Effects of vertebrate herbivores on soil processes, plant biomass, litter accumulation and soil elevation changes in a coastal marsh

1 Submergence of coastal wetlands in Louisiana is currently rapid and wides pread. A number of factors contribute to this loss of habitat, including th e activities of herbivores. The objective of this study was to examine the effects of large mammals, predominantly nutria and wild boar, on processes controlling soil elevation in coastal marshes. 2 Effects of herbivores on soil and vegetation were assessed by the use of paired fenced and unfenced plots over two successive growing seasons. Above -ground biomass, Litter production, changes in soil elevation, vertical soi l accretion, shallow subsidence, below-ground production of roots and rhizo mes, the thickness of the root zone, soil bulk density, and soil organic ma tter were measured. 3 Above-ground biomass, below-ground production, soil elevation and the exp ansion of the root zone decreased due to herbivore activity. Litter product ion, the rate of soil surface accretion and shallow soil subsidence were al l higher in grazed compared to unrated plots, while soil organic matter and bulk density did not differ significantly between treatments. 4 The results indicate that herbivores can have a negative effect on soil b uilding processes, primarily by reducing below-ground production and expans ion of the root zone. Where natural rates of mineral sediment deposition ar e high, coastal marshes are expected to persist, despite herbivore activiti es. However, where sediment inputs are substantially less, herbivores may l ead to destruction of habitat.