๐ Measuring biodiversity
Big idea: Biodiversity can be measured, not just described. Measuring biodiversity helps scientists compare ecosystems and track change over time.
๐ฟ What does species diversity mean?
Species diversity looks at how many species live in an area and how evenly individuals are shared between them.
- Richness
- Evenness
- High diversity needs BOTH richness and evenness
Lots of species โ if one species dominates โ diversity is still low.
๐ป Richness vs evenness (simple example)
Imagine a field with many lavender plants and just one sunflower.
- There is more than one species โ richness is greater than 1
- But almost all individuals are lavender โ low evenness
- The sunflower is unlikely to reproduce โ low long-term diversity
Rich but uneven = still low biodiversity.
๐งฎ Simpson's Reciprocal Index (D)
Big Idea: Simpson's Reciprocal Index turns biodiversity into a single number.
It tells you how diverse an ecosystem is by looking at how many species there are AND how evenly they're spread out.
Think of it like a classroom: if 30 students are in a room but 28 speak English and only 2 speak Spanish, there's low language diversity. But if 15 speak English and 15 speak Spanish, diversity is higherโeven though the total is the same!
๐ The Formula
Don't panic! Let's break this down into plain English.
- D = the diversity score (higher = more diverse)
- N = total number of ALL individuals (count everything!)
- n = number of individuals of ONE species
- ฮฃ (sigma) = "add up" โ you do this for each species
Higher D = more biodiversity. The minimum D is 1 (only one species). There's no maximum!
๐ป Worked Example 1: Uneven Field (Low Diversity)
Imagine a field with 9 daisies and 1 sunflower.
- Step 1: Count the total (N) โ 9 + 1 = 10
- Step 2: Calculate N(N-1) โ 10 ร 9 = 90
- Step 3: For each species, calculate n(n-1):
- โข Daisies: 9 ร 8 = 72
- โข Sunflowers: 1 ร 0 = 0
- Step 4: Add them up โ 72 + 0 = 72
- Step 5: Divide โ D = 90 รท 72 = 1.25
Result: D = 1.25 โ This is LOW diversity. Almost all plants are daisies, so the ecosystem isn't very diverse.
๐ธ Worked Example 2: Even Field (High Diversity)
Now imagine the same field with 5 daisies and 5 sunflowers.
- Step 1: Count the total (N) โ 5 + 5 = 10
- Step 2: Calculate N(N-1) โ 10 ร 9 = 90
- Step 3: For each species, calculate n(n-1):
- โข Daisies: 5 ร 4 = 20
- โข Sunflowers: 5 ร 4 = 20
- Step 4: Add them up โ 20 + 20 = 40
- Step 5: Divide โ D = 90 รท 40 = 2.25
Result: D = 2.25 โ This is HIGHER diversity! Same total plants, but they're spread more evenly between species.
๐ Comparing the Two Examples
Uneven Field
- 9 daisies, 1 sunflower
- Total: 10 plants
- D = 1.25
- Low diversity
Even Field
- 5 daisies, 5 sunflowers
- Total: 10 plants
- D = 2.25
- Higher diversity
Same number of plants, but different D values! Evenness matters just as much as the total.
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๐ The Formula
๐ฒ Worked Example 3: Three Species
A pond has 10 frogs, 10 fish, and 10 snails.
- Step 1: N = 10 + 10 + 10 = 30
- Step 2: N(N-1) = 30 ร 29 = 870
- Step 3: Each species: n(n-1) = 10 ร 9 = 90 (ร3)
- Step 4: Sum = 90 + 90 + 90 = 270
- Step 5: D = 870 รท 270 = 3.22
Result: D = 3.22 โ Even higher! More species AND perfect evenness = high biodiversity.
๐ก What D Values Mean
- D = 1 โ Only one species (no diversity at all)
- D = 1โ2 โ Low diversity (one species dominates)
- D = 2โ5 โ Moderate diversity
- D > 5 โ High diversity (many species, evenly spread)
In exams, you might need to calculate D or explain why one ecosystem has a higher D than another. Always mention evenness!
๐งช Why measuring biodiversity matters
Measuring biodiversity helps scientists and conservationists make decisions.
- Compare different habitats objectively
- Monitor changes over time (is diversity increasing or decreasing?)
- Identify ecosystems under threat
- Evaluate if conservation efforts are working