A food chain is a sequence of organisms, each of which feeds on a type of organism from the preceding trophic level. However, food chains do not give an accurate picture of the complex feeding relations that exist between each trophic level. So, to have a better understanding of these feeding relations ecological pyramids are used – for example, pyramids of biomass. A pyramid of biomass shows the relationship between biomass and trophic levels by quantifying the amount of biomass present at each trophic level. The percentage of usable energy transferred as biomass from one trophic level to another level is called ecological efficiency and it is typically of 10% i.e. 90% of biomass is lost through not all of the organism being eaten, faeces and through respiration.
To study and analyse the relation between biomass and trophic levels i.e. of ecological efficiency and to construct a pyramid of biomass, a small area in a clearing in St.Julian’s school’s wooded area was investigated.
I expected to obtain a normal upright pyramid of biomass, as biomass decreases at each trophic level. This occurs because the transfer of energy from the producers to the primary consumers and between the consumers is inefficient i.e. energy is lost between each trophic level through all of the organisms not being eaten, through faeces, through loss of energy as heat or because it has been used as a fuel in respiration. Therefore, I expected a transfer of about 10% of biomass between each trophic level, as according to official values, and so to obtain an upright pyramid of biomass, as the one shown below:
However, the results obtained might not be 100% accurate because the experiment was not carried out accurately or fairly i.e. the data was not collected randomly (the quadrat was not thrown randomly, it was placed on a selected area) and only one set of data was collected (the quadrat was only placed and observed once).
* Empty envelope
* 2 empty jars
1. Weigh empty containers (envelope for vegetation small jars for organisms). Record the weight.
2. Identify a food chain in the wooded area.
3. Remove vegetation, using the scissors, and place it inside the envelope.
4. Place herbivores and carnivores in separate jars.
5. Reweigh envelope and record weight.
6. Reweigh jars and record weight.
7. Release the herbivores and carnivores back into their habitat. *
8. Put envelope in the oven at 105C.
9. Reweigh envelope and record its weight.
* Animals released back into their habitat due to environmental and ethical reasons.
Containing fresh mass /g
Containing dry mass /g
Jar 1 (herbivores)
Jar 2 (carnivores)
Fresh mass /g
Dry mass /g
Biomass / g per 1/4 m *
Carnivores (secondary consumers)
52.36 – 18.50= 33.86g
38.21 – 18.50= 19.77g
33.06 x 1/4 = 8.47g
Primary consumers (herbivores)
52.59 – 52.55= 0.04g
0.04 x 1/4 = 0.01g
Secondary consumers (carnivores)
52.77 – 52.75= 0.02g
0.02 x 1/4 = 0.005g
The results pyramid of biomass obtained demonstrates that the analysed food chain obeys the general trend of transfer of energy as biomass between each trophic level i.e. as the pyramid goes up there is less and less biomass because energy is lost between each trophic level through not all of the organisms not being eaten, through faeces, through loss of energy as heat or because it has been used as a fuel in respiration, as stated in the hypothesis.
However, according to data in table 2, between 1st (producers) and the 2nd trophic level (herbivores) there was only a transfer of 0.12% energy. And, between the 2nd (herbivores) and the 3rd trophic level (carnivores) there was a transfer of 50% of energy.
Therefore, the results obtained do not correspond to the official data – 10% of energy passed on between each trophic level and, consequently, 90% lost. This is because the experiment was not accurately nor fairly carried out, as was mentioned in the hypothesis.
The experiment, as stated above, was extremely inaccurate due to many factors.
The experiment was not accurate because the data was not collected randomly (the quadrat was not thrown randomly, it was placed on a selected area). Also, only one set of data was obtained as the quadrat was only placed and observed once, whereas more samples should have been obtained to get mean values.
During the collection of samples, animals might have escaped from the studied area (quadrat 1/4 m ), so the data might be incomplete. Also, not all vegetation was removed – roots should have also been removed – which means that there will be an underestimation of the biomass of the vegetation.
Also, the animals collected were not dried due to ethical reasons therefore the biomass will be overestimated.
When weighing the envelopes, the weight was constantly changing as the envelopes were bigger than the balance – smaller envelopes should have been used to get more accurate results.
The pyramid of biomass was not drawn according to values of dry mass, as the animals were not dried (due to environmental and ethical reasons), which means that values are overestimated and, ultimately, inaccurate.