• The number of species living in one habitat.
  • Ecology is concerned with the question:why a population is the size it is? 
  • Populations are affected by abiotic and biotic factors.
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Abiotic Factors

Abiotic factors = the physcial or non-living aspects to an ecosystem. These include:

•  Climatic factors, such as temperature; water/humidity; light/shade;current (wind/water), frost 
•  Edaphic (soil) factors, such as pH; mineral supply; soil texture; soil moisture.
•  Topographic factors, such as altitude, slope, aspect .
•  Human factors, such as pollution.
•  Catastrophes, such as floods and fire .

Abiotic factors can vary within a habitat, giving microclimates in microhabitats.

Abiotic factors tend to be density-independent factors, i.e. the size of the effect is independent of thesize of the population. For example a low light intensity will limit plant growth regardless of the number of plants present.

 Many abiotic factors vary with the seasons, and this can cause a periodic oscillation in the population size.

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Biotic Factors

Biotic factors are all the living aspects of an ecosystem. I.e. food, competitors, predators, parasites and pathogens.

Biotic factors tend to be density-dependent factors, i.e. the size of the effect depends on the
size of the population. For example competition will be greater the greater the population.

Interspecific Competition

  • Competition for resources (such as food, space,water, light, etc.) between
    members of different species.
  • In general one species will out-compete another one.
  • Tends to have a dramatic effect on populations.
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Biotic Factors 2

Intraspecific Competitionis

  • Competition for resources between members of the same species.
  • Is more significant than interspecific competition, since member of the same species have the same niche and so compete for exactly the same resources.
  • Tends to have a stabilising influence on population size because it is density-dependent.
  • If the population gets too big, intraspecific population increases, so the population falls again.
  • If the population gets too small, intraspecific population decreases, so the population increases again.
  • is also the driving force behind natural selection, since the individuals with the “best” genes are more likely to win the competitio nand pass on their genes.
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Biotic Factors 3


  • The populations of predators and their prey depend on each other, so they tend to show cyclical changes.
  • If the population of the prey increases, the predator will have more food, so its population will start to increase.
  • This means that  more prey will be eaten, so its population will decrease, so causing a cycle in both populations.

Parasitism and Disease

  • Parasites feed on larger host organisms, harming them.
  • Parasites and their hosts have a close symbiotic relationship, so their populations also oscillate.
  • If the population of parasite increases, they kill their hosts, so their population decreases.
  • This means there are fewer hosts for the parasite, so their population decreases.
  •  This allows the host population to recover, so the parasite population also recovers.
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Biotic Factors 4

  • In harsh environments (very cold, very hot, very dry, very acid, etc.) only a few species will have  successfully adapted to the conditions so they will not have much competition from other species.
  • But in  mild environments lots of different species could live there, so there will be competition.
  • In other words in  harsh environments abiotic factors govern who survives.
  •  in mild environments biotic factors (such as competition) govern who survives.
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