Rewilding of complex ecosystems | Science

Trophic complexity

Species at higher trophic levels are often highly linked and functionally important for ecosystems (Fig. 1) (51). Large herbivores exert a strong influence on the diversity and abundance of other taxa such as birds, small mammals and insects (52, 53) and plants (54, 55). These effects occur through direct pathways, such as the supply of excrement and carrion (56) or the facilitation of dispersion (54, 55), but also by the modification of the physical environment, for example by grazing and trampling or the construction of dams by beavers (53, 57). Large carnivores can, through predation, affect the size of the population and the behavior of herbivores and create spatiotemporal heterogeneity in these processes. In the absence of downward control by carnivores, high densities of large herbivores can have adverse effects on the abundance and diversity of other groups of species (52, 53).

Humans modify species composition and modify their interactions by hunting, harvesting or planting selected species in agriculture and forestry (Fig. 1A). Large vertebrates are particularly sensitive to human-made defaunation because of their size, long reproductive cycle and high metabolic needs, hence the need for large areas of research. of food (58–62). Thus, even when large vertebrates are still present in man-dominated landscapes, they may not be able to exert the downward control they exert in wild ecosystems because of their reduced density (63, 64). Selective defaunation of top predators and large herbivores can result in cascading trophic effects and increased susceptibility of ecosystems to collapse (51, 65).

Backfilling can increase trophic complexity through various actions depending on the characteristics of the ecosystem. Passive reconstruction measures may, for example, include the creation of non-hunting areas. Where spontaneous recolonization is unlikely, translocation of species can also restore trophic complexity. The introduction of ecological substitutes may be an option if species have disappeared in the world (44). However, these replacements may involve unforeseeable environmental uncertainties and risks and should be evaluated with caution (25). Relining can also be supported by activities to promote coexistence between humans and wildlife, for example through compensation systems for damage to crops or livestock (66, 67).

Stochastic disturbances

Natural disturbances often occur stochastically at different locations, magnitudes and frequencies, thus reinforcing the spatial and temporal heterogeneity of ecosystems (48). These disturbances can trigger a reorganization and reconfiguration of ecosystems (68) and can lead to increased complexity of the ecosystem. They promote coexistence, as there is often a trade-off between the competitive capabilities of species and their resilience to events such as fires, floods or pest outbreaks (68). Species capable of surviving disturbance act as a biological heritage that promotes recovery and reorganization (for example, seed banks or small mammals surviving a fire) (48).

In human-dominated landscapes, natural disturbances are often suppressed (such as fire suppression or flood control) or altered in magnitude and frequency (Fig. 1A), which can lead to even greater disruption and potentially devastating (for example, smaller and more frequent fires). Instead, stochastic disturbances are replaced by predictable and constant perturbations.[Parexempleutilisationd'engraisetd'irrigationpourmaintenirdesintrantsconstantsdanslesécosystèmesoumobilisationannuelledessolspouréliminerlesespècesconcurrentes([Eguseoffertilizersandirrigationtomaintainconstantinputstoecosystemsorannualsoilmobilizationtoweedoutcompetingspecies([parexempleutilisationd'engraisetd'irrigationpourmaintenirdesintrantsconstantsdanslesécosystèmesoumobilisationannuelledessolspouréliminerlesespècesconcurrentes([eguseoffertilizersandirrigationtomaintainconstantinputstoecosystemsorannualsoilmobilizationtoweedoutcompetingspecies(48)]. Because these deterministic perturbations often act in the same place for a long time without the affected ecosystem having a chance to recover and reorganize (68), susceptible species may be lost (1). In addition, human efforts to repair the damage caused by natural disturbances can eliminate biological legacies (48, 68) and cause additional disturbances that hinder the processes of natural regeneration and reorganization (69). For example, salvage cutting to remove dead trees after gale or pest invasion often removes resources and habitats important for saproxylic beetles or cavity-nesting species (70).

Rewilding actions aim to release ecosystems from continuous and controlled anthropogenic disturbances to account for natural variability and sources of stochasticity (71) (Fig. 1B). Mowing grasslands can be reduced or replaced by natural grazing. Dams can be removed or their management modified to restore natural flood regimes (72). Logging can be replaced by allowing natural fire regimes and pests.

Dispersion

Populations depend on dispersal between habitats to avoid overpopulation (73), intraspecific competition and loss of genetic diversity (74). Exchange of individuals from different populations can increase gene flow, reduce inbreeding and thus lead to more viable populations (75). Habitat degradation or anthropogenic dispersal barriers reduce connectivity and dispersal of habitat (Figure 1A).

A rewilding approach includes improving connectivity within and between ecosystems to promote their dispersal. While connectivity efforts are often focused solely on corridors, a multi-scale approach should seek to identify and link opportunities ranging from local features, such as hedgerows to bird support or to other areas. insects (76), to large corridors, which allow the re-colonization by large mammals over long distances. Connectivity can also be improved by removing or increasing the permeability of dispersion barriers (Fig. 1B) such as roads, dams or fences. The permeability of inappropriate habitats, especially homogeneous agricultural areas, can be improved by the introduction of natural features of the landscape (77).

