Aim:
To schematically collect primary data from a defined area on the college campus to construct and analyze the three types of ecological pyramids: Pyramid of Numbers, Pyramid of Biomass, and Pyramid of Energy.
Ecological Pyramid link
Principle:
An ecological pyramid is a graphical representation that shows the relationship between different trophic levels (producers, consumers) in an ecosystem. There are three types:
Pyramid of Numbers: Shows the number of individuals at each trophic level.
Pyramid of Biomass: Shows the total dry mass of organisms at each trophic level.
Pyramid of Energy: Shows the flow of energy from one trophic level to the next (typically, only 10% of energy is transferred).
This practical involves a schematic (simplified) data collection method to understand the structure and limitations of these pyramids using a real-world context.
Materials Required:
1m x 1m Quadrat frame
Sweep net
Measuring tape
Laboratory balance (for dry weight measurement)
Field notebook, pencil
Calculator
(Optional) Plant identification guide or app
Procedure:
Step 1: Define the Study Area and Trophic Levels
Select a well-defined, grassy area on your college campus (e.g., a 10m x 10m lawn patch).
Define the trophic levels for your study:
Trophic Level 1 (Producers): Grass, small herbs, clover.
Trophic Level 2 (Primary Consumers): Grasshoppers, caterpillars, aphids.
Trophic Level 3 (Secondary Consumers): Ladybugs, spiders, birds.
Step 2: Schematic Data Collection
A. For Producers (Trophic Level 1):
Quadrat Sampling: Randomly place the 1m² quadrat in 5 different spots within your study area.
Pyramid of Numbers: For each quadrat, count the number of individual grass/plant shoots. Calculate the average per m², then extrapolate to your total study area (100 m²).
*e.g., Average 200 shoots/m² → Total Producers = 200 x 100 = 20,000*
Pyramid of Biomass:
Clip all the vegetation within one quadrat.
Dry the biomass in an oven (or use a standard dry:wet weight ratio of 1:4 for grass).
Weigh the dry mass. Calculate the average biomass per m², then extrapolate to 100 m².
*e.g., Dry mass = 50 g/m² → Total Producer Biomass = 50 g/m² x 100 m² = 5000 g*
B. For Primary Consumers (Trophic Level 2):
Sweep Netting: Perform 10 sweeps with the net through the grass in the study area.
Pyramid of Numbers: Count the number of herbivores (grasshoppers, etc.) caught. Estimate the total population in the 100m² area based on the sweep efficiency (a schematic estimation: assume 1 caught = 50 exist).
*e.g., 5 grasshoppers caught → Estimated Population = 5 x 50 = 250*
Pyramid of Biomass: Use an average dry weight from standard tables (e.g., grasshopper ≈ 0.5 g).
*e.g., Total Consumer Biomass = 250 individuals x 0.5 g = 125 g*
C. For Secondary Consumers (Trophic Level 3):
Observation: Observe the area for 30 minutes, noting the number of predators (e.g., spiders, birds).
Schematic Estimation: This is the hardest to count. Use a schematic ratio. (e.g., assume 1 secondary consumer for every 50 primary consumers).
*e.g., Estimated Predators = 250 (primary consumers) / 50 = 5*
Pyramid of Biomass: Use an average dry weight (e.g., spider ≈ 0.1 g).
*e.g., Total Predator Biomass = 5 individuals x 0.1 g = 0.5 g*
Step 3: Data Compilation and Pyramid Construction
Compile all collected and estimated data into a single table.
Step 4: Calculate Energy Flow (Schematic)
Use the 10% Law of Energy Transfer.
Assume the Producers have captured 100,000 kcal of energy from the sun in the study area.
Only ~10% of this energy is transferred to the next level.
