1.3 Variation in population size


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Every species has an optimum temperature, the further away from the optimum, the smaller the population can be.

In plants and cold blooded animals:

  • The lower the temperature, the slower enzymes work therefore their metabolic rate is reduced.
  • At high temp, enzymes start to denature, so again population grows slower.

In warm-blooded animals:

  • They can maintain a constant body temp, but this requires energy.
  • The further from optimum, the more energy used for homeostasis.
  • Therefore there is less energy left for growth, so they mature and reproduce slower.
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Other abiotic factors


  • Higher light intensity = faster rate of photosynthesis.
  • Faster rate of photosynthesis = faster growth and more spores or seeds produced.
  • More plants = also more animals supported by feeding on them.


  • Each enzyme has an optimum pH, further from this it works less effectively due to kinetic energy or denaturation.
  • Enzymes are used for metabolic processes, including growth. Optimum pH = more organisms.

Water and humidity

  • Water is needed for photosynthesis. No water = no plants = no animals.
  • Humidity affects transpiration, and evaporation from animals. In different humidities, differently adapted plants will grow.
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Population growth curve

1. Slow growth because a small number of organisms are reproducing rapidly.

2. Rapid growth because large number of organisms reproducing exponentially.

3. Stable state because of a limiting factor on the population i.e predation, food supply. There are small fluctations due to changes in those factors.

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