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No. 425: Biodiversity effects of rewilding

Fløjgaard, C., Buttenschøn, R.M., Byriel, F.B., Clausen, K.K., Gottlieb, L., Kanstrup, N., Strandberg, B. & Ejrnæs, R. 2021. Biodiversitetseffekter af rewilding. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 124 s. - Videnskabelig rapport nr. 425. http://dce2.au.dk/pub/SR425.pdf.

Summary

 

Background

Biodiversity is in continuous decline and Danish ecosystems have been heavily modified, which is reflected in the latest update of the Red List and the latest reporting of the conservation status of species and habitats protected by the EU Habitats Directive. In this context, we assess the potentials and challenges of rewilding as a management model.

Baseline

Rewilding rests on a biological understanding of self-managing ecosystems as the optimal management model for biodiversity. Present-day species evolved in natural ecosystems prior to human influence, and the biodiversity crisis results from our fundamental alteration of the conditions and processes in the natural ecosystems. One of the major changes that can be attributed to humans is the impoverishment of the natural mammal fauna and the replacement of wild species with domestic animals. Therefore, the question of a natural baseline for grazing in temperate ecosystems becomes decisive for the practicing of nature management and rewilding. By taking a paleoecological perspective, it is possible to compile a list of large animals that have been active in self-managing ecosystems before human influence became crucial. Some of these large animals are extinct today, a few live on as domestic animals and others have a limited distribution or live in densities below the natural carrying capacity of the landscape. Natural densities are a key factor in the establishment of baselines for nature management, and determination of such is challenged by the lack of data from ecosystems without human regulation of vegetation and megafauna. Based on analyzes and ecological interpretations of available data globally and nationally, we estimate that the natural density of herbivores in typical Danish ecosystems is in the range of 70-250 kg/ha, although more experimental evidence is needed. Despite these uncertainties, it is considered certain that the density of herbivores in unfenced nature in Denmark is far below the natural range, while the density in typical grazing management is far above the natural range. The majority of the landscape is thus either overgrown or overgrazed during the summer months with loss of biodiversity as a result.

Practical experience

Rewilding as a concept has influenced nature management in Europe over the last 10-15 years. The majority of initiatives have focused on a (re)introduction of missing keystone species, and especially cattle and horses have been used for an active restoration of natural ecosystem processes. In connection with this report, we have mapped ongoing rewilding projects in Denmark using large grazing animals. A similar mapping was conducted in 2015 (Pedersen et al. 2020) that found a total of 28 Danish projects with a minimum of rewilding – by comparison we currently find at least 85 projects - and the number of rewilding initiatives implemented has thus increased sharply over the last 5 years. The vast majority of grazing projects today involve cattle and horses, but although many have an estimated natural density of large grazing animals, only a few of the projects allow natural population dynamics. In addition, the majority of the areas are small (<100 ha) and use only a single herbivore species. Lack of resources means that only rarely does actual monitoring take place in connection with the rewilding projects, and the projects thus hardly ever contribute to an increased knowledge of the effects on biodiversity.

Effects on biodiversity

One of the keys to understanding our biodiversity is the plants' build-up of carbon and the diversification that occurs when plant material is available to animals and fungi in the form of various plant species, bark, live and dead wood and flowers as well as excrements and carrion. The large animals play an important role in this diversification by creating diverse plant communities, by damaging trees and by converting plants into excrements and ultimately carrion.

Grazing has been used as a tool in nature management for many decades, but in many places, it has not been adapted to existing conditions. This has meant that many of the open nature areas are increasingly overgrown or under development towards a species-poor vegetation with build-up of a thick layer of litter. Large herbivores reduce the layer of litter through their movements and grazing, and creates opportunities for germination of light-demanding plants. They disperse seeds of plant species, including seeds from species with a late bloom or species only eaten during winter. With year-round grazing, more aboveground biomass is produced than the animals consume during summer, which gives many plants the opportunity to flower and set seeds. The animals' choice of habitat and food varies with season, which helps to create structural variation.

