What is a community and a dominant species?
A community is all the living things of different species that live together in one place, for example all the trees, birds and insects of a forest. In plant studies we often look only at the trees or plants.
In most communities one or two species matter much more than the others. They are many, they are everywhere and they are big. The species that has the strongest effect on the community is called the dominant species. It gives the forest its name, such as a pine forest or an oak forest.
To pick the dominant species with numbers, not by guessing, we use the importance value.
Sampling with quadrats
A forest may be too large to count tree by tree. So we count a small part and use it to understand the whole. This is sampling.
A quadrat is a square frame laid on the ground. For trees it may be 10 m × 10 m; for grass it may be 1 m × 1 m. In each quadrat we record the species, the number of plants and the trunk size.
- Place quadrats at random, so that you do not choose only the nice-looking spots.
- Use enough quadrats. Few quadrats can miss rare species.
- Use the same size for every quadrat.
In our 3D forest there are 4 quadrats of 6 m × 6 m, so the sampled area is 4 × 36 = 144 m².
Density and relative density
Density tells how many plants of a species there are in a given area.
Density = number of individuals ÷ total area sampled
In the 3D forest, oak density = 8 ÷ 144 = 0.056 trees per m².
Relative density compares the species with each other:
Relative density (%) = number of individuals of the species ÷ number of individuals of all species × 100
Oak: 8 ÷ 20 × 100 = 40%. Pine: 6 ÷ 20 × 100 = 30%. Birch: 30%. The three add up to 100%.
Frequency and relative frequency
Frequency tells how widely a species is spread. A species can have many plants but all in one corner. Then its frequency is low.
Frequency (%) = quadrats in which the species occurs ÷ total quadrats × 100
Oak is in 4 of 4 quadrats, so 100%. Pine is in 3 of 4: 75%. Birch: 75%.
Relative frequency (%) = frequency of the species ÷ sum of frequencies of all species × 100
Using the count of quadrats (4, 3, 3), the sum is 10. Oak: 4 ÷ 10 × 100 = 40%. Pine: 30%. Birch: 30%.
Dominance (cover) and relative dominance
Dominance tells how much space a species takes up. For trees we use the basal area: the area of the trunk cross-section near the ground. For grasses and small plants we use the cover: the ground area under their leaves.
A trunk is nearly a circle, so basal area = π × r² (r = radius of the trunk). If the radius doubles, the area becomes four times bigger. So one thick tree can take more space than several thin trees.
Relative dominance (%) = total basal area of the species ÷ total basal area of all species × 100
In our forest the trunk area per tree is oak 2, pine 5, birch 1 (arbitrary units). Totals: oak 8 × 2 = 16, pine 6 × 5 = 30, birch 6 × 1 = 6. The sum is 52. Relative dominance: oak 30.8%, pine 57.7%, birch 11.5%.
Importance value (IV) and how to read it
Importance value (IV) = relative density + relative frequency + relative dominance
The IV of all species together is always 300. It shows the role of a species in the community in one number.
| Species | Rel. density | Rel. frequency | Rel. dominance | IV |
|---|---|---|---|---|
| Oak | 40 | 40 | 30.8 | 110.8 |
| Pine | 30 | 30 | 57.7 | 117.7 |
| Birch | 30 | 30 | 11.5 | 71.5 |
Pine has the highest IV, so pine is the dominant species, although oak has more trees. This shows why we use all three measures together. Often the community is named after the one or two species with the highest IV, for example a pine-oak forest.
Try it: use the slider under the 3D to make the pine trunks thinner. Watch the pine bar fall and see when oak becomes the dominant species.
Key formulas and definitions
- Density = number of individuals ÷ area sampled
- Relative density (%) = (individuals of the species ÷ individuals of all species) × 100
- Frequency (%) = (quadrats with the species ÷ total quadrats) × 100
- Relative frequency (%) = (frequency of the species ÷ sum of frequencies of all species) × 100
- Basal area of a trunk = π r²
- Relative dominance (%) = (basal area of the species ÷ basal area of all species) × 100
- IV = relative density + relative frequency + relative dominance (total for all species = 300)
Worked examples
1. In a sample, there are 12 teak trees out of 60 trees of all species. Find the relative density of teak.
Relative density = 12 ÷ 60 × 100 = 20%.
2. A species occurs in 6 of 10 quadrats. (a) Find its frequency %. (b) The frequencies of all species add up to 24 (counting quadrats). Find its relative frequency.
(a) Frequency = 6 ÷ 10 × 100 = 60%. (b) Relative frequency = 6 ÷ 24 × 100 = 25%.
3. A tree trunk has a diameter of 20 cm. Find its basal area. Take π = 3.14.
Radius = 10 cm. Basal area = 3.14 × 10 × 10 = 314 cm².
4. A species has relative density 30, relative frequency 25 and relative dominance 40. Find its importance value.
IV = 30 + 25 + 40 = 95.
5. Species X has 50 of 100 trees, 8 of the 20 total frequency counts, and 50 of 250 units of trunk area. Species Y has 10 trees, 5 frequency counts and 150 units of trunk area. Find the IV of X and Y and say which is more important.
X: relative density 50, relative frequency 8 ÷ 20 × 100 = 40, relative dominance 50 ÷ 250 × 100 = 20. IV = 110. Y: relative density 10, relative frequency 5 ÷ 20 × 100 = 25, relative dominance 150 ÷ 250 × 100 = 60. IV = 95. So X is more important, even though Y has the thickest trunks.
6. In a forest, oak has IV 110.8 and pine has IV 117.7. What is the IV of the third species, birch? Which species is dominant?
The IV of all species always adds up to 300. Birch = 300 − 110.8 − 117.7 = 71.5. Pine has the highest IV (117.7), so pine is the dominant species.
Common mistakes
- Thinking the species with the most trees is always dominant. Check relative dominance too: a few thick trees can cover more ground.
- Dividing by the wrong total. Relative values use the total of ALL species, not the total of one quadrat.
- Mixing up frequency and density. Density counts plants; frequency counts in how many quadrats the species is found.
- Forgetting that IV can be above 100. It is a sum of three percentages, so the total for all species is 300.