Ground-dwelling spider diversity in rare European oak and yew woodlands and the impact of grazing

Abstract

In Ireland only 2 % of the total land area is native woodland, which tends to be small and fragmented. Killarney National Park in south-west Ireland contains the most extensive semi-natural woodland in the country, and includes oak (Quercus petraea) and yew (Taxus baccata) woodlands which are internationally protected. Here, over-grazing by large populations of red deer (Cervus elaphus) and Asian sika deer (Cervus nippon) have lead to changes in overstory and understory vegetation species composition and structure. This study presents the first description of ground-dwelling spider fauna in the rare woodlands of Killarney National Park and asked (1) do these rare woodlands support rare or specialist species, (2) does deer grazing have an effect on spider abundance, richness and species composition in the park, (3) what management recommendations can be made for deer in the park? Active ground-dwelling spiders were sampled in the oak and yew woodlands of the park by pitfall trapping within deer-proof exclosures and adjacent grazed controls. Four spider species classified as vulnerable were collected from these woodlands: Agyneta subtilis occurred in the oak and yew woodlands but Saaristoa firma, Tapinocyba insecta and Walckenaeria dysderoides were collected only in the oak woodland. Killarney National Park may be important for five species, not typically found in plantation forests, which rely on nationally scarce habitats. Deer grazing was linked with decreased ground-dwelling spider abundance and species richness in the oak woodland by reducing structural diversity of the habitat. Fewer effects of grazing were detected in the yew woodland, possibly due to increased culling in the area. Results provide evidence that controlling deer grazing is important for woodland biodiversity in Killarney National Park. Incorporating deer into woodland management in the park is recommended to maintain low grazing levels which will prevent woodland closure and maintain ground vegetation diversity.