Lines around Fragments: Effects of Fencing on Large Herbivores

People construct fences to delineate land ownership and to control access to land. Fences accomplish several purposes, notable among them are containing livestock or wildlife raised for profit or subsistence, excluding use of vegetation within areas to be conserved, and reducing conflicts between wildlife and humans. In addition to these intended purposes, fences may also offer unanticipated benefits, such as vegetation within hedgerow fences providing cover to wildlife, or grazing by confined herbivores promoting native flora. However, because fences limit mobility of large herbivores, fenced areas become fragments within the landscape, sometimes with undesirable results. We review of the positive and negative consequences of fencing landscape patches for large herbivores, using examples from livestock production and wildlife conservation. Fences allow grazing to be controlled, to control grazing intensity or rest parcels. Fences may entangle or electrocute herbivores, truncate migratory routes, excise important resources needed by large herbivores, and allow resident herbivore populations to become too high and harm vegetation. More subtly, fencing parcels may reduce the carrying capacity of a landscape, irrespective of habitat loss. Eliminating access to heterogeneous forage patches within a landscape reduces options available to herbivores or their herders, both at a given time and across seasons. Normalized difference vegetation indices, derived from satellite images and reflecting green vegetation biomass, are used to suggest potential effects of fencing upon herbivore stocking rates. Ecosystem modeling quantified the decrease in herbivore stocking rate as a 300 km block of land in southern Africa was fragmented. When the block was fenced as 10 km parcels, 19% fewer cattle could be supported, compared to the block being unfenced.