Neighbourhood effects on the risk of an unpalatable plant being grazed

Most studies on the importance of the neighbourhood on a plant's risk of herbivory have focused on palatable plants and how they are protected by unpalatable neighbours. This study examined the grazing intensity of a relatively unpalatable shrub, Buxus sempervirens, in different neighbours. Exactly 2683 plants of Buxus sempervirens (including 172 controls) were sampled in 12 enclosed pastures belonging to 4 sheep farms. The enclosures were grazed at 3 different seasons (spring, summer and autumn). Plants were divided in 4 age/hight classes (first year, < 4 cm, 4-10 cm, 10-40 cm) and into 8 neighbourhoods. The first of these was characterised by the absence of any plants within a radius of 5 cm around the Buxus individual and the 7 others by the identity of the dominant species in contact with the Buxus plant. The intensity of grazing on the neighbouring plants were also recorded. At the end of one year's monitoring, 26.2% of Buxus sempervirens plants had been grazed. The proportion of plants grazed was significantly higher in spring than in the other two seasons. It decreased with increasing plant age. It was higher in neighbourhoods that were intensively grazed than in those with light grazing. The proportion grazed in the absence of a neighbour plant was intermediate between the previous two. The probability of a plant of an invading species being grazed is influenced by factors other than its life-history traits. Some neighbourhoods consisting of unpalatable plants facilitate the establishment of Buxus sempervirens by protecting the young plants from grazing, whereas other highly palatable neighbourhoods are readily grazed by sheep, thus indirectly increasing the proportion of Buxus sempervirens that are grazed. The young and short (< 4 cm in height) Buxus plants, which are less recognisable by sheep, are most sensitive to the impact of grazing.