Abiotic & Biotic Factors (Cambridge (CIE) AS Environmental Management)

Abiotic Factors

• The non-living, physical factors that influence ecosystems and the communities of organisms living within them are known as abiotic factors

• These include factors such as:

  • Temperature

  • Humidity

  • Water

  • Oxygen

  • Salinity

  • Light

  • pH

• Changes in these abiotic factors can affect the survival and reproduction of organisms, and the overall functioning of ecosystems

Examples of Abiotic Factors

Biotic Factors

• The living, biological factors that influence ecosystems and the communities of organisms within them are known as biotic factors

  • In other words, biotic factors are the interactions between the organisms within a community

• These interactions include:

  • Competition

  • Grazing (herbivory)

  • Predation

• These biotic interactions mean that each species in an ecosystem has an influence on the population dynamics and carrying capacity of the other species

Competition

• Competition can be divided into intra-specific competition (competition between members of the same species) and inter-specific competition (competition between members of different species)

  • Intra-specific competition can lower the carrying capacity of a population due to a decrease in food availability caused by high population density

  • Inter-specific competition occurs between species with similar niches, causing a decrease in the carrying capacity of one or both species

Intra-specific Competition

• Intra-specific competition occurs when individuals from the same species compete for the same resources ("intra" means within)

• For example:

  • When resources are plentiful, the population of grey squirrels (Sciurus carolinensis) increases

  • As the population increases, however, there are more individuals competing for these resources (e.g. food and shelter)

  • At some point, the resources become limiting and the population can no longer grow in size - the carrying capacity has been reached

Inter-specific Competition

• Competition between different species for the same resources is described as inter-specific competition ("inter" means between)

• A well-known example of this occurs between the grey squirrel (Sciurus carolinensis) and the red squirrel (Sciurus vulgaris)

• The introduction of the grey squirrel into the southern UK caused the native red squirrel to be outcompeted for food and shelter

• The grey squirrel also carries a disease, parapoxvirus, that is harmless to itself but can be fatal to the red squirrel

• Conservation efforts in northern England and Scotland have slowed the spread of the grey squirrel by:

  • Protecting the red squirrels' habitats and food

  • Giving the red squirrel legal protection

  • Reintroducing the pine marten, a natural predator of the grey squirrel
    

Grazing

• This is when an organism (either a herbivore or omnivore) feeds on a plant

• The carrying capacity of herbivorous species is affected by the quantity of plants they feed on

• An area with more plant resources will have a higher carrying capacity for herbivore species

  • This can also have negative feedback effects - the carrying capacity of the herbivore species may decrease if herbivory rates are too high and the plant population decreases too much due to overgrazing

Predation

• This is when one animal eats another

  • This lowers the carrying capacity of the prey species

  • This can have negative feedback effects, lowering the carrying capacity of the predator species due to a decrease in prey numbers

• Consumers that kill and eat other animals are known as predators, and those eaten are known as prey

• In a stable community, the numbers of predators and prey rise and fall in cycles, limiting the population sizes of both predators and prey

• The graph below demonstrates some of the key patterns of predator-prey cycles:

  • Initially, the number of predators increases as there is more prey available

  • The number of prey then decreases as there are now more predators

  • Then the number of predators decreases as there is now less prey available

  • As a result, the number of prey increases as there are now fewer predators

  • This cycle repeats continuously