🔍 Identification of Organisms
Big idea: Scientists need reliable ways to identify organisms so they can study biodiversity, monitor ecosystems, and detect environmental change.
🤔 Why does identification matter?
Imagine this scenario: You're counting butterflies in a meadow for a conservation project. You see 50 butterflies — but are they all the same species? Or 5 different species?
Without correct identification, your data is useless!
- Correct identification = accurate biodiversity measurement
- Helps track population changes over time (is a species declining?)
- Essential for conservation decisions (which species need protection?)
- Wrong ID = wrong conclusions about ecosystems
If you don't know what species you're looking at, you can't study it properly!
👀 How do we identify organisms?
Organisms are identified using observable physical characteristics — things you can see without special equipment.
What features do scientists look at?: 🐦 Bird: Beak shape, feather colour, wing pattern, size
🌿 Plant: Leaf shape, flower colour, number of petals
🐛 Insect: Number of legs, wings, antennae, body segments
🐟 Fish: Fin shape, scale pattern, body colour
- Body shape and size
- Number of legs, wings, or petals
- Leaf shape and arrangement
- Colour and patterns
- Presence or absence of key features (Does it have a tail? Spots? Thorns?)
In ESS, identification focuses on visible features you can observe in the field — not DNA testing!
🔑 What is a dichotomous key?
A dichotomous key is like a "choose your own adventure" book for identifying organisms!
Think of it like 20 Questions: You ask yes/no questions to narrow down the answer:
"Is it a mammal?" → Yes
"Does it have stripes?" → Yes
"Is it a cat?" → Yes
"It's a tiger!" 🐯
At each step, you choose between two contrasting options. Each choice removes some possibilities until only one organism fits.
Dichotomous = "di" (two) + "chotomy" (division)
= TWO CHOICES only at each step!
📋 How to use a dichotomous key
- Read both options at step 1 carefully
- Choose the option that matches your organism
- Follow the instruction to the next step
- Repeat until you reach a species name
- Double-check — does the description match your organism?
🌳 Example: Identifying a tree leaf
Simple dichotomous key for leaves: Step 1: Is the leaf needle-shaped or broad?
→ Needle-shaped: Go to Step 2
→ Broad: Go to Step 3
Step 2: Are needles in bundles or single?
→ In bundles: Pine tree 🌲
→ Single: Spruce tree
Step 3: Does the leaf have smooth or jagged edges?
→ Smooth edges: Magnolia
→ Jagged edges: Go to Step 4
Step 4: Is the leaf lobed (like fingers)?
→ Yes: Oak tree 🍂
→ No: Birch tree
Always read BOTH choices before deciding — don't rush or you'll make mistakes!
✅ Strengths of dichotomous keys
- Simple — anyone can learn to use them
- Quick — identification in minutes
- Low cost — just need eyes and the key (no lab equipment)
- Portable — great for fieldwork
- Systematic — reduces guesswork
⚠️ Limitations of dichotomous keys
When keys go wrong: 🦋 Damaged specimen: A butterfly with torn wings — you can't count the wing spots!
🐛 Young organism: A caterpillar looks nothing like the adult butterfly
👯 Similar species: Two beetles look identical but are different species
❌ User error: You accidentally chose "6 legs" when it had 8
- Damaged organisms may be missing key features
- Young/immature organisms don't look like adults
- Similar species can be hard to tell apart
- User mistakes — one wrong choice = wrong answer
- Limited scope — keys only work for species they include
Exam tip: Always give one strength AND one limitation when evaluating dichotomous keys!
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IB-style question — Using a dichotomous key to identify species
A researcher studying beetles in the Carpathian Forest photographs four species. Using the dichotomous key below, identify Species B and Species C. [2]
1a. Wing cases have spots → go to 2
1b. Wing cases have no spots → go to 3
2a. Spots arranged in two rows → Spotted ground beetle (Carabus maculosus)
2b. Spots in a single row → Carpathian longhorn (Morimus funereus)
3a. Antennae longer than body → Alpine longhorn (Rosalia alpina)
3b. Antennae shorter than body → Forest click beetle (Athous vittatus)
Species B has spotted wing cases in a single row. Species C has no spots and antennae shorter than its body.
How to answer it, step by step
- Follow each couplet from 1
• Species B: spots present → go to 2 → single row → Carpathian longhorn (Morimus funereus).
• Species C: no spots → go to 3 → antennae shorter → Forest click beetle (Athous vittatus). - Write the answer clearly
• Common name OR binomial both accepted.
• Never skip couplet 1 — always start at the top.
Final answer
Work through every couplet in order — jumping to an answer that 'looks right' is the classic mistake. Both species must be correct to earn both marks.
IB-style question — Lincoln Index (mark–release–recapture) calculation
A field ecologist is estimating the population of river crayfish (Austropotamobius pallipes) in a 2 km stretch of the Brenna River. In the first capture, 48 crayfish were caught, marked with a small paint dot on the shell, and released. After two days, a second capture yielded 60 crayfish, of which 12 bore the paint mark. Estimate the total population size using the Lincoln Index. Show your working. [3]
How to answer it, step by step
- Formula → substitution → answer
• N = (n₁ × n₂) / m₂ = (48 × 60) / 12 = 2880 / 12 = 240 crayfish
• n₁ = first capture; n₂ = second capture total; m₂ = marked in second capture. - Show every step
• A correct formula with wrong arithmetic still earns 2 of 3 marks.
• Most common error: dividing by n₂ instead of m₂.
Final answer
Always show three lines: formula → substitution → answer with units. A correct method scores even if the arithmetic slips.
IB-style question — Describe a sampling method to estimate population size (7-mark essay)
Describe the practical methods a researcher would use to estimate the population size of water snails in a highland lake, including how the Lincoln Index is applied. [7]
How to answer it, step by step
- Capture, mark, release → re-sample
• Catch snails at random locations (standardised effort); count = n₁. Mark harmlessly (e.g. corrector fluid dot); release all.
• Wait 24–48 h for mixing; re-sample same effort → count total (n₂) and marked individuals (m₂). Apply N = (n₁ × n₂) / m₂. - State at least one assumption
• Marks must not affect survival or behaviour.
• Population must be closed (no births, deaths, migration) during the study.
Final answer
To reach 7 marks, cover the full procedure AND state the formula AND address at least one assumption. Giving only the formula without describing the practical steps scores 3 at most.