birth rate minus the crude death rate; both are per 1,000 people, so divide the difference by 10 to get a percent. Dividing 70 by the growth rate in percent estimates how many years the population takes to double (the rule of 70), and growth is held back by density-dependent factors like disease and density-independent ones like droughts.
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The demographic transition model describes how birth and death rates change as a country develops. Death rates drop first thanks to better food, medicine and sanitation, so the population grows fast, and later birth rates drop too, moving through the pre-industrial, transitional, industrial and post-industrial stages.
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