Carrying capacity at Higher Level: The same ideas as SL. At HL, practise carrying a change, such as warming, through to the carrying capacity step by step.
Practise this as you read
- Link each change to a resource, then to the carrying capacity.
- Judge whether a change raises or lowers it, and why.
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The most an area can support: The carrying capacity is the largest population of one species that the resources of an area can support.
The points to remember
- Carrying capacity is the maximum number of individuals of one species an area can support sustainably.
- It is set by competition for limited resources: food, water, space, light, nest sites.
- Below it, births outnumber deaths, so the population grows.
- Above it, deaths outnumber births, so the population falls back.
- Most populations rise and fall around their carrying capacity (K).
Remember it as: Carrying capacity: one species, most individuals, for the long term.
One species, not many: Write 'the maximum number of individuals of a species'. 'The maximum number of species' describes something else and is wrong.
Real example: on St Kilda, wild Soay sheep increase until the grass runs short, then many die in late winter.
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Every species needs some resources from the non-living world and some from other living things.
The points to remember
- Abiotic resources: water, light, space, nest sites, soil nutrients, oxygen, a suitable temperature.
- Biotic resources and factors: food or prey, partners in mutualism, and fewer predators, competitors or pathogens.
- The resource in shortest supply sets the limit: it is the limiting factor.
- Improve the limiting resource and the carrying capacity rises; reduce it and the carrying capacity falls.
- Farmers raise it for crops with fertiliser and irrigation.
Remember it as: Shortest supply sets the limit: fix it and the limit rises.
Abiotic: the non-living world
- Water
- Light
- Space and nest sites
- Soil nutrients and oxygen
- Temperature
Biotic: other living things
- Food or prey
- Mutualist partners, such as pollinators
- Predators
- Competitors
- Pathogens
Real example: brown trout in a Highland stream need cold water rich in oxygen (abiotic) and insects to eat (biotic); in a hot summer the oxygen falls and the stream supports fewer trout.
Too many for the land: When nothing keeps a population in check, it can overshoot its carrying capacity and damage its own habitat.
The points to remember
- With no predators and no culling, a population can grow beyond its carrying capacity.
- It overgrazes: plants are eaten faster than they regrow, and plant diversity falls.
- Bare ground erodes; young trees die when animals eat them or strip their bark.
- Food runs out, so animals starve or catch disease, and the population crashes.
- Managers cull (kill a set number) to keep the population at its carrying capacity.
Remember it as: Overshoot, overgraze, erode, starve, crash.
Real example: before wolves returned to Yellowstone in 1995, elk grazed river valleys so hard that young willows and aspens could not grow, and river banks eroded.
Say what actually happens: 'The ecosystem becomes unstable', 'it collapses' or 'resources are used up' are too vague. Name the change: overgrazing, soil erosion, bark stripping, starvation, a crash.
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Carrying capacity is easy to define and hard to measure. Scientists can only estimate it, and it keeps changing.
Why it is hard to estimate
- There are many limiting factors, and they act together.
- Conditions change: weather, climate change, new diseases, introduced species.
- Species' needs can change as they evolve; finding them takes long-term study.
- Counts are estimates with a large error; a large range is hard to sample.
- Future prey, competitors and policies (such as a hunting ban) are hard to predict.
- Humans use many resources, substitute one for another, import resources and export wastes.
Remember it as: Many limits, always changing, hard to count.
Real example: the Scottish government estimates wild deer numbers from helicopter counts, which miss animals hidden in forests.
Humans are a special case: A city can live beyond its local carrying capacity by importing food and energy and exporting its wastes, or by using new resources, such as turning sea water into drinking water.
Warming can move the limit: Global warming changes the resources a species depends on, so it can raise or lower its carrying capacity.
The points to remember
- Warmer weather means more plant growth: more food, so carrying capacity rises.
- But more plant growth can also feed more competitors, so it can fall.
- Glaciers retreat, exposing new habitat: carrying capacity rises.
- Droughts and wildfires destroy food and habitat: carrying capacity falls.
- Less rain means less plant growth and less food: carrying capacity falls.
Remember it as: Warming, then the resource, then the carrying capacity: three links.
Real example: emperor penguins breed on sea ice that must last from April to December; where warming breaks the ice up early, chicks drown, so fewer penguins can breed.
Start from warming: 'Less food lowers carrying capacity' is not enough: first say how warming causes the change in food or habitat.
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How this comes up: Paper 1: describe how warming could change a species' carrying capacity [3]; predict what happens to an unmanaged population [2]. Paper 2: define it [1] and explain the S-curve [7].
The mountain nyala is a large antelope that lives only in the high forests and grasslands of Ethiopia's Bale Mountains. Climate models predict warmer temperatures and longer dry seasons there.
Describe how global warming could affect the carrying capacity of the mountain nyala.
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