Human influence is a global driver of ecological processes on the planet, on par with climatic trends, geological forces, and astronomical variations. The Wildlife Conservation Society (WCS) and the Center for International Earth Science Information Network (CIESIN) at Columbia University joined together to systematically map and measure the human influence on the Earth’s land surface today. In 2002 they produced version one of The Last of the Wild and Human Footprint data sets (Sanderson et al. 2002).
The Human Influence Index and Human Footprint are produced through an overlay of a number of global data layers that represent the location of various factors presumed to exert an influence on ecosystems: human population distribution, urban areas, roads, navigable rivers, and various agricultural land uses. The combined influence of these factors yields the Human Influence Index. The Human Influence Index (HII), in turn, is normalized by global biomes to create the Human Footprint (HF) data set. HF values range from 1 to 100. A score of 1 in moist tropical forests indicates that that grid cell is part of the 1% least influenced or “wildest” area in its biome, the same as a score of 1 in temperate broadleaf forests (although the absolute amount of influence in those two places may be quite different). The areas that have the least influence (Human Footprint grid values less than or equal to 10) are included in The Last of the Wild data set.
Changes Since The Last of the Wild, Version One
Gridded Rural Urban Mapping Project (GRUMP) Urban Extent
The GRUMP urban extent is used in place of USGS Urban built-up polygon data and the DCW populated settlements point data. Both the USGS urban built-up and the DCW populated settlement data sets used in version one had issues better addressed by the GRUMP data set. The USGS had fewer polygons showing only population centers greater than one million compared to the GRUMP data. Also, the DCW populated settlements was limited to points indicating location of places (cities, towns, and permanent camp sites) without reference to size. We used 2km buffers around these points in version one to reflect the area of influence. The GRUMP urban extent used a combination of polygons created from nighttime light polygons and circles created around populated places (relative to population size).
Gridded Population of the World, Version Three (GPW v3)
GPW v3 global population density (UN-adjusted) is used in place of similar variables from GPW v2.
The DCW roads used in version one had some of the segments missing in South America and Central Africa. To address this in version two, Latin American roads data were updated using roads data from the International Center for Tropical Agriculture (CIAT), Latin America. CIAT compiled Latin American roads data from national roads data for all the Latin American countries. The Africa section of the DCW roads was replaced by CIESIN transportation data compiled by Andy Nelson.
Version two has more navigable rivers than version one. The number of navigable rivers now includes inland waterways and some less-known rivers in West Africa, Russia, and North America.
Land Cover Data
In this version we used the Global Land Cover 2000 product of the European Commission Joint Research Centre (JRC) Global Vegetation Monitoring Unit (GVM).The data set’s 22 land cover classes were reclassified into four main categories: Urban Areas, Irrigated Croplands, Rainfed Croplands, and Forests and Other Cover Types.
As a result of the changes, in version two the maximum HII value possible is 64 compared to 72 in version one.
Comparing Versions One and Two
About four out of eight input layers used in The Last of the Wild, version two are dated around 2000 compared to 1995 in version one. The GRUMP Urban Extent is is not only more recent (circa 2000) but also a better representation of urban extents than the DCW Settlement point layer and USGS Urban polygons combined. Similarly, GPW v3 population data has a number of improvements over GPW v2, including a three-fold increase in number of input administrative units, with population estimated around 2000. The roads layer is also more complete particular in Africa and Latin America.
Although Last of the Wild, version two data has a number of improvements over version one, the two cannot be used to derive temporal changes in Human Footprint. This is in part the result of input layers not being directly comparable. While the GRUMP Urban Extent layer is a better representation of urban influence, it cannot be directly compared to USGS urban polygons and DCW Settlement points. The land cover data sets used are also not directly comparable due to differences in methodologies and sensors used in deriving the two.
Calculating Human Influence Index
Using procedures developed by Sanderson and others (Sanderson 2002) we computed Wild/Not Wild scores for each of the eight input layers and combined them to created the composite human influence index (HII). For details on influence scores see table below.
|Variable category||Influence Score|
|Influence of Population Density/sq. km|
|0 – 0.5||0|
|0.6 – 1.5||1|
|1.6 – 2.5||2|
|2.6 – 3.5||3|
|3.6 – 4.5||4|
|4.6 – 5.5||5|
|5.6 - 6.5||6|
|6.6 – 7.5||7|
|7.6 – 8.5||8|
|8.6 – 9.5||9|
|Influence Score of Railroads|
|Within 2 km of railroads||8|
|Beyond 2 km of railroads||0|
|Influence Score of Major Roads|
|Within 2 km of roads||8|
|Within 2 to 15 km of major roads||4|
|Beyond 15 km of major roads||0|
|Influence Score of Navigable Rivers|
|Within 15 km of navigable rivers||4|
|Beyond 15 km of navigable rivers||0|
|Influence Score of Coastlines|
|Within 15 km of coastlines||4|
|Beyond 15 km of coastlines||0|
|Influence Score of Nighttime Stable Lights Values|
|39 - 88||6|
|Inside urban polygons||10|
|Outside urban polygons||0|
|Land Cover Categories|
|Other cover types including forests, tundra, and deserts||0|
Composite Human Influence Index (HII) was calculated by adding influence scores of all eight input variables. The HII values range from 0 (no human influence) to 64 (maximum human influence possible under the method).
Calculating the Human Footprint Score
The Human Footprint (HF) score was calculated by normalizing the Human Influence Index across the 15 World Wildlife Fund terrestrial biomes using the equation below:
Z = Human Footprint value
Xb = Input HII value in a biome
Xbmin = Minimum HII in a biome
Xbmax = Maximum HII value in a biome
Ymin = Minimum HII on Earth (0)
Ymax = Maximum HII on Earth (64)
The normalization assigns zero to minimum HII values and 100 to maximum HII values within each biome. Biome HF grids were merged to create the global Human Footprint data set.
The Last of the Wild Data
The Last of the Wild data is a vector representation of the world’s 10% least influenced areas. The Human Footprint grid was reclassified into Wild, HF values less than or equal to 10 and Not Wild HF value greater than 10. The reclassified grid was converted to a vector layer. Only those wild areas that are greater than 5 sq. km were retained in the final vector layer.
You can read more about The Last of the Wild methodology and related analyses in:
- Sanderson, E.W., M. Jaiteh, M.A. Levy, K.H. Redford, A.V. Wannebo, and G. Woolmer. 2002. The Human Footprint and The Last of the Wild. BioScience 52(10) 891-904, http://dx.doi.org/10.1641/0006-3568(2002)052[0891:THFATL]2.0.CO;2
- Scott M. 2003. The Human Footprint. NASA Earth Science Enterprise Data and Services.http://earthobservatory.nasa.gov/Study/footprint/
- Mayell H. 2002. ‘Human Footprint’ Seen on 83 Percent of Earth’s Land. National Geographic News.http://news.nationalgeographic.com/news/2002/10/1025_021025_HumanFootprint.html