Simpson's Index of Diversity

Hey,

The Simpson's Diversity Index is a measure of biodiversity.
In order to measure biodiversity, we must consider ''evenness'' and ''species richness''.
Evenness is a measure of the relative abundance of individuals in each species. In other words, it's how many of each species there are. A habitat where there are even numbers of individuals in each species is likely to be more diverse than one in which individuals of one species outnumber all the others
Species richness is just how many different species there are in a habitat.
A high Simpson's Diversity Index number indicates a diverse habitat. Simpsons diversity is just an equation used to work out the assumed biodiversity of an area. A high number happens to represent a biodiverse habitat..

hope thats useful :-)

Textbook confusing me :

"Values of D range from 0 to 1. A Diversity Index value of 1 represents very low species richness. A value of 0 represents very high species richness -- the value you get if there is only one species present, for example."

If species richness is "the number of different species per unit area" then surely only one species present would amount to LOW species richness? Is there a mistake in the textbook somewhere, or am I being thick?

Later on it says "the higher the number we get for D, the greater the diversity." I would have thought that this generally means a high species richness, as in lots of different species present (though I realise it depends on the abundance of each species too, not just species richness), so in the first extract I posted have they just got "low" and "high" the wrong way round? Oh I don't know, I give up, if anyone could de-confuse at all me I'd be grateful!