Major Abiotic Factors NCERT (NEET)

13.1.1 Major Abiotic Factors

1. Temperature: 

• Temperature is the most ecologically relevant environmental factor. 
• You are aware that the average temperature on land varies seasonally, decreases progressively from the equator towards the poles and from plains to the mountain tops. 
• It ranges from subzero levels in polar areas and high altitudes to >50°C in tropical deserts in summer. 
• There are, however, unique habitats such as thermal springs and deep-sea hydrothermal vents where average temperatures exceed 100°C. 
• It is general knowledge that mango trees do not and cannot grow in temperate countries like Canada and Germany, 
• snow leopards are not found in Kerala forests 
• and tuna fish are rarely caught beyond tropical latitudes in the ocean. 
• You can readily appreciate the significance of temperature to living organisms when you realise that it affects the kinetics of enzymes and through it the basal metabolism, activity and other physiological functions of the organism. 

Eurythermal organisms: 

• A few organisms can tolerate and thrive in a wide range of temperatures. e.g., Tardigrades (water bears or moss piglets) at  –273 °C to 150 °C, The desert pupfish  from 8º to 42 °C, The green crab 8°C to 35 °C, 

Stenothermal organisms: 

• a vast majority of them are restricted to a narrow range of temperatures. 
• The levels of thermal tolerance of different species determine to a large extent their geographical distribution. 
• ex. polar bear, snow leopard, tropical mango tree, tuna fish, penguins etc

2. Water: 

• Next to temperature, water is the most important factor influencing the life of organisms. In fact, life on earth originated in water and is unsustainable without water. 
• Its availability is so limited in deserts that only special adaptations make it possible to live there. 
• In the aquatic environment, the sediment-characteristics often determine the type of benthic animals that can thrive there.
• The productivity and distribution of plants is also heavily dependent on water.
• You might think that organisms living in oceans, lakes and rivers should not face any water-related problems, but it is not true. 
• For aquatic organisms the quality (chemical composition, pH) of water becomes important. The salt concentration (measured as salinity in parts per thousand), is less than 5 in inland waters, 30-35 in the sea and > 100 in some hypersaline lagoons. 

• The salt concentration  (measured as salinity in parts per thousand)
  Inland waters	less than 5 ppm
  Sea water	30-35 ppm
  Hypersaline lagoons.	more than 100 ppm

Euryhaline organisms: 

• Some organisms are tolerant of a wide range of salinities. 
• ex. the molly (Poecilia sphenops) which can live in fresh water, brackish water, or salt water.

Stenohaline organisms: 

• are restricted and are tolerant to a narrow range of salinities. 
• most of fresh water and marine fish.
• Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems, they would face.

3.  Light: 

• light loving plants are known as heliophytes.
• shade loving plants are called sciophytes. 
• Since plants produce food through photosynthesis, a process which is only possible when sunlight is available as a source of energy, we can quickly understand the importance of light for living organisms, particularly autotrophs. 
• Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesise optimally under very low light conditions because they are constantly overshadowed by tall, canopied trees. 
• Many plants are also dependent on sunlight to meet their photoperiodic requirement for flowering. 
• For many animals too, light is important in that they use the diurnal and seasonal variations in light intensity and duration (photoperiod) as cues for timing their foraging, reproductive and migratory activities. 
• The availability of light on land is closely linked with that of temperature since the sun is the source for both. 
• But, deep (>500m) in the oceans, the environment is perpetually dark and its inhabitants are not aware of the existence of a celestial source of energy called Sun. What, then is their source of energy?
• Chemosynthesis of bacteria provide the energy and organic matter for the whole food web in vent ecosystems. using energy from  Hydrothermal vents and  Cold seeps.

Hydrothermal vents

• Hydrothermal vents were discovered in 1977 by scientists from Scripps Institution of Oceanography. So far, the discovered hydrothermal vents are all located at the boundaries of plates: East Pacific, California, Mid-Atlantic ridge, China and Japan. 

Cold seeps

• A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs, often in the form of a brine pool.
• The spectral quality of solar radiation is also important for life. 
• The UV component of the spectrum is harmful to many organisms while not all the colour components of the visible spectrum are available for marine plants living

4. Soil: 

• study of soil is called as Pedology.
• factors related to soil are called as Edaphic factor. 
• The nature and properties of soil in different places vary; it is dependent on the climate, the weathering process, whether soil is transported or sedimentary and how soil development occurred. 
• Pedogenesis: the bio-chemical changes leading to development of soil from parent rock is termed as pedogenesis. 
• Various characteristics of the soil such as soil composition (mineral composition and organic composition), grain size (sand, silt and clay) and aggregation determine the percolation and water holding capacity of the soils. 

Soil	Composition	Organic (biomass)
		Inorganic minerals
	Grain size	Sand:         0.05 to 2.0 mm
		Silt:            0.002 to 0.05 mm
		Clay:          less than 0.002 mm
	pH	pH is a measure of how acidic/basic water is. The range goes from 0 to 14, with 7 being neutral.
	Percolation	percolation refers to the movement and filtering of fluids through porous materials (soil).
	Water holding capacity (WHC)	soil water holding capacity is the amount of water that a given soil can hold for crop use.

• percolation is inversely proportional to water holding capacity.
• These characteristics along with parameters such as pH, mineral composition and topography determine to a large extent the vegetation in any area. 
• This in turn dictates the type of animals that can be supported. 
• Loam soil is the mixture of sand and clay, and is the best soil for the development and growth of plants.

THINK/ANSWER

• Can you think of a few eurythermal and stenothermal animals and plants? 
• In recent years, there has been a growing concern about the gradually increasing average global temperatures. If this trend continues, would you expect the distributional range of some species to be affected? (YES/NO)
• Which is the most ecologically relevant environmental factor? what is its significance?
• Next to temperature, what is the most important factor influencing the life of organisms?
• For aquatic organisms what becomes very important?
• Many freshwater animals cannot live for long in sea water and vice versa. why?
• arrange the grain size of soil in ascending and descending order.
• what would be the water holding capacity of the water, if the soil has more amount of clay and less amount of sand in its composition? or vice-versa?  (more/less)