Figure 3: The vegetation-derived clusters as recognized over the western Palaearctic. The image was obtained from a yearly series of monthly satellite images, capturing the Normalized Difference Vegetation Index (NDVI): a measure of the photosynthetic activity of the vegetation. These images were subjected to a cluster analysis according to the monthly NDVI features to obtain 10 categories (category 1 is water and not displayed in the picture). (Estrada-Peña et al. 2006 [48], with permission from Medical and Veterinary Entomology).