uses very large herds on land left fallow longer than usual. Environmental considerations Ecology A number of ecological effects derive from grazing, and these may be either positive or negative. Negative effects of grazing may include overgrazing, increased soil erosion, compaction and degradation, deforestation, biodiversity loss, and adverse water quality impacts from run-off. Sometimes grazers can have beneficial environmental effects such as improving the soil with nutrient redistribution and aerating the soil by trampling, and by controlling fire and increasing biodiversity by removing biomass, controlling shrub growth and dispersing seeds. In some habitats, appropriate levels of grazing may be effective in restoring or maintaining native grass and herb diversity in rangeland that has been disturbed by overgrazing, lack of grazing (such as by the removal of wild grazing animals), or by other human disturbance. Conservation grazing is the use of grazers to manage such habitats, often to replicate the ecological effects of the wild relatives of domestic livestock, or those of other species now absent or extinct. Grazer urine and faeces "recycle nitrogen, phosphorus, potassium and other plant nutrients and return them to the soil". Grazing can reduce the accumulation of litter (organic matter) in some seasons and areas, but can also increase it, which may help to combat soil erosion. This acts as nutrition for insects and organisms found within the soil. These organisms "aid in carbon sequestration and water filtration". When grass is grazed, dead grass and litter are reduced which is advantageous for birds such as waterfowl. Grazing can increase biodiversity. Without grazing, many of the same grasses grow, for example brome and bluegrass, consequently producing a monoculture. The ecosystems of North American tallgrass prairies are controlled to a large extent by nitrogen availability, which is itself controlled by interactions between fires and grazing by large herbivores. Fires in spring enhance growth of certain grasses, and herbivores preferentially graze these grasses, producing a system of checks and balances, and allowing higher plant biodiversity. In Europe heathland is a cultural landscape which requires grazing by cattle, sheep or other grazers to be maintained. Conservation An author of the Food and Agriculture Organization (FAO) report Livestock's Long Shadow, stated in an interview: Much grazing land has resulted from a process of clearance or drainage of other habitats such as woodland or wetland. According to the opinion of the Center for Biological Diversity, extensive grazing of livestock in the arid