Energy available to Primary Consumers = 10% of 100,000 = 10,000 kcal
Energy available to Secondary Consumers = 10% of 10,000 = 1,000 kcal
Observations & Data Analysis:
Table 1: Compiled Data for Ecological Pyramids (100 m² Area)
| Trophic Level | Organism | Pyramid of Numbers (No. of individuals) | Pyramid of Biomass (Dry weight in grams) | Pyramid of Energy (kcal) |
|---|---|---|---|---|
| T3: Secondary Consumer | Spiders, Birds | 5 | 0.5 g | 1,000 kcal |
| T2: Primary Consumer | Grasshoppers | 250 | 125 g | 10,000 kcal |
| T1: Producer | Grass | 20,000 | 5,000 g | 100,000 kcal |
Graph 1: Pyramid of Numbers
*(A
hand-drawn inverted pyramid would be here, showing a wide base of
20,000 producers, a smaller bar of 250 primary consumers, and a very
small bar of 5 secondary consumers)*
Graph 2: Pyramid of Biomass
*(A
hand-drawn upright pyramid would be here, showing a wide base of 5000
g, a medium bar of 125 g, and a very small bar of 0.5 g)*
Graph 3: Pyramid of Energy
*(A
hand-drawn upright pyramid would be here, showing a wide base of
100,000 kcal, a medium bar of 10,000 kcal, and a small bar of 1,000
kcal. The 90% energy loss at each step should be noted on the arrows
between levels.)*
Result:
The Pyramid of Numbers was inverted for this grassland ecosystem due to the very high number of individual grass plants compared to the number of consumers.
The Pyramid of Biomass was upright, showing a clear decrease in biomass at higher trophic levels (5000 g → 125 g → 0.5 g).
The Pyramid of Energy was always upright and cannot be inverted. It clearly demonstrated the 10% law, with 90% of energy being lost as heat at each transfer.
Discussion:
Why the Pyramid of Numbers is Inverted: A single producer (a large tree) can support many herbivores, leading to an upright pyramid. In this case, countless small grass plants support fewer herbivores, creating an inverted pyramid of numbers. This shows the limitation of using only numbers.
Biomass as a Better Measure: The Pyramid of Biomass gives a more accurate picture of the total amount of living matter at each level, which is why it is upright.
Energy is the Fundamental Concept: The Pyramid of Energy is the most accurate representation of ecosystem structure because it shows the inevitable energy loss at each step, explaining why food chains rarely have more than 4 or 5 trophic levels. There is simply not enough energy to support higher levels.
Limitations: This data collection is schematic and estimated. Accurate biomass measurement requires drying organisms, and counting mobile animals is extremely difficult. The estimates used (e.g., 1:50 ratio) are for educational purposes to illustrate the concept.
Conclusion:
This practical successfully demonstrated the schematic collection of field data to construct and compare the three types of ecological pyramids. It highlighted that while the Pyramid of Numbers can be inverted, the Pyramids of Biomass and Energy are always upright due to the laws of thermodynamics and the inefficiency of energy transfer. Understanding these pyramids is crucial for appreciating the flow of energy and the structure of ecosystems, which has implications for conservation and resource management.
Viva Voce Questions:
Why can the Pyramid of Numbers be inverted, but the Pyramid of Energy never can be?
The Pyramid of Numbers depends on the size of individuals (e.g., one tree vs. many insects). The Pyramid of Energy is governed by the laws of thermodynamics; energy cannot be created, and much is lost as heat at each transfer, making an inverted energy pyramid impossible.
What is the 10% Law?
On average, only about 10% of the energy stored in one trophic level is converted into biomass in the next higher trophic level. The remaining 90% is lost primarily as metabolic heat.
Why are food chains limited to 3-5 trophic levels?
Due to the massive energy loss at each transfer (10% Law), there is insufficient energy to support a viable population of organisms at higher trophic levels.
What is the main advantage of a Pyramid of Biomass over a Pyramid of Numbers?
It accounts for the size of organisms, providing a better representation of the total amount of living material available at each trophic level.
Name one reason for the high energy loss between trophic levels.
Energy is lost as heat through respiration, used for movement, excreted as waste, or simply not consumed.
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