Integration in ecological processes

The three ecological processes can influence and promote each other (Fig. 1). Disturbances can, for example, promote habitat heterogeneity and increase resource availability for less competitive species, which can lead to increased species diversity (78). Significant habitat dispersal supports the recovery of ecosystems after (major) disturbance by allowing recolonization and recovery of affected populations (Figure 1B). Large vertebrates in complex ecosystems often act as plant dispersal agents (54, 55) and can introduce stochasticity into a system, for example by predation or grazing (79). Therefore, restoring one of these processes can positively affect the levels of functionality of the other two processes (Figure 1B). Interactions between processes can increase ecosystem resilience by jointly promoting, for example, functional redundancy or recolonization.

Rewilding efforts can be evaluated by representing ecosystems in their degraded and restored states in a three-dimensional pyramid-shaped space where each axis corresponds to an ecological process and where faces represent interaction between processes (Fig. 2). When restoring a process, the respective vertices of the pyramid move away from the origin and the volume of the pyramid increases. The difference in volume between the restored ecosystem and the degraded ecosystem is therefore an indirect indicator of the effect of renewal of the woodland on the resilience of this ecosystem. Since processes interact, restoring all three dimensions simultaneously will restore only one dimension but not consider the other two (for example, the change in the volume of the pyramid is very small when 39, one of the axes is reversed). fully restored but the other two axes remain severely degraded).

Restoration of the natural flood regime in the Leipziger Auwald city forest, Germany

The Leipziger Auwald is an alluvial forest surrounding and crossing the city of Leipzig in Germany. Since the middle of the 19th century, flood suppression and changes in forest management have led to a well-documented shift in tree community composition with increasing dominance of sycamore (Acer pseudoplatanus), Norway maple (Acer platanoides), and common ash (Fraxinus excelsior), mainly at the expense of hornbeam (Carpinus betulus) and oak (Quercus robur) (105). In this state, connectivity between the Auwald’s waterbodies is severely diminished, so active management is necessary to restore the flood regime (Fig. 2A).

After identifying flood disturbance as a major missing component of this ecosystem, city managers initiated yearly experimental flooding of a pilot area in the early 1990s (106). Results of concomitant monitoring confirmed the effectiveness and suitability of this management action. Flooding led to an increase of flood-tolerant species, such as oak and hornbeam, and a decrease or local extinction of some plant species that are intolerant to flooding but had become dominant after flooding had been suppressed (e.g., sycamore and Norway maple) (106). At the same time, (re)colonization by moisture-tolerant slug species and several ground beetle species associated with alluvial forest systems was observed (106). The findings of this long-term experiment thus inform the project’s implementation phase, in which the natural flood regime is restored in several drained branches of the Luppe River (Lebendige Luppe project) (72) (Fig. 2A).

The implementation phase is accompanied by an extensive outreach strategy that offers several opportunities for the public to engage with the ecosystem in the Auwald. Examples include multimedia teaching material to support environmental education and tools (e.g., magnifying glasses, landing nets, and maps) for interactive experiments that allow children to learn about the ecology and topography of the alluvial forest and explore its flora and fauna. A local conservation nongovernmental organization (NGO) organizes excursions to inform residents about ongoing management activities, and regular public discussion forums offer the opportunity to engage actively in the project. Two concomitant research programs assess the ecological outcomes of the project and monitor and evaluate the acceptance and perception of natural processes in the Auwald, respectively (107).

Nonintervention policy in the Swiss National Park

Established in 1914, the Swiss National Park is the oldest national park in Europe and the largest protected area in Switzerland (108). In 1909, the park’s founders—botanists and naturalists concerned with the widespread development of tourism infrastructure threatening the area’s unique flora and fauna—identified the region around the Pass dal Fuorn as a suitable target area, owing to its remoteness and species richness (108).

Making space for natural processes and conducting research on their development are central missions of the park’s management (108). The establishment of the park and management decisions were advised by cartographers and naturalists with extensive knowledge of the area and its ecosystems (109). The protection status of the area was secured by a lease agreement negotiated with the local municipalities and financed through the foundation of the Swiss Federation of Nature Conservation.

Since its establishment, the National Park has been subject to a strict nonmanagement approach and has been fully protected from human activities such as hunting, agriculture, and forestry. Trophic complexity was promoted through targeted reintroductions of ibex (Capra ibex) in 1920, 1923, and 1926 and bearded vultures (Gypaetus barbatus) from 1991 to 2007 (110). Natural disturbances are not managed, and dispersal potential is high for most species (Fig. 2B). Ecosystem development has been monitored continuously, and many of the monitoring schemes have been in place for decades (109). Conflicts with local communities were mitigated via selected active management measures. For example, public discontent over sapling damage caused by red deer (Cervus elaphus) was alleviated by organizing hunting events outside the borders of the park (109). The nonmanagement approach has resulted in the recovery of large populations of red deer, chamois (Rupicapra rupicapra), ibex, and roe deer (Capreolus capreolus), species that were nearly extinct or very rare in Switzerland when the park was established (111). The increased red deer population density has resulted in higher plant species richness in subalpine grassland (112). Additionally, wolves (Canis lupus) and brown bears (Ursus arctos) have recently been sighted, suggesting the imminent recolonization of the area by large predators. Socioeconomic studies show that the park attracts ~150,000 visitors per year, contributing substantially to the economic prosperity of the region (109, 113, 114).

