Ecosystem Test - 20

• Maximum reproductive capacity or biotic potential is realised when environmental resources are non limiting and conditions favour minimum mortality. However, environment has a limiting effect on the rise of population.
• The sum of abiotic (e.g., temperature, water, space) and biotic factors (e.g., food, competition, disease, predation), which checks the rise in population size and prevents the species to realise its biotic potential are called  environmental resistance. 
• It is inversely related to the difference between carrying capacity and number of existing population.

• Crystalline rocks are the natural source of phosphorus. It is not found free in nature, but it is widely distributed in many minerals, mainly phosphates.
• Inorganic phosphate rock, which is partially made of apatite (an impure tri-calcium phosphate mineral), is today the chief commercial source of this element. 
• It is an essential mineral nutrient for all plants and animals.
• It acts as  building blocks of certain parts like bones and teeth in humans and other animals. 

• The term biogeochemical cycle is used for the exchange or circulation of biogenetic nutrients between living and nonliving components of the biosphere.
• The nutrient cycle can be a gaseous cycle in which the materials involved in circulation between living and nonliving components are gases or vapors, e.g., carbon, hydrogen, oxygen.
• In sedimentary cycles of matter; the materials involved in circulation between biotic and abiotic components of the biosphere are non-gaseous e.g., phosphorous, calcium, magnesium.
• The reservoir of phosphorous is phosphates present in rocks.
  
  Thus, the correct answer is option B.

• Lichens stage is the first stage in Lithosere (or succession on bare rock). Bare rock is invaded first by crustose lichens e.g: Graphis, Rhizocarpon. They corrode the rock at places causing foliose lichen e. g: Parmelia,
• Dermatocarpon to invade and eliminate crustose lichens and create conditions for invasion by mosses. In the tropics, blue-green algae are pioneers instead of lichens in lithosere.
• So, the correct answer is option C.

• For the sedimentary cycle, the major reservoir is the lithosphere which releases the elements through weathering. 
• The mineral elements required by living organisms are obtained from inorganic sources like salts dissolved in water or from rocks. 
• The sedimentary cycles are represented by phosphorus, sulphur, and iodine cycles.

• The pyramid of biomass is more fundamental, they represent the quantitative relationships of the standing crops. In this pyramid, there is a gradual decrease in the biomass from the producers to the higher trophic levels. 
• In an ocean, as the producers are small organisms, their biomass is the least, and this value gradually shows an increase towards the apex of the pyramid, thus making the pyramid inverted in shape.

Solution:

• Correct answer is option D.
• Explanation for correct option:
• Xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
• Pioneer species in xerarch succession are usually lichens.
• Lichen secretes acid to dissolve rock, helping in weathering and soil formation.
• The correct order of succession in xerarch is:

• In the phosphorus cycle, weathering makes phosphate available to the soil. 
• Phosphorus is a critical element. It is obtained from the soil as orthophosphate. 
• Plants or producers absorb phosphorus and use it to produce several biological molecules like nucleic acids, nucleotides, etc. 
• From plants, the organic phosphorous transfers to other organisms, and organic phosphorous circulate in nature from plants to animals. 
• Phosphate is released by decomposers back to the soil. 

• Most of the energy of the sun that falls on earth is reflected and scattered. 
• Plants absorb only a small fraction of the energy and out of that also lost a lot of energy as radiation and actually convert only a small fraction of the absorbed energy (2 %) into organic molecules or biomass.