Lines around fragments: effects of fencing on large herbivores

Abstract

People construct fences to delineate land ownership and to control access to land. Fences accomplish several purposes, notable among these 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, fences may 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. 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 damage vegetation. More subtly, fencing parcels may reduce the carrying capacity of a landscape irrespective of habitat loss by eliminating access to heterogeneous forage patches. Normalised difference vegetation indices, derived from satellite images and reflecting green vegetation biomass, are used to suggest effects of fencing upon stocking rates. Ecosystem modeling quantified the decrease in herbivore stocking rate as a 300km2 parcel was fragmented. When the parcel was fenced as 10km2 sub-parcels, 19% fewer cattle could be supported, compared to the parcel being unfenced.



Keywords: ecosystem modeling, fence design, fragmentation, NDVI, ungulates



African Journal of Range & Forage Science 2004, 21(3): 147–158