The perils of having tasty neighbors: grazing impacts of large herbivores at vegetation boundaries

The boundaries between vegetation patches are focal points for dynamic interactions between plant communities, particularly in,grazed ecosystems where vegetation types may differ in their acceptability to herbivores. Here we show that key vegetation resources attract herbivores, and the surrounding vegetation receives a higher impact than if it is associated with patches of less preferred vegetation (an example of apparent competition). We studied the influence of proximity to preferred grass patches on utilization of the less preferred dwarf shrub, heather (Calluna vulgaris) by red deer (Cervus elaphus) and sheep (Ovis aries) at a range of spatial scales in the Cairngorm Mountains of Scotland, UK. There was a sharp decline in heather utilization with increasing distance from the edges of grass patches. The proportion of grass in the local landscape (within I km) had a significant positive effect on heather utilization both at the grass-heather boundary and beyond 5 m from the grass patch. There was also a significant effect of dominant grass species on the utilization of heather within 50 cm of the grass-patch edge, with utilization around Agrostis/Festuca patches (most preferred) being greater than around Nardus-dominated patches, and lowest around patches of Molinia. The greatest contribution to variation in heather utilization was at the smallest scale, and variance components decreased as spatial scale increased, making it impossible to predict local heather utilization (i.e., at the scale of individual plants and of individual bites by foraging ungulates) from large-scale parameters alone, such as herbivore density. These findings emphasize that vegetation-herbivore interactions are localized within the landscape, and that it is these hot spots which are the key fulcrum for vegetation dynamics