Restoring ecological interactions in the Tijuca National Park, Rio de Janeiro city, Brazil

The Atlantic Forest of Brazil is a globally important biodiversity hotspot. However, most of the protected areas containing Atlantic Forest remnants have been defaunated (115). One of these remnants is the Tijuca National Park in Rio de Janeiro. During the 17th and 18th centuries, deforestation for agricultural purposes and hunting pressure led to severe losses of native fauna. Because the forest is completely surrounded by urban infrastructure, the affected animal species could not fully recover after the area was reforested in the 19th century (116), and dispersal of mammal species to other ecosystems is still inhibited.

The REFAUNA Project was established in 2012 to restore the mammal community via gradual reintroductions of species that have disappeared from the Atlantic Forest (116). Tijuca was considered suitable for first reintroductions because its relatively small size and urban location would allow for easy monitoring and control of the released animals (116). Researchers identified two native, locally extinct candidate species, the red-rumped agouti (Dasyprocta leporina) and the howler monkey (Alouatta guariba), both of which were expected to promote ecological interactions. Agoutis are important dispersers of large seeded plants (117) and increase seed survival by transporting them to locations with lower densities of conspecific tree species. Howler monkeys influence dung beetle abundance, and the decomposition of howler dung by the beetles can enhance nutrient cycling and soil fertilization (118).

Concomitant monitoring revealed that the presence of agoutis and howler monkeys enhanced ecological interactions in the park. Agoutis broadened their diet and improved the dispersal and germination success of several large-seeded plants. By interacting with the dung beetle community, howler monkeys promoted the dispersal of large seeds, with likely positive effects on forest regeneration (116) (Fig. 2C). Although Tijuca is Brazil’s most popular national park (119), there is little emotional connection between the park and people living in adjacent communities (120). To improve the linkage between the park and local communities, the park administration has installed a park council through which representatives of governmental institutions, NGOs, and the private sector aim to reach satisfactory management decisions for all stakeholders (121). A community-based, cooperative project has trained locals as tourist guides and offers tours through the park and a neighboring favela. Additionally, the cooperative runs a restaurant that offers local cuisine prepared with products growing in the forest and community gardens (122, 123).

Ecosystem and wildlife recovery in the Chernobyl exclusion zone, Belarus

The meltdown of the nuclear reactor in Chernobyl on 26 April 1986 resulted in massive contamination, especially in the immediate surrounding area (124–126). The evacuation of the entire local population within a 30-km exclusion zone around the reactor and the most strongly contaminated areas outside this zone resulted in the abandonment of ~1400 km² of agricultural land (40, 41). The breakdown of the Soviet Union, with widespread outmigration and an additional 36% of all farmland abandoned in Belarus and Ukraine, further lowered human pressure in the region surrounding the Chernobyl site (41).

Two years after the meltdown, the Belarusian part of the exclusion zone and adjacent areas were turned into the strictly protected 1300 km² Polesie State Radioecological Reserve. In 1993, the reserve was extended by 850 km², making it the largest nature reserve in Belarus (127). Management of the exclusion zone on both sides of the border has since followed a paradigm of minimal to no intervention. Targeted reintroductions of European bison (Bison bonasus) in the Polesie State Radioecological Reserve and of Przewalski’s horses (Equus ferus przewalskii) in the Ukrainian exclusion zone to restore trophic interactions in the Chernobyl area were exceptions to this passive approach. Recognizing the growing ecological and conservation value of the Chernobyl area, the Ukrainian government established the 2300-km² Chornobyl Radiation and Ecological Biosphere Reserve—an almost 5000 km² contiguous rewilding area in the heart of Eastern Europe—in 2016 (128). Management activities in the biosphere reserve aim to recover biodiversity and ecosystem resilience and include monitoring of the ecological, medical, and radiation status of the area, as well as educational activities (128).

The region now harbors the entire portfolio of extant European large carnivores[wolflynx([wolflynx([wolflynx([wolflynx(Lynx lynx), and brown bear], large herbivores[Europeanbisonwildhorsemoose([Europeanbisonwildhorsemoose([Europeanbisonwildhorsemoose([Europeanbisonwildhorsemoose(Alces alces), red deer, roe deer, and wild boar (Sus scrofa)], a rich mesopredator community[egEuropeanbadger([egEuropeanbadger([egEuropeanbadger([egEuropeanbadger(Meles meles), raccoon dog (Nyctereutes procyonoides), and red fox (Vulpes vulpes)], and key ecosystem engineers, such as the Eurasian beaver (Castor fiber). The Chernobyl exclusion zone is the only area where these species interact in sizable numbers in a large wilderness complex and can thus be considered one of the most iconic natural experiments on rewilding in recent history.