Populations of major wild grazing animals that are the heart and soul of Kenya's cherished and heavily visited Masai Mara National Reserve—including giraffes, hartebeest, impala, and warthogs—have "decreased substantially" in only 15 years as they compete for survival with a growing concentration of human settlements in the region, according to a new study published today in the May 2009 issue of the British Journal of Zoology.