Large herbivores as ‘keystone species’ in ecosystems

It is clear from many chapters in this book that large herbivores may have a very significant impact in the ecological systems of which they are a part – with a major shaping effect on the structure and dynamics of vegetational systems – even at a landscape scale. These effects include consumption of vegetation, redistribution of nutrients via deposition of urine and dung, soil compaction and erosion via trampling, dispersal of seeds via fur or dung, and secondarily, through these effects on the vegetation, alteration and creation of habitats or microenvironments for other plants and animals.

Some of these effects are direct and straightforward: consumption of vegetation by large herbivores obviously leads to profound changes of the morphology of individual plants (height, stature, structure), but also to changes of the physical three-dimensional architecture of the community. The stunted and ‘hedged’ shrubs in the understorey of grazed woodlands or the distinct browseline on the underside of the canopy trees form nice examples. Consumption also leads to changes in species composition, with elimination from the community of grazing-intolerant species, and increase of species which are tolerant to defoliation, or have specific defences against attack: spines, thorns, or chemical defences rendering them less palatable (Callaway et al., 2000). Together with dunging and trampling, consumption further creates gaps within closed swards for the establishment of ephemerals.