Introduction

Understanding how energy flows through ecosystems is fundamental to ecology. Energy is lost at every step as it moves from producers to herbivores and then to carnivores. This article explains how to calculate the percentage of energy that primary carnivores assimilate from herbivores, using a practical example and clear calculations.

The Problem

Imagine a scenario where primary carnivores consume 40% of the production of herbivores and assimilate 70% of the energy they consume. The question is: What percentage of the energy available from herbivores do these carnivores actually assimilate?

Step-by-Step Calculation

Let’s break down the problem using a simple example:

1. Assume herbivore production = 100 units.

   • This makes calculations straightforward and easy to follow.

2. Carnivores consume 40% of herbivore production:

   • $100\times 0.40=40$ units consumed.

3. Carnivores assimilate 70% of what they consume:

   • $40\times 0.70=28$ units assimilated.

4. Percentage of herbivore production assimilated by carnivores:

   • $28$ units out of 100 units is 28%.

Why Is This Important?

• Energy Efficiency:
  The average energy transfer from one trophic level to the next is typically around 10%157. However, this is a general rule and actual values can vary depending on consumption and assimilation efficiencies.

• Ecosystem Dynamics:
  Understanding how much energy is actually assimilated helps ecologists model food webs and predict the biomass and population sizes of different organisms.

• Human Implications:
  Knowing energy transfer efficiencies can inform sustainable agriculture and fisheries management.

Common Misconceptions

• Energy Transfer vs. Assimilation:
  Energy transfer refers to the overall movement of energy from one trophic level to the next. Assimilation is the portion of consumed energy that is actually absorbed and used by the organism.

• Not All Consumed Energy Is Assimilated:
  A significant portion of consumed energy is lost as feces, urine, or metabolic heat. Only a fraction is assimilated and available for growth and reproduction2.

• The 10% Rule:
  While the 10% rule is a useful approximation, actual efficiencies can vary depending on the organisms and ecosystem157.

Real-World Example

Suppose a field produces 1000 kg of plant material (herbivore food). Herbivores might consume 400 kg (40%), but only 280 kg (70% of 400 kg) is assimilated by the carnivores. Relative to the original 1000 kg, this is 28% of the herbivore’s production, but this interpretation is only valid if “herbivore production” is the energy available to carnivores. In standard ecological terms, the question is ambiguous, but based on the wording, the calculation above is correct for the scenario described.

Clarification:
In standard ecological models, “production” is often the energy available to the next level, but if the question refers to herbivore production as the total energy herbivores have, then the calculation is correct as shown. If the question means carnivores consume 40% of what herbivores assimilate (not produce), then the calculation would be different. However, based on the wording and the options, the most logical answer is 28%.

Conclusion

When primary carnivores consume 40% of herbivore production and assimilate 70% of what they consume, they assimilate 28% of the energy available from herbivores (assuming “herbivore production” is the total energy available to carnivores).

Summary Table

Correct answer:
(2) 28