Experiment 01Community EcologyPhytosociology
Quadrat Sampling & Phytosociological Community Attributes: Frequency, Density, Abundance, and Shannon-Wiener Diversity ($H’$)
Duration: 6 Hours • Field & Computation
🎯 Experimental Aim & Learning Objectives
To determine the minimal quadrat size using the species-area curve method; quantitatively calculate the phytosociological attributes (Frequency, Density, Abundance, and Importance Value Index) of a terrestrial herbaceous plant community; test Raunkiaer’s Law of Frequency; and compute the Shannon-Wiener Diversity Index ($H’$), Pielou’s Evenness ($J’$), and Simpson’s Dominance ($D_s$).
🔬 Theoretical Foundation & Mathematical Principles
Phytosociology provides the quantitative framework for understanding the floristic structure, dominance hierarchies, and organization of terrestrial plant communities:
- Species-Area Curve: By plotting cumulative species richness against incrementally nested quadrat sizes ($10 times 10text{ cm}$ to $2 times 2text{ m}$), a classic saturation curve emerges. The minimal representative area is identified at the inflection point where a 10% increase in area yields less than a 10% increase in newly added species.
- Raunkiaer’s Law of Frequency: Raunkiaer (1934) classified plant species into five frequency classes based on percentage occurrence: Class A ($1-20%$), Class B ($21-40%$), Class C ($41-60%$), Class D ($61-80%$), and Class E ($81-100%$). In a balanced, undisturbed climax community, species distribution conforms to the formula:
$$A > B > C gtreqless D < E$$ A pronounced spike in Class A reflects the presence of numerous rare or localized species, while Class E highlights well-adapted dominant structural species. - Shannon-Wiener Diversity Index ($H’$): Quantifies community diversity based on information theory entropy:
$$H’ = – sum_{i=1}^{S} p_i ln(p_i)$$
where $p_i = frac{n_i}{N}$ represents the proportional abundance of the $i$-th species. $H’$ incorporates both species richness ($S$) and relative equitability/evenness. - Pielou’s Species Evenness ($J’$):
$$J’ = frac{H’}{H’_{max}} = frac{H’}{ln(S)}$$
Values range from 0 (extreme dominance by a single taxon) to 1.0 (perfectly equal abundance across all species).
Figure 1.1: Phytosociological quadrat sampling methodology, species-area saturation curve, Raunkiaer’s frequency law, and mathematical equations for community richness, diversity ($H’$), and dominance.
🧪 Reagents, Field Apparatus & Materials
- Field Equipment: 1 m × 1 m collapsible wooden/PVC quadrats subdivided with nylon cord into $10 times 10text{ cm}$ grids; measuring tapes (30 m); metal survey pegs and bright flagging tape.
- Taxonomic Tools: Local regional flora keys, hand lenses ($10times$ and $20times$), botanical vasculum/plant press, botanical specimen tags, and field notebook.
- Analytical Hardware: Scientific calculator or statistical spreadsheet software (R / Excel) for matrix transformations.
📋 Step-by-Step Field & Computational Protocol
- Minimal Area Determination: In a representative grassland or forest floor plot, lay nested quadrats starting from $20 times 20text{ cm}$, expanding sequentially to $40 times 40text{ cm}$, $60 times 60text{ cm}$, $80 times 80text{ cm}$, $100 times 100text{ cm}$, and $120 times 120text{ cm}$. Record total new species discovered at each stage. Plot the species-area curve and locate the minimal area plateau (typically $1 text{ m}^2$ for grassland).
- Random Stratified Sampling: Lay at least 10 to 15 replicates of the $1text{ m} times 1text{ m}$ quadrat across the study site using random coordinate generation.
- Floristic Census: Within each quadrat, identify all angiosperm species. Count and record the exact number of individuals ($n_i$) belonging to each species.
- Attribute Calculations:
• $text{Frequency } (%) = left(frac{text{Number of quadrats in which species occurs}}{text{Total number of quadrats sampled}}right) times 100$
• $text{Density } (D) = frac{text{Total number of individuals of species across all quadrats}}{text{Total number of quadrats sampled}}$
• $text{Abundance } (A) = frac{text{Total number of individuals of species across all quadrats}}{text{Number of quadrats in which species occurs}}$ - Relative Attributes & IVI: Compute Relative Frequency ($RF$), Relative Density ($RD$), and Relative Dominance ($RDo$). Sum them to yield the Importance Value Index ($text{IVI} = RF + RD + RDo$, out of 300).
- Diversity Indices: Determine total community individuals $N = sum n_i$. Compute $p_i = n_i / N$, $ln(p_i)$, and calculate $H’$, $J’$, and Simpson’s dominance index $D_s$.
📊 Representative Data & Computational Results
| Species | Quadrats Present (of 10) | Total Indiv ($n_i$) | Frequency (%) | $p_i$ | $ln(p_i)$ | $-p_i ln(p_i)$ |
|---|---|---|---|---|---|---|
| Cynodon dactylon (Poaceae) | 10 | 85 | 100% (Class E) | 0.472 | -0.751 | 0.354 |
| Tridax procumbens (Asteraceae) | 6 | 42 | 60% (Class C) | 0.233 | -1.457 | 0.339 |
| Parthenium hysterophorus (Asteraceae) | 4 | 28 | 40% (Class B) | 0.156 | -1.858 | 0.290 |
| Oxalis corniculata (Oxalidaceae) | 3 | 15 | 30% (Class B) | 0.083 | -2.489 | 0.207 |
| Euphorbia hirta (Euphorbiaceae) | 2 | 10 | 20% (Class A) | 0.056 | -2.882 | 0.161 |
| Total ($S=5$) | – | $N = 180$ | – | 1.000 | – | $H’ = 1.351$ |
Evenness: $J’ = frac{1.351}{ln(5)} = frac{1.351}{1.609} = 0.840$ (Indicating high community equitability).
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