Question

Given below is a pyramid showing various trophic levels in an ecosystem:

(a) From the organisms listed below, identify which one is to be placed at which trophic level:
Deer, Grass, Lion, Snake, Rabbit
(b) Discuss the reason why primary consumers will have more energy as compared to secondary consumers?
(c) Why is the base of the pyramid broad?

Hint:

Ecological pyramid: Broad base (producers) \(\Rightarrow\) narrow top (tertiary consumers). Reason: 10% energy transfer law. Primary consumers \(>\) secondary consumers in energy because energy decreases at each trophic level.

Answer 1

Part (a): Placing organisms at correct trophic levels
Step 1: Understand the trophic levels.

• Producers (Trophic level 1): Autotrophs that produce their own food through photosynthesis (green plants).
• Primary consumers (Trophic level 2): Herbivores that eat producers.
• Secondary consumers (Trophic level 3): Carnivores that eat primary consumers.
• Tertiary consumers (Trophic level 4): Top carnivores that eat secondary consumers.

Step 2: Identify each organism's role.

• Grass: Green plant, produces food by photosynthesis \(\Rightarrow\) Producer
• Deer: Herbivore that eats grass \(\Rightarrow\) Primary consumer
• Rabbit: Herbivore that eats grass/plants \(\Rightarrow\) Primary consumer
• Snake: Carnivore that eats primary consumers (rabbits, rats) \(\Rightarrow\) Secondary consumer
• Lion: Top carnivore that eats secondary consumers (snakes) and other animals \(\Rightarrow\) Tertiary consumer

Step 3: Arrange in pyramid form.
\[ \begin{array}{c} \text{Tertiary consumers: Lion
\text{Secondary consumers: Snake
\text{Primary consumers: Deer, Rabbit
\text{Producers: Grass
\end{array} \] Part (b): Why primary consumers have more energy than secondary consumers
Step 1: Recall the 10% law of energy transfer.
According to the 10% law given by Lindeman, only about 10% of the energy from one trophic level is transferred to the next trophic level. The remaining 90% is lost in various processes.
Step 2: Energy loss at each trophic level.
Energy is lost at each trophic level through:

• Metabolic activities (respiration, digestion)
• Heat loss (according to second law of thermodynamics)
• Undigested material (excretion)
• Movement and other life processes

Step 3: Compare primary and secondary consumers.

• Primary consumers obtain energy directly from producers. They receive about 10% of the energy fixed by producers.
• Secondary consumers obtain energy by eating primary consumers. They receive only about 10% of the energy present in primary consumers.

Therefore: \[ \text{Energy in secondary consumers} = \frac{10}{100} \times \text{Energy in primary consumers} \] \[ \text{Energy in secondary consumers} = 0.1 \times \text{Energy in primary consumers} \]
Step 4: Numerical example.
If producers have 10,000 J of energy:

• Primary consumers receive: \(10,000 \times 10% = 1,000\) J
• Secondary consumers receive: \(1,000 \times 10% = 100\) J

Thus, primary consumers have 10 times more energy than secondary consumers.
Step 5: Final answer for part (b).
\[ \boxed{\text{Primary consumers have more energy than secondary consumers because only 10% of energy is transferred from one trophic level to the next (10% law). The rest is lost in respiration, heat, and other metabolic activities.}} \] Part (c): Why the base of the pyramid is broad
Step 1: Understand the ecological pyramid.
An ecological pyramid is a graphical representation of the relationship between organisms at different trophic levels. The base represents producers, and the apex represents top consumers.
Step 2: Reason for broad base - Energy availability.
Producers (plants) capture solar energy through photosynthesis and convert it into chemical energy. They have the maximum amount of energy in an ecosystem.
Step 3: Reason for broad base - Biomass and numbers.

• Biomass: Producers have the highest biomass in an ecosystem to support the energy needs of all consumers above them.
• Numbers: A large number of producers are required to support a smaller number of primary consumers, which in turn support an even smaller number of secondary consumers, and so on.

Step 4: Energy flow principle.
Since energy decreases at each successive trophic level (10% law), a large base of producers is necessary to sustain the energy requirements of higher trophic levels.
Step 5: Final answer for part (c).
The base of the pyramid is broad because producers have the maximum energy, biomass, and numbers to support the energy needs of all higher trophic levels. As energy decreases at each trophic level (10% law), a broad base is essential for ecosystem stability.