Herbivores as mediators of their environment: the impact of large and small species on vegetation dynamics

<FONT FACE="Arial">Regarding the scale at which grasslands are grazed and the use of large grazers as a tool in nature conservation, it is important to understand the impact that herbivores may have on grasslands. Over the last decades evidence has accumulated that herbivores can have a strong impact on plant communities and vegetation dynamics, but the direction and strength of herbivore effects differs much between studies. One reason could be that different types or sizes of herbivores have different effects. This study compares the effects of large and small herbivores on plant species diversity, vegetation and nutrient dynamics and grassland-woodland transitions. The data presented result from field experiments on a moderately nutrient rich riverine floodplain grassland, grazed by cattle, European rabbits and common voles.The herbivores studied showed strong interactions: cattle facilitated for rabbits, i.e. most rabbits grazed where cattle also grazed, whereas voles preferred vegetation where cattle were excluded. Cattle grazing created a short sward where colonization rates of plant species were enhanced, resulting in more species per square meter than in ungrazed vegetation. However, most species within this short sward were found where rabbits created bare soil patches through digging, thus providing colonizing species with a suitable regeneration site. Grazing by cattle and rabbits resulted in lower nitrogen availability for plants compared to treatments where only voles grazed. This can be due to a different scale of returning nutrients through faeces: cattle create a few very rich patches and remove nutrients from most of the vegetation, whereas voles redistribute nutrients at a very fine scale, returning small amounts to many plants. The grazing pressure of voles was calculated to be roughly similar to that of cattle and rabbits together. However, grazing by cattle and rabbits caused a short sward and a very stable plant composition with regard to the dominant species, whereas vole grazing caused the dominant plants to show large year-to-year fluctuations in abundance and a high average vegetation height. These differences are probably induced by the size of the herbivores and thus whether they graze the vegetation from above or from below.In the transition zone of grassland to woodland, palatable trees (Oaks) could invade grazed grassland through the association with unpalatable thorny shrubs (Blackthorn), a process called associational resistance. Associational resistance did work effectively against cattle, but not against rabbits, that both consumed Blackthorn sprouts and young Oaks by going under the shrubs to consume the tree seedlings. Therefore rabbits inhibited tree regeneration, whereas under cattle grazing a mosaic of shrubs, trees and grassland could develop.Concluding, herbivores can strongly affect their environment, but not in a standard way, i.e. different herbivores have different potentials that are explained both by differences in herbivore size and in habits, as burrowing or consumption of woody plants