Numerical Problems/ICSE/Class 10
Created by Titas Mallick
Biology Teacher • M.Sc. Botany • B.Ed. • CTET (CBSE) • CISCE Examiner
BioNotesBioNotesCreated by Titas Mallick
Biology Teacher • M.Sc. Botany • B.Ed. • CTET (CBSE) • CISCE Examiner
Online
Ecological pyramids (Pyramid of Numbers, Biomass, and Energy) graphically represent the trophic structure and function of an ecosystem. A fundamental concept tested in numerical problems is Lindeman's 10% Law, which states that only about 10% of the energy from one trophic level is transferred to the next.
[!IMPORTANT] Crucial Rule: Plants only capture 1% of the sunlight (solar energy) that falls on their leaves. Once energy enters the food chain at the producer level, the 10% rule applies for all subsequent trophic level transfers.
In an ecosystem, the total solar energy falling on the leaves of producers is . Calculate the energy available to the secondary consumers in this food chain.
Energy Flow: Sunlight (5,000,000 J) ➔ (1% absorbed) ➔ Producers [T1] (50,000 J) ➔ (10% transferred) ➔ Primary Consumers [T2] (5,000 J) ➔ (10% transferred) ➔ Secondary Consumers [T3] (500 J)
Step 1: Calculate the energy captured by Producers (T1) Producers convert only 1% of the solar energy falling on them into chemical energy (Gross Primary Productivity).
Step 2: Calculate the energy at Primary Consumers (T2) Apply Lindeman's 10% Law.
Step 3: Calculate the energy at Secondary Consumers (T3) Apply the 10% Law again.
Final Answer: The energy available to the secondary consumers is .
If a tertiary consumer (top carnivore) receives of energy, how much energy was originally present at the producer level? Express your answer in Joules (J).
Step 1: Identify the trophic levels and target. Let the energy at Producers (T1) be . Trophic levels: Producer (T1) Primary Consumer (T2) Secondary Consumer (T3) Tertiary Consumer (T4) Energy at T4 = . (Always convert to consistent units first!)
Step 2: Work backward using the 10% rule. Since each level receives 10% of the previous level, we must multiply by 10 as we move backward down the pyramid.
Alternative Formula Method: Where is the trophic level number. For tertiary consumers, .
Final Answer: The energy present at the producer level was .
In a pond ecosystem, phytoplankton have a total biomass of , zooplankton have a biomass of , and small fishes have a biomass of .
Biomass Pyramid (Inverted):
- [Small Fishes: 12 g/m²] (Top)
- [Zooplankton: 21 g/m²] (Middle)
- [Phytoplankton: 4 g/m²] (Base) (Note: An inverted pyramid structure where the base is smaller than the middle trophic level)
Part 1: Pyramid of Biomass The pyramid of biomass is Inverted. In aquatic ecosystems, phytoplankton have a very short lifespan and high turnover rate. Even though their standing crop (biomass at any given moment) is small, they reproduce fast enough to support a larger mass of zooplankton.
Part 2: Energy Calculation Energy flow always strictly follows the 10% law, regardless of biomass.
Final Answer: The energy available to small fishes is . The pyramid of energy is Upright, which contradicts the inverted pyramid of biomass. The pyramid of energy is always upright because energy is lost as heat at every trophic level (Second Law of Thermodynamics).
A crop field is sprayed with a non-biodegradable pesticide (DDT). The producer level absorbs of energy and has a DDT concentration of (parts per million). If the food chain is: Crop Grasshopper Frog Snake Hawk. Calculate:
1. Energy Available to the Hawk: The food chain has 5 trophic levels (T1 to T5).
2. DDT Concentration in the Hawk: This is the trap! While energy decreases at each trophic level (10% law), the concentration of non-biodegradable toxins increases at each successive trophic level. This phenomenon is called Biological Magnification (or Biomagnification). Since the Hawk is the apex predator (T5), it will have the highest concentration of DDT, significantly greater than .
[!CAUTION] Common Pitfall! Students often mistakenly apply the 10% rule to toxins, thinking the Hawk receives 10% of the pesticide. Remember: Energy diminishes (10%), but toxins magnify!