Question

(a) What is primary productivity ? Why does it vary in different types of ecosystems ?

(b) Why is the assimilation of energy at the herbivore level called secondary productivity ?

Hint:

Primary productivity is the biomass made by plants per unit area per unit time, and it changes with species, climate and nutrients. Herbivores only reuse plant matter, so it is secondary.

Answer 1

Step 1: Define primary productivity:
Primary productivity is the amount of biomass or organic matter produced per unit area over a given time by plants during photosynthesis.
It is expressed as \( \mathrm{g\,m^{-2}\,yr^{-1}} \) or as \( \mathrm{kcal\,m^{-2}\,yr^{-1}} \).

Step 2: Why it varies between ecosystems:
It depends on the plant species living in the area and on their photosynthetic capacity.
It depends on environmental factors such as light, temperature, rainfall and water.
It depends on the availability of nutrients in the soil or water.
These factors differ in a forest, a grassland, a desert and an ocean, so productivity differs too.

Step 3: Part (b): Secondary productivity:
Herbivores cannot make food from sunlight and inorganic matter.
They take in organic matter already made by producers and convert part of it into their own tissue.
The rate of formation of new organic matter by consumers is called secondary productivity.
The energy assimilated at the herbivore level is the first such conversion by consumers, so it is called secondary productivity.

Final Answer:
Primary productivity is the biomass made by plants per unit area per unit time. It varies with plant species, climate and nutrients. Herbivores only convert existing plant matter into new tissue, so their assimilation is secondary productivity.