# Measuring Species Richness & Species Evenness ## What is Species Richness & Species Evenness? - One measure of biodiversity is the **species diversity** in a given area - There are two key components of species diversity: - Species richness - Species evenness ### Species richness

Species richness is a measure of the number of different species within a given area

- An area with a greater number of species will have a greater species richness - Think of a meadow with 50 different wildflower species - this would be described as **species-rich** - However, species richness can be a misleading indicator of diversity as it does not take into account the number of individuals of each species - A woodland could have 20 tree species, but if 19 of them are represented by only a single individual, the ecosystem isn't as diverse as the number suggests ### Species evenness

Species evenness is a measure of the relative abundance of the different species within a given area

- An area in which all the species have similar abundances will have a greater species evenness - Consider the following comparison between two gardens: - **Garden A** has 4 shrub species: hawthorn (25 individuals), elder (22 individuals), blackthorn (24 individuals), and dog rose (29 individuals) - **Garden B** has 4 shrub species: hawthorn (6 individuals), elder (12 individuals), blackthorn (70 individuals), and dog rose (1 individual) - Although both gardens contain 4 species, Garden B is actually dominated by one species and one of the species is very rare (only one individual) - Although the two areas have exactly the same species richness, Garden A has a higher species evenness (and therefore a higher overall species diversity) than Garden B **Garden A - High Species Evenness:**

**Garden B - Low Species Evenness:**

Species diversity takes both richness and evenness into account

## Comparing species richness and evenness The following table shows data from two areas with identical species richness but different species evenness: | Feature | Area 1 | Area 2 | |---------|--------|--------| | Total species | 4 butterfly species | 4 butterfly species | | Species A count | 25 | 6 | | Species B count | 25 | 12 | | Species C count | 25 | 70 | | Species D count | 25 | 12 | | **Species richness** | Same | Same | | **Species evenness** | Higher | Lower | | **Overall diversity** | Higher | Lower | **Visual comparison of Area 1 vs Area 2:** **Area 1 - High Species Evenness:**

**Area 2 - Low Species Evenness:**

- Area 1 and Area 2 have the same species richness but different species evenness - As it has a higher species evenness, the overall species diversity of Area 1 is higher than that of Area 2 - The charts clearly show how Area 1 has an even distribution (all bars similar height) while Area 2 is dominated by one species (one very tall bar) ## Why both measures matter - **Ecosystems with high species diversity** are usually more stable than those with lower species diversity as they are more resilient to environmental changes - Consider a heathland ecosystem dominated by one or two plant species: - If a fungal pathogen appears that targets one of the two dominant species of plants, then the whole population could be wiped out and the ecosystem it is a part of could collapse - All the invertebrates, birds, and mammals that depend on that plant would lose their food source - Conservationists often favour the use of an **index of diversity** (known as Simpson's Index) as it takes into account both species richness and evenness ## How to measure each component

To measure species richness you simply count the number of different species present. To measure species evenness you count the number of different species present and the number of individuals of each species

- **Species richness measurement:** - Identify and count the number of different species in the sample area - Record the total number (this is your species richness value) - **Species evenness measurement:** - Identify and count the number of different species in the sample area - Count the number of individuals of each species - Compare the abundance of each species to assess how evenly distributed they are - Any sampling method that takes into account the abundance of each species can be used to measure evenness