Survival Factors


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•  The distribution of a species is affected by limiting factors

In ecology, a limiting factor is a component of an ecosystem which limits the distribution or numbers of a population

  • A limiting factor defines optimal survival conditions according to its effect on a species when in deficiency or excess

Limiting factors can be either biotic (living) or abiotic (non-living):

  • Biotic factors include interactions between organisms – either intraspecific (within species) or interspecific (between species)
  • Abiotic factors include environmental conditions – such as light, temperature, salinity, rainfall, wind velocity, soil pH, etc.

Law of Tolerance

The law of tolerance was proposed by an American zoologist – Victor Ernest Shelford – in 1911

  • According to the law of tolerance, populations have optimal survival conditions within critical minimal and maximal thresholds
  • As a population is exposed to the extremes of a particular limiting factor, the rates of survival begin to drop

The distribution of a species in response to a limiting factor can be represented as a bell-shaped curve with 3 distinct regions:

  • Optimal zone – Central portion of curve which has conditions that favour maximal reproductive success and survivability
  • Zones of stress – Regions flanking the optimal zone, where organisms can survive but with reduced reproductive success
  • Zones of intolerance – Outermost regions in which organisms cannot survive (represents extremes of the limiting factor)

Shelford’s Law of Tolerance

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•  Distribution of one animal and one plant species to illustrate limits to tolerance and zones of stress

Plant Example

Plant growth varies greatly in response to concentrations of salt within the soil (salinity levels)

  • Plant species that are not particularly salt tolerant are called glycophytes – these plants are easily damaged by high salinity
  • Plant species that are salt tolerant are called halophytes – these plants may become stressed in freshwater environments

Most plant species are considered to be glycophytes – relatively few plant species are halophytes (~ less than 2%)

  • Cultivation of land for agriculture (e.g. irrigation or grazing) causes the water table to rise and concentrates salt at the roots
  • This makes it harder for glycophytes to extract water from the soil (also, the uptake of salt can be toxic to the plant)
  • Understanding salt tolerance for different plant species is therefore critically important to effective crop farming

Animal Example

Coral species form connected reefs that are greatly impacted by changes in oceanic temperature

  • Coral polyps receive nutrition from photosynthetic zooxanthellae (an algae) that lives within the polyp’s endodermis
  • The zooxanthellae cannot survive in lower ocean temperatures (i.e. < 18ºC)
  • Increases in ocean temperature cause zooxanthellae to leave the coral tissue, leading to coral bleaching (i.e. > 35ºC)

Reef-building coral species therefore have a typical optimal growth range in temperate waters between 20 – 30ºC

  • This correlates to the tropical and sub-tropical regions of the world (i.e. near the equator)

Global Distribution of Reef-Building Coral

coral distribution