Class 12 Biology Notes Chapter 16 (Chapter 16) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 16 from your Biology Lab Manual. This chapter deals with ecological studies, specifically focusing on populations and environmental analysis, which are important practical aspects for your exams.

Chapter 16: Ecological Studies - Key Experiments and Concepts

This chapter typically covers experiments related to studying population attributes and analysing environmental components like soil and water.

Experiment 1: Population Density and Frequency of Plants using Quadrat Method

• Aim: To determine the population density and frequency of different plant species in a given area using the quadrat method.
• Principle:
  • A quadrat is a square (or sometimes rectangular/circular) frame of a known area used as a sampling unit in ecological studies.
  • Population Density: Represents the number of individuals of a species per unit area. It indicates the abundance of the species.
  • Population Frequency: Represents the percentage of quadrats in which a particular species occurs. It indicates the distribution pattern of the species.
• Procedure:
  1. Select a study area (e.g., grassland, field).
  2. Measure the size of the quadrat (e.g., 1m x 1m = 1 sq m).
  3. Lay the quadrat randomly at multiple spots (e.g., 10 spots) within the study area. Random sampling ensures unbiased data.
  4. For each quadrat placement:
     • Identify the different plant species present within the quadrat.
     • Count the number of individuals of each species.
     • Record whether each species is present or absent.
  5. Record the observations systematically in a table.
• Calculations:
  • Density:

    Density = (Total number of individuals of the species in all quadrats) / (Total number of quadrats studied × Area of one quadrat)
    

    Unit: Individuals per square meter (or the unit of area used).
  • Frequency:

    Frequency (%) = (Number of quadrats in which the species occurred / Total number of quadrats studied) × 100
    

• Inference: Comparing the density and frequency values helps understand the structure of the plant community. High density indicates abundance, while high frequency indicates wide distribution.

Experiment 2: Study of Water Parameters (e.g., Pond Water)

• Aim: To study the physical and chemical characteristics (pH, clarity/turbidity) and presence of living organisms in given water samples (e.g., from a pond, river, tap).
• Parameters Studied:
  1. pH:
     • Principle: Measures the acidity or alkalinity of water. Neutral pH is 7. Values below 7 are acidic, above 7 are alkaline. Important for aquatic life survival.
     • Procedure: Use pH paper (compare colour change with standard chart) or a digital pH meter for more accurate readings. Dip the paper/electrode into the water sample and note the reading.
  2. Clarity/Turbidity:
     • Principle: Measures how clear the water is. Turbidity is caused by suspended particles (silt, clay, plankton, pollutants). High turbidity reduces light penetration, affecting photosynthesis.
     • Procedure:
       • Visual Observation: Note if the water is clear, slightly cloudy, or very muddy.
       • Secchi Disk Method: A circular disk (usually black and white quadrants) is lowered into the water until it is no longer visible. The depth at which it disappears (Secchi depth) is recorded. Lower Secchi depth indicates higher turbidity.
  3. Presence of Living Organisms:
     • Principle: Water bodies contain various microscopic organisms (plankton) and sometimes larger forms. Their type and abundance indicate water quality.
     • Procedure: Take a drop of the water sample on a clean glass slide, cover with a coverslip, and observe under a compound microscope. Look for phytoplankton (algae like Spirogyra, Volvox, diatoms) and zooplankton (protozoans like Amoeba, Paramecium; small crustaceans like Daphnia, Cyclops).
• Inference:
  • pH values outside the optimal range (usually 6.5-8.5) can indicate pollution.
  • High turbidity/low clarity can suggest pollution (e.g., sewage influx, soil erosion) or high plankton bloom (eutrophication).
  • The presence of diverse plankton usually indicates a healthy ecosystem, while the dominance of certain pollution-tolerant species (like some blue-green algae) or absence of sensitive species can indicate pollution.

Experiment 3: Study of Soil Parameters

• Aim: To study the texture, moisture content, pH, and water holding capacity of different soil samples (e.g., garden soil, roadside soil, field soil).
• Parameters Studied:
  1. Soil Texture:
     • Principle: Refers to the relative proportion of sand, silt, and clay particles in the soil. Texture affects water retention, aeration, and root penetration.
     • Procedure (Sedimentation Method): Take a known amount of dry soil in a measuring cylinder, add water, shake vigorously, and allow it to settle undisturbed. Observe the distinct layers formed: coarse sand settles first at the bottom, followed by silt, and finally clay particles remain suspended or form the topmost layer. The relative thickness of these layers indicates the texture (e.g., sandy soil, clayey soil, loamy soil).
  2. Moisture Content:
     • Principle: Measures the amount of water present in the soil sample. Crucial for plant growth.
     • Procedure: Weigh a known amount of fresh soil sample (Initial weight). Dry the soil completely in a hot air oven (at 105-110°C) until constant weight is achieved (Dry weight).
     • Calculation:

