Biology · Organisms and Populations · NEET
Eury means 'wide' and steno means 'narrow'. Eurythermal organisms can survive and grow in a WIDE range of temperatures (example: humans, crows, dogs). Stenothermal organisms can survive only in a NARROW range of temperatures (example: fish like trout, most tropical corals). NEET tip: remember eury = wide by linking 'eury' with 'Europe is large/wide'.
These describe tolerance to SALT (salinity), not temperature. Euryhaline organisms tolerate a wide range of salinity (example: salmon, which move between rivers and sea). Stenohaline organisms tolerate only a narrow range of salinity (example: most freshwater fish die in sea water and most marine fish die in fresh water). 'Haline' = salt, 'thermal' = heat — do not mix these two word-pairs.
Because temperature controls the rate of metabolic (enzyme) reactions inside the body and directly affects growth, activity and even geographic distribution of organisms. Temperature on Earth decreases from the equator to the poles and from plains to mountain tops, which is why different regions have different plants and animals. This is why NCERT lists temperature first.
Photoperiod means the DURATION of light in a day (day length). Plants need light for photosynthesis, and many plants depend on the photoperiod to decide when to flower — this is called photoperiodism. Animals also use photoperiod as a cue for foraging, reproduction and migration. Small plants of the forest floor are adapted to grow in low (diffuse) light because tall trees block sunlight above them.
Soil is the base on which plants grow and it decides the vegetation, which in turn decides the animals. The nature of soil depends on climate, the weathering process, whether it is transported or sediment-based, and how the soil was developed. Soil characteristics like grain size, aggregation, pH, mineral content and water-holding capacity together decide the type of plants that can grow, and therefore the whole community.
Abiotic factors are the NON-LIVING physical parts of the environment: temperature, water, light and soil. Biotic factors are the LIVING parts: other organisms such as competitors, predators, parasites and pathogens. Both together decide whether an organism can survive in a place. In this concept you only study the four major abiotic factors.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Temperature, Water, Light and Soil. NCERT discusses them in this exact order in the 'Organism and its Environment' section of Organisms and Populations.
Temperature, because it affects the rate of enzyme (metabolic) reactions, activity and the geographic distribution of organisms across the Earth.
Eurythermal: humans (can live in many climates). Stenothermal: many fish and corals that survive only within a narrow temperature range.
For aquatic organisms the quality (chemical composition, pH) of water is important, and salinity level decides which organisms can live there (euryhaline vs stenohaline).
This topic is mostly asked as direct term-matching (eurythermal/stenothermal, euryhaline/stenohaline, photoperiod). In this chapter most NEET PYQs test population growth and interactions, so master the definitions here and practise those PYQs separately.
