The Role of Vertebrate Herbivores in Regulating Shrub Expansion in the Arctic: A Synthesis

Shrubs are expanding in Arctic ecosystems, and herbivores may be influencing this expansion by reducing the growth of preferred forage species. We synthesized new and published data to evaluate the relative influence of climate and vertebrate herbivory on different shrub species and groups. Variation in chemistry across shrub species translates to a strong preference for (and damage to) palatable deciduous shrubs compared with evergreen shrubs when herbivores are at low densities, but differences in palatability matter less when herbivores are at high densities and/or food limited. Long-term observational and experimental studies indicate that herbivores moderate the expansion of fast-growing deciduous shrubs such as willows (Salix spp.), although more research is needed to address the relative strength of climate and herbivory at larger scales. Well-defended shrubs such as Siberian alder (Alnus viridis) and resinous dwarf birch (Betula nana exilis) are generally not preferred by herbivores and may therefore outpace the expansion of more palatable species.