When an ecologist compares the diversity of different communities by counting the number of species within each community, the measure of diversity being used is called

When an ecologist compares the diversity of different communities by counting the number of species within each community, the measure of diversity being used is called "Species richness".

Explanation:

The species diversity can be measured by:

Species richness is the number of species present in a sample, within a community, habitat or microhabitat, ecosystem, landscape, region or any well defined spatial unit.

It is the standard metric for measuring biodiversity because of its simplicity and for being easy of interpretation. It is the simplest and straightforward measure of biodiversity. It is just a count of species, with no need for abundance data. In some cases, this can be the cause of criticism, as it ignores information about the species-abundance distribution.  

the answer is 100% density i've answered this question before and done the quiz myself.