Ungulates,rodents, shrubs: interactions in a diverse Mediterranean ecosystem

Ungulate abundance has increased dramatically worldwide, having strong impacts on ecosystem functioning. High ungulate densities can reduce the abundance, diversity and/or body condition of small mammals, which has been attributed to reductions in cover shelter and food availability by ungulates. The densities of wild ungulates have increased recently in high-diversity Mediterranean oak ecosystems, where acorn-dispersing small rodents are keystone species. We analysed experimentally ungulate effects on seed-dispersing rodents in two types of oak woodland: a forest with dense shrub layer and in dehesas lacking shrubs. Ungulates had no significant effects on vegetation structure or rodent body mass, but they reduced dramatically rodent abundance in the lacking-shrub dehesas. In the forest, ungulates modified the spatial distribution and space use of rodents, which were more concentrated under shrubs in the presence than in the absence of ungulates. Our results point to the importance of shrubs in mediating ungulate–rodent interactions in Mediterranean areas, suggesting that shrubs serve as shelter for rodents against ungulate physical disturbances such as soil compaction, trampling or rooting. Holm oak seedling density was reduced by ungulates in dehesa plots, but not in forests. Acorn consumption by ungulates may reduce oak recruitment to a great extent. Additionally, we suggest that ungulates may have a negative effect on oak regeneration processes by reducing the abundance of acorn-dispersing rodents. Given that shrubs seem to mediate ungulate effects on acorn dispersers, controlled shrub encroachment could be an effective alternative to ungulate population control or ungulate exclusion for the sustainability of the high-diversity Mediterranean oak ecosystems.