Question

1. An ecosystem consists of one oak tree on which 10,000 herbivores are feeding. These herbivores are prey to 500 spiders and carnivorous insects. There birds of the same species are eating these spiders and carnivorous insects. The oak tree has a mass of 4000 kg, the herbivores insects have an average mass of 0.05 g, the spiders and carnivorous insects have an average mass of 0.2 g and the three birds have an average mass of 10 g.
a. Construct, to scale, a pyramid of numbers.
b. Construct, to scale, a pyramid of biomass.
c. Explain the differences between these two pyramids.

Answer 1

A) Scale: 1 mm² = 1 organism
The base of the pyramid will be a square that is 1 mm by 1 mm
The next tier of herbivores will be 100 mm by 100 mm
The next tier of carnivorous insects and spiders will be 50 mm by 10 mm
The final tier of birds will be 3 mm by 1 mm

b) Biomasses:
Tree = 4000 kg
Herbivores = 10,000 x 0.05/1000 = 0.5 kg
Carnivores = 500 x 0.2/1000 = 0.1 kg
Birds = 3 x 10/1000 = 0.03 kg
Scale: 1 mm² = 0.1 kg
The bottom tier will be 400 mm by 100 mm
The next tier will be 5 mm by 1 mm
The next tier will be 1 mm by 1 mm
The final tier will be 0.3 mm by 0.1 mm

The pyramid of numbers and pyramid of biomass are the opposites of one another because energy is lost as we go up the tiers of a food pyramid. Only 10% of the energy in a trophic level is passed to a higher one. This is why the overall numbers may increase, due to smaller organisms, but the main place where organisms utilize energy, production of biomass, reduces drastically.