Understanding Pyramids of Biomass in Terrestrial Ecosystems

The question asks us to evaluate two statements regarding the pyramid of biomass in terrestrial ecosystems like grasslands and forests. Let's analyze each statement carefully.

Analyzing Assertion (A)

Assertion (A) states: "In terrestrial ecosystems (grassland, forests) the pyramid of biomass is upright with broad base".

In ecology, a pyramid of biomass represents the total mass of living organisms (biomass) at each trophic level in an ecosystem. Trophic levels include producers (plants), primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and tertiary consumers (carnivores that eat other carnivores).

In most terrestrial ecosystems, the total biomass at the producer level (plants) is much larger than the total biomass of the primary consumers, which in turn is larger than the biomass of the secondary consumers, and so on. This decrease in biomass at successive trophic levels happens because only a fraction of the energy (and thus biomass) from one level is transferred to the next. The majority of energy is lost as heat or used for metabolic processes.

When we plot these biomass values for each trophic level, with producers at the base and higher consumers at the top, the pyramid shape is typically wide at the bottom and narrows towards the top. This shape is described as 'upright' with a 'broad base'. Therefore, Assertion (A) is correct for terrestrial ecosystems.

Analyzing Reason (R)

Reason (R) states: "In terrestrial ecosystems the biomass increases from primary to tertiary consumers".

As discussed above, the typical pattern in terrestrial ecosystems is that biomass decreases as you move up the trophic levels, from producers to primary consumers, to secondary consumers, and finally to tertiary consumers. Primary consumers have less total biomass than producers, secondary consumers have less than primary consumers, and tertiary consumers have the least biomass among these levels.

Reason (R) claims the opposite – that biomass increases from primary to tertiary consumers. This statement contradicts the fundamental concept of the pyramid of biomass in terrestrial ecosystems. Therefore, Reason (R) is not correct.

Comparing Assertion (A) and Reason (R)

We found that Assertion (A) is a correct statement describing the typical pyramid of biomass in terrestrial ecosystems. We also found that Reason (R) is an incorrect statement, describing a trend opposite to what is observed in terrestrial biomass pyramids.

Therefore, Assertion (A) is correct, but Reason (R) is not correct.

Evaluating the Options

• Option 1: Both (A) and (R) are correct and R is the correct explanation of A. This is incorrect because (R) is not correct.
• Option 2: Both (A) and (R) are correct and R is NOT the correct explanation of A. This is incorrect because (R) is not correct.
• Option 3: (A) is correct but (R) is not correct. This matches our analysis.
• Option 4: (A) is not correct but (R) is correct. This is incorrect because (A) is correct and (R) is not correct.

Based on our analysis, the most appropriate answer is the one stating that Assertion (A) is correct but Reason (R) is not correct.

Conclusion on Pyramid of Biomass

Pyramids of biomass show the total mass of living matter at each trophic level. In terrestrial ecosystems, energy transfer efficiency between levels is relatively low (around 10%), causing a significant decrease in biomass at each successive level. This forms an upright pyramid.

Revision Table: Ecology Concepts

Additional Information: Pyramids of Energy and Numbers

Besides the pyramid of biomass, there are other types of ecological pyramids:

• Pyramid of Energy: This represents the flow of energy through each trophic level. It is always upright because energy is lost at each transfer (typically 90% loss), meaning less energy is available at higher levels. It cannot be inverted.
• Pyramid of Numbers: This represents the number of individual organisms at each trophic level. It is usually upright (e.g., many plants support fewer herbivores, which support even fewer carnivores). However, it can be inverted or spindle-shaped in some cases, such as when a single large producer (like a tree) supports many insects (primary consumers).

The pyramid of biomass is generally upright in terrestrial ecosystems, but can sometimes be inverted in aquatic ecosystems (e.g., a small biomass of phytoplankton supporting a larger biomass of zooplankton, which reproduce faster).