Biodiversity confusion! (Shannon Weaver diversity index)

[AliceT]

Hi everyone, i really find this forum helpful as i am just finishing my first year (foundation science) in order to go on and do an Environmental health degree. i didnt exactly do well in my science GCSE 7 years ago.

 

so my question for now is, im doing an assignment on biodiversity (which is due in very soon...my bad for not asking earlier), we went to some woods, from a 30x30m plot of woodland, we noted down all of the species and how many there were of each species. i have calculated the shannon weaver diversity index of every groups results, the groups who were in the woods with less species and less trees in general have a higher diversity number....does that make it more diverse?

 

one group had 6 species in their plot...and 45 trees in total...their shannon weaver result was: 0.971

 

another group from another wood had 5 species and only 23 trees in total and their result was: 1.422.

 

i dont understand why its a higher number....i have a feeling i have missed something very obvious but i just cant figure out why the number is higher in the wood with less trees and less species??

 

i hope someone can help???

 

cheers guys

Alice

[CharonY]

If you can just post the formula we can quickly scan over it to see how the relation between the index and diversity is.

[AliceT]

Thankyou,

 

the shannon weaver diversity index equation? the equation is:

 

H'= - ∑(Ni/N) × ln (Ni/N)

 

hope thats what you were after! thanks again for your help

[Mokele]

It's a higher number because it's a measure of how diverse the system is - systems where one species massively predominates get low numbers, systems with a more even distribution get higher numbers.

 

The reason the groups with fewer trees got higher numbers is because the index doesn't take into account total population, only relative proportion of species. Thus, two populations of the same size and same number of species can have different results if one has almost complete dominance by a single species, while the other has equal numbers of each. Similarly, a smaller population can have a higher number if the numbers of each species are more similar to each other.

[AliceT]

thankyou so much, i thought that might be a reason but i really wasnt sure. the first group has alot of the same tree and only one or two of the others and thats why its low diversity because it is mainly populated by one species. i get it!

 

Thankyou so much, im getting the jist of it.

 

really appreciated!