20.2 - Species Diversity of a Community

species diversity

composition of a local ecological community with respect to its richness, evenness or both

species richness

number of unique species in an ecosystem

species evenness

how species in an ecosystem are distributed

Shannon Wiener index 

formula used to measure diversity of a community that accounts for both species richness and evenness

Shannon Wiener formula

H = - (S -above sum symbol) ∑ (i = 1 - under sum symbol)pi ln pi

• H = Shannon diversity index 

• S = total number of species

• Pi = proportion of individuals of species i

formula e.g. with population of 4 species each with 25% evenness

-(0.25xln0.25 + 0.25xln0.25 + 0.25ln0.25 + 0.25ln0.25) = 1.39

when is H maximised 

when all species are equally abundant, leading to maximum evenness

• Hmax = lnS

evenness formula

J = H/Hmax

• J = evenness

• H = Shannon Wiener Index

• Hmax = maximum diversity index

what doesn’t Shannon index measure

doesn’t measure actual abundance of species - if proportion across different species are the same then the Shannon Index is the same

• A = 25% for all 4 species - total pop is 20

• B = 25% for all 4 species - total pop is 4

ecosystem

a community of living and non-living components that interact in an area. Health is measured by biodiversity

biodiversity

a measure of the variety of biological species in an ecosystem

core habitats

a specific area within an animal's or plant's range that is essential for its long-term survival and recovery

edge habitat

the transitional area between two different ecosystems, often containing a mix of species from both habitats.

how to measure biodiversity when it includes an edge community between 2 core habitats

e.g. meadow borders a forest - core forest, core meadow and edge

1. random quadrat sampling in core areas and calculate H value

2. transect line across each core extending into edge

3. quadrats along lines for richness and evenness = calculate each H

what is another way to assess ecosystem health

measure chemical components

• check for pollution

• examine soil quality by measuring nutrients present

• useful info to help explain changes in biodiversity patterns in different regions of a community

how to determine Shannon index without calculating

higher richness and evenness = higher Shannon diversity