Exclosure Experiment in the Swiss National Park

Trophic interactions of herbivores of different body sizes were assessed using exclosures in different vegetation types in grassland ecosystems of the Swiss National Park. The focus was on changes in plant, root and microbial biomass and nutrient content (nitrogen and phosphorus) as well as on the composition of plant and microbial communities over the course of several growing seasons.

The aboveground plant biomass produced in grassland ecosystems supports a multitude of vertebrate and invertebrate herbivores. Vegetation productivity, vegetation nutritional quality, plant species richness, soil physical, chemical and biological properties or ecosystem nutrient cycling, in turn, can be affected by the quantity, quality and the composition of forage that herbivores ingest. The response of an ecosystem to grazing, is however, highly dependent on the productivity of the ecosystem studied and the grazing intensity it receives.

The grazing intensity and the amount/quality of biomass consumed by a herbivore group, in turn, are generally related to body size: large ungulates, for example, consume greater proportions of the total biomass produced, but generally are less selective than small vertebrate and invertebrate herbivores. Consequently, the sum of the effects of herbivores on ecosystem parameters depends on the density and composition of the herbivore community as well as the productivity of an ecosystem.

However, most of the research conducted on grazer-grassland interactions only considered one or two herbivore groups of different body size. We are aware of only four studies that assessed how three or more herbivore groups of different body size interacted with the grassland ecosystem they inhabit. In these studies the investigations were restricted to the effects of herbivores i) on plant-animal interactions (two trophic levels) in one or several vegetation types only or ii) plant-animal-soil interactions in a single vegetation type only. Consequently, our understanding on the functioning of grazer-grassland interactions considering the combined effect of several herbivore groups on vegetation types of different productivity and across several trophic levels is still very limited.

To assess a multitude of potential interactions in a grassland ecosystem we established an exclosure experiment in the Swiss National Park in spring 2009 that lasted until the fall of 2013. With this experiment we determined single and combined effects of four herbivore groups of distinctively different body size and foraging behavior in two different vegetation types on plant, root and microbial biomass, nutrient content (nitrogen and phosphorus) and mineralization rates (nitrogen) as well as on the composition of plant and microbial communities. The herbivore groups include large (red deer and chamois), medium (marmots), and small (small rodents) vertebrate as well as invertebrate herbivores. During the last field season we assesed, in addition, how excluding the different mammalian herbivore species affects the above- and belowground invertebrate community composition and population size.