Short-term effects of red deer overabundance on herbaceous communities in Mediterranean woody ecosystems

Herbaceous communities are vital for biodiversity in forests and scrublands. Herbivory influences their cover, richness, and diversity. Overabundant herbivores disrupt these communities and ecosystem processes. In recent decades, vast territories (e.g., North America, Europe) have experienced a remarkable increase in deer (Cervidae) populations. However, few studies have examined the effects of increasing ungulate densities in different Mediterranean habitats. This study explores the short-term impacts of red deer overabundance on the cover, richness, and diversity (taxonomic and functional) of herbaceous layers in Mediterranean scrublands and forests to guide management. In three enclosures across two habitats (scrubland and oak forest), we manipulated deer densities: control (no deer), high density (18–63 deer/km²), and hyper density (58–113 deer/km²). Herbaceous species occurrence and cover were recorded in 90 quadrats (50 × 50 cm), alongside bibliographic data for six functional traits. Herbaceous cover, richness, and diversity (taxonomic and functional) were analysed with Generalized Linear Mixed Models. Hyper deer density negatively affected herbaceous cover, taxonomic richness, and diversity in both forests and scrublands. The high density treatment already showed a clear trend towards reducing these three variables that were heavily increased for the hyper density scenario (e.g. 84.2 % cover loss in the scrubland and 64.2 % in the forest). Functional diversity remained unchanged, likely due to a > 40-year legacy of deer herbivory favoring traits typical of grazing areas. Deer overabundance is causing rapid, drastic changes in herbaceous communities, even in short term. Managers should reduce deer populations to protect these communities, which offer high-quality forage and essential ecological roles.