Humans move their own limit: For most species, carrying capacity is set by local resources, and numbers settle around it. The human niche is broad and keeps changing, so our limit keeps moving.
The points to remember
- Humans use a very wide range of resources, so no single limiting factor can be found.
- Ingenuity: humans substitute one resource for another when one runs short.
- Technology keeps changing which resources are needed and how much is available.
- Lifestyles and consumption vary hugely between places and over time.
- Imports: trade moves resources, so a population is not limited by its own land.
- Climate change can quickly change which resources are available.
- So estimates are disputed and are true only for now.
Remember it as: Wide niche, clever, changing and trading: humans move their own limit.
A herd of deer
- Eats only the plants around it.
- Numbers settle at an equilibrium.
A human city
- Imports food from other continents.
- Numbers keep growing with trade and technology.
Real example: Singapore imports more than 90% of its food, so its population is limited by trade, not by its own land.
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Carrying capacity differs between countries, because they have different resources and use them differently.
Why it differs between countries
- Higher where resources are used efficiently, e.g. productive farms and efficient energy.
- Higher with more land and natural resources, or more imports.
- Higher with advanced technology, sanitation and health care, and money for good management.
- Lower where consumption per person is high: each person needs more.
- Lower where land is degraded or the climate is harsh.
Remember it as: Efficient, rich in land, high-tech, importing: higher. Heavy consumers, degraded: lower.
Real example: Singapore grows vegetables in vertical farms on rooftops and in old buildings, adding food without using new farmland.
Population size is not a reason: A bigger population does not change how many people the land can support; only resources and their use do.
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Food in, waste out: An environment supports a population by providing its food and by absorbing its wastes. Both set the limit.
The points to remember
- Food production raises carrying capacity: fertilisers, irrigation and new crops.
- Poor farming lowers it over time: soil erosion, salty soils, pesticides, desertification.
- Wastes lower it: sewage raises BOD, toxins poison food chains.
- Greenhouse gases cause climate change, which can cut harvests.
- Management raises it again: recycling, crop rotation, renewable energy, waste treatment.
- The ecological footprint is the inverse: bigger footprints mean fewer people supported.
Remember it as: Food in, waste out: both set the limit.
Real example: in the 1930s Dust Bowl, poor farming and drought blew away the topsoil of the US Great Plains, and millions of people had to leave.
How this comes up: Paper 2, Section B (a) [4]: outline why human carrying capacity is hard to find, or why it varies. Section B (c) [9]: how far food production and wastes set it.
A report says the Earth could support anywhere between 2 billion and 40 billion people.
Outline the difficulties in determining the carrying capacity of the human population.
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