"For example, if you have 100 hectares of arble land divided into 100 subsistence farmers, and then 99 of those farmers sell their land and move to the city, whilke the remaining one guy you bought all that land still cultivates it - you will have a large migration figure and perhaps depopulation of the village, but no depopulation of the land which is still used for farming. "
But between 1981 and 1999 the world's grain harvested area has shrunk from 732 million hectares to 690 million hectares, so the population movement is accompanied by a reduction in grain farming.
This is the situation in China, where "rural-urban migrants, young people who have abandoned farms caught between rising costs and falling prices, and migrate to coastal provinces towork in factories" http://migration.ucdavis.edu/mn/more.php?id=2476_0_3_0
Liu Chengwu Li Xiubin write about the way "the total sown area shrinks notably in the eastern region" and "most cases of abandoned farmland are reported in the central region, the second in the eastern region and the least in the western region [which by contrast saw some growth]" 'Regional disparity in the changes of agricultural land use intensity in China during 1980-2002' Journal of Geographical Sciences 2006 Vol.16 No. 3
Also, as indicated, a great deal of land is being ear-marked for national parks, as I wrote about in the Review of Radical Political Economics a few years ago:
"In a key step toward restoration of America's fabled River of Grass President Clinton and Secretary of the Interior Bruce Babbitt have committed $133 million in federal funds to purchase more than 50,000 acres of sugarcane in the Everglades.The government of Gabon,under the leadership of President Omar Bongo, declared 1.4 million acres of tropical moist forest at Minebe as a protected area for lowland gorillas, forest elephants, chimpanzees, bongo, and other rare species. The Minkebe forest, the largest forest reserve in Gabon untouched by logging, now links up with a 10 000-square-mile network of protected areas extending throughout central Africa. The World Bank committed to increase by 120 million acres the area of forests around the world under formal protection to bring an additional 450 million acres of forests under independent certification as "sustainably managed" by 2005. Brazil's President Fernando Henrique Cardoso has committed to triple the area of Amazon forests under formal protection by the year 2000. The 62 million acres of new parks and preserves is the largest conservation step ever in the Amazon-greater in size than the entire U.S. National Parks system outside Alaska (World Wildlife Fund 1999)" James Heartfield, 'The Politics of Food', Review of Radical Political Economics 32, 2 (2000)