Worldwide, the rapid decline of biodiversity due to intensive land-use, overexploitation and pollution poses a critical threat to ecosystems, impacting the provision of essential ecosystem services. The United Nations decade on ecosystem restoration 2021-2030 seeks to halt the biodiversity decline. One promising restoration strategy is rewilding, which has received increasing scientific attention over the past 10 years. Rewilding aims at restoring human-dominated ecosystems into their natural state, for instance by re-establishing keystone species such as large herbivores, and habitat connectivity. It promotes trophic cascades, ecosystem services and resilience against environmental changes with the goal of halting biodiversity loss and habitat degradation.
Since rewilding represents a nascent scientific concept, publications are mostly opinion papers or short-term experiments. Analyses with long-term empirical data are missing. To fill this knowledge gap, we explore a unique rewilding experiment that was initiated 110 years ago: In 1914, stringent regulations were implemented in a 170 km2 mountain area in the European Alps, banning any human use such as alpine farming, hunting, mining, and forestry. This action led to the establishment of the Swiss National Park, a protected area with the highest possible level of protection (IUCN Ia), where human presence is restricted to a network of trails. As a result, the Swiss National Park is the world's second-oldest IUCN Ia protected area larger than 12 km2. To monitor the expected natural succession from grassland into secondary natural forest, permanent vegetation plots were set up in 1917 and marked with wooden posts, allowing to re-survey the plots within centimetre-precision the following years. Until today, 130 permanent plots were re-surveyed, on average every 9 years.
This results in 1256 vegetation surveys, all including abundance estimations on species level, and a total of 338 observed vascular plant species. Annual observations of ungulates revealed that at the beginning of the rewilding experiment, red deer where locally extinct due to overexploitation and hunting. Only three years after, the first red deer returned to the park area and the population exponentially increased until the 1980s, with a slight decline thereafter until today. The return of this keystone species suppressed forest succession and increased plant diversity. This effect was strongest in permanent plots with highest local red deer grazing intensity, which was mainly where cow densities were highest before farming was banned (cattle resting areas). There, the rewilding strategy counteracted recent global change effects on plant communities and diversity by impeding the increase of thermophilic, nitrophilic and competitive species. Under intensive red deer grazing, the presence and abundance of non-thermophilic plant species increased because thermophilic species tend to be taller and more nutritious, hence preferably eaten by red deer.
Contrary to common assumptions, our results suggest that restored biotic interactions following rewilding have the potential to overrule the effects of global change on plant communities in the long run. Moreover, our contiguous, very long-term time series allows us to explore the particularly slow vegetation shifts in mountain ecosystems that are typically dominated by clonal, long-lived plant species. Finally, our approach advances opinion-based papers with data-based analyses to quantify the effectiveness of long-term rewilding as restoration strategy.
Figure 1: Year of foundation and size of the world's terrestrial highly protected areas (IUCN Ia, Ib, II). The Swiss National Park, indicated as SNP, is the world's second-oldest IUCN Ia protected area larger than 12 km2.
Figure 1: Year of foundation and size of the world's terrestrial highly protected areas (IUCN Ia, Ib, II). The Swiss National Park, indicated as SNP, is the world's second-oldest IUCN Ia protected area larger than 12 km2.
Figure 2: Temporal distribution of the studied permanent vegetation plots within the Swiss National Park. Crosses indicate vegetation surveys. Visualised on top of the graph are the three botanists which conducted the vegetation surveys (1917-1938 Josias Braun-Blanquet; 1939-1989 Balthasar Stüssi; 1992-today Martin Schütz).
Figure 2: Temporal distribution of the studied permanent vegetation plots within the Swiss National Park. Crosses indicate vegetation surveys. Visualised on top of the graph are the three botanists which conducted the vegetation surveys (1917-1938 Josias Braun-Blanquet; 1939-1989 Balthasar Stüssi; 1992-today Martin Schütz).