Impact of herbivory by red deer ( Cervus elaphus L.) on patterns and processes in subalpine grasslands in the Swiss National Park

Abstract

The spatial grazing patterns of red deer ( Cervus elaphus L.) in a subalpine grassland ecosystem of the Swiss National Park were analysed with reference to the phosphorus content in the topsoil and to the former agricultural management system. Changes in the composition of the vegetation and in plant species richness were studied on permanent plots set up between 1917 and 1945 in areas which today are heavily grazed by red deer, and were related to the development of the red deer population. The spatial grazing patterns of red deer are, 80 years after the Park’s foundation, still determined by former agricultural use. Red deer prefer the most phosphorus-rich, formerly irrigated parts for grazing. Grazing by red deer was found to be the driving force behind temporal changes in the vegetation observed on the subalpine grasslands. Plant succession has proceeded faster in the preferred grazing areas and the vegetation has adapted to the grazing pressure with a decrease in tall-growing plant species. At the same time, the abundance of unpalatable species has increased due to (i) morphological defences, (ii) spatial avoidance, i.e. by growing close to the ground and (iii) temporal avoidance because they have a short life-span (annuals). Changes in vegetation composition have coincided with a sharp increase in species richness. Between 1917 and 1999, the number of plant species has doubled on average. We found a highly significant correlation between the changes in the numbers of plant species and red deer, with species richness increasing with increasing deer density.