Human Modification

Human Modification, Santa Barbara County, California

For each indicator, two factors were calculated at a given location (cell): intensity and footprint. Intensity (I) is the degree to which an activity at a location generally modifies terrestrial and aquatic ecosystems, which is useful to differentiate effects of different types of land uses. For example, using a patch of land as pasture is likely to have a lower overall effect on the physical integrity of ecosystems than conversion to a parking lot. The second is the footprint (F), or the areal extent of a given human activity. In practice, the footprint is measured as the proportion of a raster cell (here 30 m) that is occupied by a given land use. Thus, the overall degree of human modification (H) at a location is calculated as: H = I × F, where a value of 0.0 has no human modification and a value of 1.0 has high modification. Estimates of I and F for each indicator were made from two different sources: expert opinion or empirical datasets. For the empirically-based stressors, I was estimated as a value from 0.0 to 1.0 based on the relative amount of energy required to maintain a particular land use type, obtained from Brown and Vivas (2005). Thus, H accounts for a gradient of impact of human activities, has a direct physical interpretation, and the value remains a ratio data type so that differences within the range are meaningful (i.e. a value of 0.8 is twice the effect of 0.4), unlike most index-based approaches where values are converted to nominal or interval values. Note the H value was set to No Data (i.e., masked out) for locations (30-m cells) that intersected lakes, reservoirs, or rivers (represented by the USGS National Hydrography Dataset as waterbodies and river area maps; http://nhd.usgs.gov/; accessed June 2015).