Population growth curves at Higher Level: The same curves as SL. At HL, practise switching between numbers and rates, and explaining each phase with its cause.
Practise this as you read
- Read a growth rate graph as well as a numbers graph.
- Explain each phase of a curve with the factor behind it.
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Growth with no brakes: With nothing to limit it, a population grows in an exponential way, drawn as a J-shaped curve.
The points to remember
- With no limiting factors, a population grows exponentially: a J-curve.
- Growth is slow at first, because few individuals are breeding.
- Then it gets faster and faster: more individuals breed, so even more young are born.
- J-curve growth happens when a species reaches a new place with plenty of resources and no predators.
- It cannot last: resources run out, so the population often crashes.
Remember it as: J for 'just keeps going': until the food runs out.
Real example: 101 cane toads were released in Queensland, Australia, in 1935; with no predators able to eat them, there are now over 200 million.
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Growth with brakes: When density-dependent factors start to act, growth slows and the population levels off: an S-shaped curve.
The points to remember
- When density-dependent limiting factors start to act, the curve becomes S-shaped.
- Lag phase: few individuals, growth is slow.
- Exponential phase: plenty of resources, fast growth.
- Transitional phase: competition, predation and disease grow (environmental resistance), so growth slows.
- Plateau: the population levels off at the carrying capacity (K), with small rises and falls.
Remember it as: S for 'slows and settles': lag, fast, slowing, level.
Real example: sheep brought to Tasmania in the early 1800s increased fast, then levelled off at under two million by the 1850s: an S-curve.
A growth curve can show how many individuals there are, or how fast the number is changing. The exam can give you either.
The points to remember
- The number of individuals is how big the population is; the growth rate is how many are added each year.
- On a J-curve, the growth rate keeps rising.
- On an S-curve, the growth rate rises, peaks at about half of K, then falls to zero at K.
- The number keeps rising as long as the growth rate is above zero, even while the rate is falling.
- A negative growth rate means the population is shrinking.
Remember it as: Numbers: how many. Rate: how fast. Fastest at half K, zero at K.
Read the axis first: A falling growth rate does not mean a falling population. The population falls only when the rate is below zero.
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Boom and bust: Some populations do not level off. They boom, overshoot their carrying capacity, and crash.
The points to remember
- A population with plenty of food and no predators grows as a J-curve.
- The limiting factors act too late, so it overshoots its carrying capacity.
- It eats its food faster than the food can regrow and damages its habitat.
- Then it crashes: many starve (the bust).
- The damaged habitat now supports fewer than before: the carrying capacity has fallen.
Remember it as: Boom, overshoot, bust: and less food afterwards.
The guide's example: In 1944, 29 reindeer were released on St Matthew Island, Alaska, where no predators lived. They ate the slow-growing lichen faster than it could regrow. In the hard winter of 1963-64 most starved; by 1966, 42 were left.
Real example: 25 reindeer released on St Paul Island, Alaska, in 1911 rose to about 2000 by 1938, then crashed to 8 by 1950 after they had eaten the lichen.
How this comes up: Paper 2, Section B (b): explain why a named species follows an S-curve [7]. Data questions give J- and S-curves to identify, a carrying capacity to read off, or a boom and bust to explain.
Studies of wildebeest in the Serengeti, Tanzania have followed the herd's numbers for many years.
With reference to a named herbivore species, explain why its population growth curve follows an S-shape.
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