Species richness is proportional to productivity, spatial heterogeneity, habitat (island) area, succession stage, frequency of disturbance.
Cautionary: (feeding animals) Tuttle, 1979. Status, causes of decline, and management of endangered gray bats. Journal of Wildlife Management.
What kind of "diversity" do/should we value? Leaving aside direct and indirect economic value. Hawksworth, 1994. Biodiversity: Measurement and estimation. Philosophical Transactgions of the Royal Society of London. Series B.
How many species? May 1990 Philosophical Transactgions of the Royal Society of London. Series B. : How do we estimate the extent of our uncertainty?