Introduction

Aquatic and terrestrial ecosystems differ in many ways, including the structure of their food webs, the efficiency of energy transfer, and the productivity of their primary producers. This article compares aquatic and terrestrial ecosystems with respect to the number of trophic levels, productivity/biomass (P:B) ratios, and herbivore assimilation efficiency.

Number of Trophic Levels

• Aquatic Ecosystems (A):

  • More trophic levels are possible, especially in oceans, where food chains can be long (e.g., phytoplankton → zooplankton → small fish → large fish → top predators).

• Terrestrial Ecosystems (T):

  • Fewer trophic levels are typical, as terrestrial food chains are shorter and energy is lost more quickly due to higher respiration and indigestible plant material.

Productivity/Biomass (P:B) Ratio

• Aquatic Ecosystems (A):

  • Higher P:B ratio: Phytoplankton are microscopic, have rapid turnover, and low standing biomass, resulting in a high ratio of new production to existing biomass.

• Terrestrial Ecosystems (T):

  • Lower P:B ratio: Plants are large, have high standing biomass, and slow turnover, resulting in a low ratio of new production to existing biomass.

Herbivore Assimilation Efficiency

• Aquatic Ecosystems (A):

  • Higher assimilation efficiency: Phytoplankton are highly digestible and lack indigestible tissues, so herbivores can assimilate more of the energy they consume.

• Terrestrial Ecosystems (T):

  • Lower assimilation efficiency: Plants contain indigestible tissues (lignin, cellulose), making them harder for herbivores to digest.

Summary Table

Conclusion

Aquatic ecosystems have more trophic levels, higher productivity/biomass ratios, and greater herbivore assimilation efficiency than terrestrial ecosystems.

Correct answer:
(4) Number of trophic levels is more in A than in T. Productivity/Biomass ratio is greater in A than in T. Herbivore assimilation efficiency is higher in A than in T.