people- estimate that humans have modified >50% of Earth's land surface.
The grass family, Poaceae, is one of the largest plant families, and, to humans, perhaps the most important, as it includes agricultural grains such as wheat, rice, maize (corn), and sugar cane. The Poaceae comprise about 600 genera and perhaps ten thousand species. It is estimated that grasslands cover about 20 percent of the earth's land area.
so the answer is humans