The BII represents the average remaining proportion of the abundance of indigenous faunal and floral populations in an area relative to an intact reference state. In this case, it means pre-colonial and pre-industrial conditions4. Calculation of the BII requires three sets of information: (1) a map of human impacts across the area (typically proxied by land use, which captures major land covers, uses and associated activities); (2) the richness of indigenous species that occur in the area; and (3) intactness scores—estimates of the remaining proportion of intact reference populations of these indigenous species under different conditions of human impact—on a scale from 0 (no remaining individuals) to 1 (same abundance as the reference) and, in rare cases, to 2 (two or more times the reference population)4,5. For species that thrive in human-modified landscapes, scores can be greater than 1 but not exceed 2 to avoid extremely large scores biasing aggregation exercises4,5. The intact reference is the population abundance that would have occurred in the area before alteration by modern industrial society (around 1700 ce)4. In most parts of sub-Saharan Africa, this corresponds to populations before the substantial alteration of the landscape triggered by colonial settlement, although we recognize that some declines would already have occurred by this point in time. Because information on species populations from this era is almost non-existent, standard protocol is to reference a remote protected or wilderness area with a natural disturbance regime (a hybrid–historical approach51) where necessary4.
The BII for a unit of land (pixel) is calculated by averaging, across the richness R of indigenous species that should occur in that pixel, the intactness score I sk of species s given the human impacts (that is, land use) in that pixel k, such that each species counts equally. That is,
$${\mathrm{BII}}_{\mathrm{pixel}}=(1/R){\Sigma }_{s}{I}_{{sk}}$$ (1)
The index can accommodate data scarcity. In the absence of intactness scores per species per land use, the BII can use intactness scores that represent groups of species that are expected to respond similarly to human impacts (functional response groups) and species richness information for the broader area (for example, ecoregion)4. In such cases, the BII for a pixel is determined as follows:
$${\mathrm{BII}}_{\mathrm{pixel}}=(1/{\Sigma }_{i}{R}_{i}){\Sigma }_{i}{R}_{i}{I}_{{ik}}$$ (2)
where I ik is the intactness score for functional response group i given the land use in that pixel k, weighted by the richness R i of functional response group i (number of species or proportion of total species) in the relevant area (commonly, the ecoregion).
BII can be averaged across n pixels in an area of interest (for example, continent, biome, ecoregion or country) to provide a single BII score that accounts for the composition of both species and land uses:
$$\mathrm{BII}=(1/n){\Sigma \mathrm{BII}}_{\mathrm{pixel}}$$ (3)
BII scores can be calculated for all species or a subset (for example, vertebrates, amphibians, among others). Here we made use of equations (2) and (3) to accommodate data scarcity. BII estimates were converted to percentages by multiplying by 100.
Intactness scores co-produced by experts
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