Biodiversity and productivity vary strongly among ecosystems: understanding the causes of these variations is a primary objective of ecology. To date a few overarching principles have been established. One is the species‐area relationship: the species diversity of a system depends principally on its area, and some major mechanisms underlying this principle have been identified (Rosenzweig 1995). The structure and dynamics of plant communities also affect biodiversity profoundly. Edaphic conditions set the bounds for plant communities, and fire can be a key determinant of their structure and diversity. In addition, at least in some ecosystems, large herbivores are ‘keystone’ species, so the systems have very different structures according to whether large herbivores are present or absent. There is also some evidence that large herbivores affect plant productivity, from modelling (de Mazancourt et al. 1998) as well as empirical work (McNaughton 1985).

Understanding the role of large herbivores is therefore important for ecology, and also because the abundance of these animals can have profound effects on the conservation status of other species, through their impact on plant communities. However, the literature on these questions is rather difficult to access especially for people who are not academic ecologists. Reviewing the impact of large herbivores on ecosystems was identified as a priority in the Action Plan for the Large Herbivore Initiative for Europe and Central Asia (see http://www.largeherbivore.org). In some areas the ungulate populations are ‘overabundant’ and have serious negative impacts on forestry, agriculture and biodiversity.