With more than 18,000 species, insects form an important and ecologically diverse group. Only a limited number of evidence-based studies exist, that assess the effects of rewilding-initiatives with large herbivores on insects. The presence of large herbivores can benefit the insect fauna by increased availability of excrements, dead wood, mud holes and other physical influences that creates variation and thus habitats and resources for insects. In addition, large herbivores can help reduce overgrowth and accumulation of dead plant matter on the soil surface (litter), which is a significant threat to many insects in open habitats. However, a uniformly high density of large herbivores might have negative effects on the insect fauna, e.g. via unintentional predation and a reduction in the availability of food resources for herbivorous insects, including flower-seeking insects, as well as decom-posers.

Overall, it is expected that re-establishment of natural grazing and disturbance dynamics will lead to an increased heterogeneity in vegetation structure, natural processes and hence habitats, resulting in an expected positive effect on the diversity of birds. Compared to current practice, rewilding will change the occurrence of different habitats, and therefore probably also lead to both winners and losers in comparison to the current situation. Positive effects are expected among others for insectivorous species and meadow birds associated with low vegetation, while the impact may be negative for species that prefer a dense undergrowth or tall herbaceous vegetation. The diversity of birds often peaks at an intermediate grazing pressure, and in this respect, rewilding might represent a middle ground between the current extremes in the form of absence of large herbivores and intensive summer grazing. An important point seems to be the advantage of landscape heterogeneity in areas where rewilding is established, and the positive effects of natural grazing, hydrology and disturbance on the birds, is probably conditional on the right ecological framework (in terms of size and heterogeneity) that allow these processes to develop with natural dynamics.

Like animals, fungi are dependent on carbon sources, and respond positively to ecosystems with diverse vegetation, especially ecosystems with a mixture of large trees, dense scrub and open clearings with grassland vegetation. Therefore, the hypothesis is that rewilding will be able to support a rich funga, as long as the density of animals and the variation in the grazing can support the development of a structurally diverse vegetation.

Wild mammals and hunting

Denmark has only a few wild species of large grazers (primarily deer) that can contribute to ecosystem functions. In the national administration, there are no vision to allow these species to build up populations to a level potentially constituting the role of keystone species. On the contrary, management is based on the view that these animals are conflict species that cause damage to cultivated fields and plantations, and a notion that there are too many individuals. The national administration is thus today a barrier to rewilding. Likewise, the regional and local management precludes sufficient densities of animals and prevents the animals from having a behavior that supports rewilding. Within the core areas of the relevant species, the objectives are too many and too diverse to co-exist with rewilding purposes. Hunting plays a crucial role in limiting the number and distribution of wild deer, and obstructs the animals' natural behavior. On the other hand, hunting is a valued economic and recreational asset and, hence, the development of rewilding with wild species of large grazers might require hunting to be considered as part of the management.

The directives

Denmark is obliged by the EU's nature directives to ensure a favorable conservation status for species and habitats. Immediately, rewilding is expected to be able to ensure greater spatial and temporal continuity in nature management than today, where agrienvironmental schemes have failed. Rewilding might result in challenges in relation to traditional “box thinking” and conservatism regarding static habitat types in certain successional stages. Here, we emphasize the potentials of mapping and reporting mosaic nature, and stress that rewilding does not exclude targeted interventions to preserve special species or habitats if rewilding proves unable to safeguard specific interest, e.g. during a start-up period.

Knowledge

The effects of rewilding on natural processes and on species within different taxonomic or trophic groups have been studied only sporadically. In particular, long-term studies are lacking. Therefore, many of the conclusions in the report are based on a relatively limited empirical basis. On the other hand, the same can be said about the instruments of current nature management, and it would therefore not be fair to call rewilding more of an experiment than traditional nature conservation.