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 pyramid of numbers represents the raw count of organisms at each trophic level and can be inverted or diamond-shaped, while a pyramid of biomass shows the cumulative mass of organisms, which can also be inverted in some ecosystems. The energy pyramid, which is always upright, indicates how energy is reduced at each successive trophic level.

Step-by-step explanation:

Constructing an Ecosystem's Energy Pyramids

An ecosystem can be represented by different types of pyramids: a pyramid of numbers and a pyramid of biomass. These pyramids help in understanding the relationship between the different trophic levels and the flow of energy through the ecosystem.

a. Construct, to scale, a pyramid of numbers: This pyramid would have a broad base representing the one oak tree, followed by a much larger layer above for the 10,000 herbivores, then the 500 spiders and carnivorous insects, and finally at the top, the three birds.

b. Construct, to scale, a pyramid of biomass: The oak tree with a mass of 4000 kg would form the base, the herbivores insects (500 kg total) would form the second level smaller than the base, and the spiders and carnivorous insects (100 kg total) would be above that. Lastly, the three birds at the top would represent a very small fraction (0.03 kg).

c. Explain the differences between these two pyramids: The pyramid of numbers can sometimes be inverted or diamond-shaped due to the sheer number of smaller organisms. However, the energy pyramid is always upright because energy diminishes at each successive trophic level due to energy loss, primarily as heat. Pyramids of biomass can also be inverted when lower trophic levels have a high turnover rate and are consumed rapidly, thus, temporarily reducing their biomass despite high energy conversion and productivity.

Pyramids of biomass and energy are particularly important in ecosystem modeling because they depict the raw mass of organisms and amount of energy present at each trophic level, respectively.

Answer 2

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.