18.15 Estimating Abundance of Mobile Animals and Measuring Species Diversity
Estimating Abundance of Mobile Animals
Challenges
- Mobile animals cannot be accurately sampled using quadrats because they can move in and out of the area being studied.
- Alternative techniques, such as the Mark–Release–Recapture (Lincoln Index) method, are employed to estimate population sizes.
Mark–Release–Recapture Technique
- Capture and Mark:
- Capture as many individuals as possible from the target population.
- Mark each individual in a way that is non-toxic, non-invasive, and does not affect survival or behavior (e.g., non-toxic paint, small tags).
- Release the marked individuals back into their habitat and allow time for them to mix randomly with the unmarked population.
- Recapture:
- After sufficient mixing time, capture another large sample.
- Record the total number of individuals captured in this second sample and count how many are marked.
- Population Estimate:
- Use the Lincoln Index Formula to estimate population size:
Worked Example: Brown Planthoppers
| Sample | Count |
|---|---|
| Number marked in first sample (n1n_1n1) | 247 |
| Total captured in second sample (n2n_2n2) | 259 |
| Marked individuals in second sample (m2m_2m2) | 16 |
Calculation:
Assumptions of Mark–Release–Recapture:
- Marked individuals mix randomly with the population.
- Marking does not affect the animals’ behavior, survival, or likelihood of recapture.
- Population size remains constant (no births, deaths, immigration, or emigration).
- All marks are permanent and visible during the study.
Simpson’s Index of Diversity
Definition
- Simpson’s Index of Diversity (( D )) quantifies species diversity in a habitat.
- Takes into account:
- Species richness: Number of different species in the community.
- Species evenness: Distribution of individuals across species.
Formula:
Steps to Calculate ( D ):
Worked Example: Diversity on Shore A
Table: Species Abundance on Shores A and B
| Species | Shore A (nnn) | Shore B (nnn) |
|---|---|---|
| Painted Topshells | 24 | 51 |
| Limpets | 367 | 125 |
| Dogwhelks | 192 | 63 |
| Snakelocks Anemones | 14 | 0 |
| Beadlet Anemones | 83 | 22 |
| Barnacles | 112 | 391 |
| Mussels | 207 | 116 |
| Periwinkles | 108 | 93 |
| Total (NNN) | 1107 | 861 |
Table:
| Species | ( n ) | n/N | (n/N)2 |
|---|---|---|---|
| Painted Topshells | 24 | ( 0.022 ) | ( 0.000 ) |
| Limpets | 367 | ( 0.332 ) | ( 0.110 ) |
| Dogwhelks | 192 | ( 0.173 ) | ( 0.030 ) |
| Snakelocks Anemones | 14 | ( 0.013 ) | ( 0.000 ) |
| Beadlet Anemones | 83 | ( 0.075 ) | ( 0.006 ) |
| Barnacles | 112 | ( 0.101 ) | ( 0.010 ) |
| Mussels | 207 | ( 0.187 ) | ( 0.035 ) |
| Periwinkles | 108 | ( 0.098 ) | ( 0.010 ) |
| Total ( N = 1107 ) | ∑ = 0.201 ) |
Calculation:
[
D = 1 – 0.201 = 0.799
]
Thus, Shore A has a Simpson’s diversity index of 0.799.
Practice Example: Shore B
From Table:
| Species | ( n ) | n/N | (n/N)2 |
|---|---|---|---|
| Painted Topshells | 51 | 51/861 = 0.059 | 0.0592 = 0.003 |
| Limpets | 125 | ( 0.145 ) | ( 0.021 ) |
| Dogwhelks | 63 | ( 0.073 ) | ( 0.005 ) |
| Snakelocks Anemones | 0 | ( 0.000 ) | ( 0.000 ) |
| Beadlet Anemones | 22 | ( 0.026 ) | ( 0.001 ) |
| Barnacles | 391 | ( 0.454 ) | ( 0.206 ) |
| Mussels | 116 | ( 0.135 ) | ( 0.018 ) |
| Periwinkles | 93 | ( 0.108 ) | ( 0.012 ) |
| Total ( N = 861 ) | ∑ = 0.266 ) |
Calculation:
D = 1 – 0.266 = 0.734
Thus, Shore B has a Simpson’s diversity index of 0.734.
Comparison of Shores
- Shore A (( D = 0.799 )) has higher diversity than Shore B (( D = 0.734 )).
- Explanation:
- Shore A has a more even distribution of individuals among species.
- Shore B is dominated by barnacles, reducing species evenness.
Advantages of Simpson’s Index
- Does not require exact species identification.
- Different morphotypes (e.g., “anemone A” and “anemone B”) can be used as distinct species.
- Considers both species richness and species evenness, making it more comprehensive than simple richness measures.
Limitations of Simpson’s Index
- Subjectivity in Grouping:
- If species are not well-defined (e.g., morphotypes), results may be inconsistent.
- Habitat-Specific Comparisons Only:
- Unsuitable for comparing vastly different ecosystems (e.g., forests vs. coral reefs).
Summary
- Mark–Release–Recapture:
- Best for mobile animals; estimates population size using the Lincoln Index.
- Key assumptions: random mixing, constant population, and visible marks.
- Simpson’s Index of Diversity:
- Quantifies species diversity by combining richness and evenness.
- Useful for comparing similar ecosystems.
- Higher ( D ) indicates greater biodiversity.