WC Archive

BII is formally defined as the average, across all species chosen for consideration (typically, the well-known taxonomic groups: plants, mammals, birds, reptiles and amphibians), of the change in population size relative to a reference population. The reference population is conceived as that occurring in the landscape before it was altered by modern industrial society during the colonial period (i.e. pre-1700). In the southern African context, the current populations in large protected areas in each ecosystem type serve as a proxy for the reference populations.

In the Nature piece, they apply this Index to South Africa:

The BII score in the year 2000 is about 84%: in other words, averaged across all plant and vertebrate species in the region, populations have declined to 84% of their presumed pre-modern levels. The taxonomic group with the greatest loss is mammals, at 71% of pre-modern levels, and the ecosystem type with the greatest loss is grassland, with 74% of its former populations remaining. During the 1990s, a population decline of 0.8% is estimated to have occurred.

One of the values of the BII is that it scales relatively well -- the Index provides a useful gauge even at the broadest level, while still providing insight when used at a finer resolution. The four maps of South Africa shown here demonstrate how the BII scales with changing focus. It's worth noting (even if it's not surprising) that the finest resolution map (D, lower right) shows low biodiversity mapping almost perfectly to urbanization patterns.

The BII isn't perfect, but it's a useful addition to our toolkit for understanding the changes we're making to the planet's systems.