Fig. 1

The diversity of the herbivore community (here expressed as richness of species or groups of herbivores) can have different impacts on processes, functions or properties of tundra ecosystems. a Taking as an example the effects of herbivores on vegetation, if the herbivores present in the herbivore community have similar diet we can expect additive effects on vegetation, where the combined effects of herbivores promote directional changes in plant community composition. On the other hand, if herbivores have different diets, we can expect compensatory effects, where the effects of different herbivores on plant species may balance each other by reducing competition among plant species and lead to stabilizing effects on plant community composition. b We hypothesize that herbivore diversity will influence ecosystem processes, functions or properties directly (e.g. consuming biomass, trampling) and indirectly (via effects on plant community composition as in panel a). Through both direct and indirect effects, we expect the combined effects of multiple herbivores on processes, functions and properties to be greater compared to the effects of single herbivores, although the slope of this relationship can change depending on the ecosystem process, function or property and the herbivore assemblage being considered. At very high herbivore diversity this relationship may saturate (dashed line) but we expect that in tundra ecosystems (shaded area), with relatively low numbers of species of herbivores, the relationship will not saturate