The United Nations Millennium Development Goals now include provision for monitoring diversity of the world's plants and animals. Using the IUCN Red List of Threatened Species, the latest Annual Report on progress towards the Millennium Development Goals, released by the UN in late September 2008, includes in Target 7.B:
Reduce biodiversity loss, achieving, by 2010, a significant reduction in the rate of loss
7.1 Proportion of land area covered by forest
7.2 CO2 emissions, total, per capita and per $1 GDP (PPP)
7.3 Consumption of ozone-depleting substances
7.4 Proportion of fish stocks within safe biological limits
7.5 Proportion of total water resources used
7.6 Proportion of terrestrial and marine areas protected
7.7 Proportion of species threatened with extinction.
You can read the 2008 MDG report online.