Contextual intactness of habitat for biodiversity: global extent, 30 arcsecond resolution

Brief description

This global spatial layer of contextual intactness aims to identify priority areas around the world where protection and management will best promote biodiversity persistence. This layer was derived by integrating both the condition of each focal location and the condition of all other locations expected to have supported shared species with the focal location prior to any habitat degradation. The contextual intactness of each location (grid cell) is the proportion of habitat predicted to have once supported a similar assemblage of species but is now in worse condition than the focal location. This was derived using the BILBI global biodiversity assessment system, by integrating: (1) an updated map of the terrestrial human footprint on natural systems, and; (2) generalized dissimilarity models of species assemblage turnover for terrestrial vertebrates, invertebrates, and plants.

Lineage

Source Data
The input datasets are:
(1) an updated map of the terrestrial human footprint on natural systems to describe current habitat condition (Venter et al. 2016);
(2) generalized dissimilarity models of species assemblage turnover for terrestrial vertebrates, invertebrates, and plants, derived using more than 100 million occurrence records from more than 400,000 species (Hoskins et al. 2019). These models enable the estimation of the similarity in species assemblages between any pair of locations, and subsequently the expected uniqueness of the biodiversity within any terrestrial location globally.

Preparation Method
To determine the contextual intactness for each location across the globe, we combined the revised human footprint spatial layer and the predicted similarity in species assemblages between pairs of locations using the BILBI framework. For each grid cell i, we selected a spatially regular randomly positioned selection of n other grid cells j to compare to cell i . A sample of comparison cells is required because there are >200 million cells on the 1 km terrestrial grid of the planet, and comparing each grid cell with every other grid cell is computationally prohibitive. For this assessment, the number of other grid cells j was a minimum of 1% of the total grid cells within each of the world’s 7 biogeographic-realms (Antarctica being excluded) (Olson et al. 2001).

We then determined the expected similarity (sij) in species assemblages between cell i and each comparison cell j using the BILBI framework (Hoskins et al, 2019). The human footprint (HFP) value for cell i (HFPi) and all comparison cells j (HFPj) was also extracted. We then derived a histogram of the summed species assemblage similarity to grid cell i, within integer bands of the human footprint value for all the comparison cells j .

From this histogram, we then calculated: (a) the sum of the assemblage similarities to i where the comparison cell j had a higher human footprint to i, and; (b) the total sum of the all the assemblage similarities between i and j across all human footprint scores . The contextual intactness for grid cell i (CIi) was then calculated as the sum of assemblage similarities to i with a higher human footprint divided by the total sum of assemblage similarities to i:

This calculation was repeated for every terrestrial grid cell globally to derive a spatial map of contextual intactness for each taxonomic group (vertebrates, invertebrates, plants). The spatial layers for these three taxonomic groups were then averaged to derive a single contextual intactness layer for biodiversity, though future analyses could consider each taxonomic group separately.

References
A. J. Hoskins et al., (2019) Supporting global biodiversity assessment through high-resolution macroecological modelling: Methodological underpinnings of the BILBI framework. bioRxiv, 309377.
D. M. Olson et al. (2001) Terrestrial Ecoregions of the World: A New Map of Life on Earth: A new global map of terrestrial ecoregions provides an innovative tool for conserving biodiversity. BioScience 51, 933-938.
O. Venter et al., (2016) Sixteen years of change in the global terrestrial human footprint and implications for biodiversity conservation. Nature Communications 7, 12558.