Abiotic Factors & Species Distribution (HL IB Biology)

Abiotic factors

• Abiotic factors are non-living factors that affect organisms within their habitat e.g.
  • Light intensity and wavelength
  • Temperature
  • Turbidity, or cloudiness, of water
  • Humidity
  • Soil or water pH
  • Soil or water salinity
  • Soil composition
  • Oxygen or carbon dioxide concentration

Adaptions to abiotic factors: sand dune grass species

• Marram grass is commonly found on sand dunes, a habitat where abiotic factors include:
  • Low water availability
  • High salinity
  • Low nutrient levels
• Marram is an example of a xerophyte, a group of plants that are adapted to survive in dry conditions; xerophytes need to deal with the following challenges
  
  • Dry air means that there is a steep concentration gradient between the inside of leaves and the surrounding air; this means that water evaporates quickly from the surface of cells in the leaf and is lost to the surrounding environment
  • Soil water availability is low, meaning that any water lost by evaporation may not be easily replaced
• Some marram grass leaf adaptations raise the humidity of the air surrounding the leaf, reducing the steep concentration gradient and so reducing water loss by evaporation
  • Leaves are rolled up to reduce the exposure of surfaces to the wind; this traps water vapour inside the rolled leaf
  • The stomata are sunken in pits to trap water vapour
  • The inner surface of the leaf is covered in tiny hairs which trap water vapour
• Marram grass leaves also have a thick waxy cuticle on their outer surface to reduce evaporation

Marram grass adaptation diagram

Marram grass is an example of a xerophyte, and has adaptations that reduce water loss by evaporation

Adaptions to abiotic factors: mangrove tree species

• Mangrove swamps are tropical, coastal habitats that are frequently submerged in sea water, so abiotic factors include:
  • High salinity
  • Low fresh water availability
  • Low oxygen availability (due to being underwater)
• Mangrove trees grow with their roots submerged in sea water, so they face challenges such as:
  • The cells of the roots that are underwater are unable to take in oxygen for respiration
  • The surrounding salty water means that the availability of fresh water is low
  • The surrounding sea water has a higher solute concentration than the contents of the mangrove root cells, so there is a risk of the mangrove roots losing water from its submerged roots by osmosis
• Mangroves are adapted to deal with low oxygen levels by having some form of aerial root system; the parts of the roots that are above the water take in oxygen for respiration
  • Red mangroves have prop roots
    • These roots also provide stability in unstable soil
    • Prop roots are partially under the water, and these underwater networks of roots provide crucial shelter for marine animals
  • Black mangroves have structures known as pneumatophores that grow vertically upwards out of the water-logged soil
• Mangrove trees also have different methods of dealing with low fresh water and high salinity levels
  • Red mangroves have cells that do not allow the entry of salt into their water-transport systems, allowing the trees to take up fresh water
    • These cells also prevent the outward movement of water, so prevent water loss by osmosis
  • Black mangroves take salt water into their cells and excrete excess salt through salt glands in their leaves

Mangrove adaptations diagram

Mangroves have adaptations that allow them to survive in environments with low oxygen levels and high salinity

• The abiotic, or non-living, factors in the environment can influence the distribution of species
• Abiotic factors affect living organisms in a variety of ways:

Effect of abiotic factors table

Abiotic factor	Effect of factor on living organisms
Light intensity	Light is required by plants for photosynthesis
Temperature	Temperature affects the rate of enzyme-controlled reactions
Water availability	Water is required by all living organisms for survival
Soil pH and mineral content	Different plant species require different pH levels and nutrient concentrations
Wind speed	High wind speeds can increase water loss by evaporation from the leaves of plants
Carbon dioxide concentration	Carbon dioxide is needed by plants for photosynthesis
Oxygen concentration	Oxygen is needed by all organisms that carry out aerobic respiration

Abiotic factors & species distribution

• Abiotic factors in a habitat can act as limiting factors for species distribution
• Species exist with a range of tolerance, meaning that certain conditions are ideal, or optimum, but some amount of variation from these ideal levels can be tolerated
  • This range within which the environmental conditions are tolerable is known as the range of tolerance
  • E.g. mangrove trees grow best in a salinity range between 3-27 parts per thousand (ppt), but they can survive in fresh water, and in salt concentrations of up to 75 ppt
• Species will not be found in areas with abiotic conditions that are outside their range of tolerance
• Species which are adapted to live in extreme conditions may have an especially wide range of tolerance, allowing them to live in areas where other species cannot survive
  • Such species will often have an environmental optimum that is higher or lower than average, e.g. marram grass has a lower optimum level for water availability than other plant species
  • These extreme species will do well in these extreme environments where competition from other species is low
  • Such extreme species may do less well in more moderate environments due to competition from species with an average environmental optimum
• Species will have a range of tolerance for all abiotic factors, and while some factors may have a more significant effect than others, the abiotic factors interact to determine the distribution of a species

Range of tolerance and population size graph

The cricket population shown here will be largest when they live within the optimum range for the abiotic factor in question, e.g. water availability. If the abiotic factor goes below the critical minimum (e.g. not enough water) or above the critical maximum (e.g. too much water) then the crickets will not survive and the population size will be zero