2. What are the ways to describe a community? abundance: total number of individuals (sometimes called community size) relative abundance: abundance of a species relative to the total abundance (Pi) species richness: count of the number of species diversity: combines both richness and relative abundance
Evenness: relative commonness or rarity of species relative to the other species in the community
3. How do you calculate the relative abundance (Pi) of a species?
The abundance of a species (by any measure), divided by the total abundance of all species combined.
4. Species richness
Count of the number of species
5. Shannon diversity- what does it measure and how do you calculate it?
pi = proportion of individuals in the ith species where:
H = the Shannon diversity index
Pi = fraction of the entire population made up of species i
S = numbers of species encountered
∑ = sum from species 1 to species S
Note: The power to which the base e (e = 2.718281828.......) must be raised to obtain a number is called the natural logarithm (ln) of the number.
To calculate the index:
1. Divide the number of individuals of species #1 you found in your sample by the total number of individuals of all species. This is Pi
2. Multiply the fraction by its natural log (P1 * ln P1)
3. Repeat this for all of the different species that you have. The last species is species “s”
4. Sum all the - (Pi * ln Pi) products to get the value of H
- (Pi * ln Pi)
H = 0.223
High values of H would be representative of more diverse communities. A community with only one species would have an H value of 0 because Pi would equal 1 and be multiplied by ln Pi which would equal zero. If the species are evenly distributed then the H value would be high. So the H value allows us to know not only the number of species but how the abundance of the species is distributed among all the species in the community.
6. What is Evenness and how do we measure it?
Evenness is the relative commonness or rarity of species relative to the other species in the community
Divide Shannon's diversity index H by natural logarithm of species richness ln(S) to calculate the species evenness. Note that species evenness ranges from zero to one, with zero signifying no evenness and one, a complete evenness.
7. Rank-abundance plots
1. Each dot is a species
2. Each Pi is plotted against its rank
8. Plant communities at Rothamsted Manor in England
9. What is ecological stability in a community?
Stability means no change in X.
Resistance: undergoes little change in the face of disturbance
Resilience: returns rapidly following a disturbance
10. How does diversity influence stability?
Diversity should be positive according to logical argument
Example: the portfolio effect
11. What is the portfolio effect?
It compares biodiversity to stock holdings, where diversification minimizes the volatility of the investment, aka the risk in stability of ecosystem services
12. Are diverse systems more stable?
13. Are stable systems more diverse?
14. Intermediate disturbance hypothesis
The Intermediate Disturbance Hypothesis (IDH) states that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At low levels of disturbance, more competitive organisms will push subordinate species to extinction and dominate the ecosystem. At high levels of disturbance, due to frequent forest fires or human impacts like deforestation, all species are at risk of going extinct.
15. Sousa’s study of invertebrates in the rocky intertidal zone