       Moisture Content (%) = [(Initial weight - Dry weight) / Initial weight] × 100
       

  3. Soil pH:
     • Principle: Measures the acidity or alkalinity of the soil, affecting nutrient availability to plants.
     • Procedure: Take a known amount of soil, mix it with a specific volume of distilled water (e.g., 1:2 soil:water ratio). Shake well and let it stand. Filter or use the supernatant liquid. Test the pH using pH paper or a pH meter.
  4. Water Holding Capacity (WHC):
     • Principle: Measures the ability of the soil to retain water against gravity. Important for supplying water to plants between irrigations/rainfall. Clayey soils generally have higher WHC than sandy soils.
     • Procedure: Place a filter paper in a funnel. Put a known weight of dry soil sample on the filter paper. Place the funnel over a measuring cylinder. Pour a known volume of water gently over the soil until it starts dripping from the funnel. Allow water to drain completely. Measure the volume of water collected in the measuring cylinder (percolated water).
     • Calculation: The amount of water retained by the soil is (Volume of water added - Volume of water collected). WHC can be expressed as the percentage of water retained per unit weight of dry soil.

        WHC (%) = [(Volume of water added - Volume of water collected) / Weight of dry soil] × 100
       

       (Note: Calculation methods can vary slightly, focus on the principle of water retained).
• Inference: Different soil types exhibit different properties. Loamy soils (balanced mix of sand, silt, clay) are generally considered best for agriculture due to good texture, aeration, WHC, and moderate pH. Comparing these properties for different soil samples helps assess their suitability for plant growth.

Multiple Choice Questions (MCQs)

1. In the quadrat method for studying plant populations, 'Frequency' is calculated as:
   a) (Total individuals of a species / Total quadrats studied) × 100
   b) (Number of quadrats where species occurred / Total quadrats studied) × 100
   c) (Total individuals of a species / Area of one quadrat) × 100
   d) (Area of one quadrat / Total individuals of a species) × 100

2. Population density is best defined as:
   a) The distribution pattern of a species in an area.
   b) The number of quadrats in which a species is found.
   c) The number of individuals of a species per unit area or volume.
   d) The total number of species found in all quadrats.

3. A Secchi disk is used in aquatic studies to measure:
   a) Water pH
   b) Water temperature
   c) Dissolved oxygen
   d) Water clarity/turbidity

4. While observing a drop of pond water under the microscope, you find organisms like Amoeba, Paramecium, Daphnia, and Cyclops. These primarily belong to:
   a) Phytoplankton
   b) Zooplankton
   c) Benthos
   d) Nekton

5. A soil sample is mixed with water and allowed to settle in a measuring cylinder. The layer that settles at the very bottom consists of:
   a) Clay
   b) Silt
   c) Humus
   d) Sand

6. To determine the moisture content of a soil sample, the first step after collecting the fresh sample is usually:
   a) Drying it in an oven.
   b) Mixing it with distilled water.
   c) Weighing the fresh sample accurately.
   d) Testing its pH.

7. A soil sample tested with universal pH indicator solution turns yellowish-orange. This indicates the soil is likely:
   a) Strongly alkaline
   b) Neutral
   c) Slightly acidic
   d) Strongly acidic

8. Which soil type is expected to have the highest water holding capacity?
   a) Sandy soil
   b) Gravel
   c) Clayey soil
   d) Silty soil

9. When calculating population density using a 1m x 1m quadrat, if a total of 50 individuals of species 'X' were found across 10 quadrat samples, the density of species 'X' is:
   a) 50 individuals/m²
   b) 10 individuals/m²
   c) 5 individuals/m²
   d) 0.5 individuals/m²

10. Random sampling using quadrats is important in ecological studies to:
    a) Ensure all species are counted.
    b) Make the counting process faster.
    c) Avoid bias and obtain representative data of the study area.
    d) Focus only on the most abundant species.

Answer Key for MCQs:

1. b
2. c
3. d
4. b
5. d
6. c
7. c (Universal indicator colours: Red=Strong Acid, Orange/Yellow=Weak Acid, Green=Neutral, Blue/Violet=Alkaline)
8. c
9. c (Density = 50 individuals / (10 quadrats * 1 m²/quadrat) = 5 individuals/m²)
10. c

Study these concepts and procedures thoroughly. Understanding the 'why' behind each step is as important as knowing the step itself. Good luck with your preparation!