Science for a wilder Anthropocene: Synthesis and future directions for trophic rewilding research

Abstract

Trophic rewilding is an ecological restoration strategy that uses species introductions to restore top-down trophic interactions and associated trophic cascades to promote self-regulating biodiverse ecosystems. Given the importance of large animals in trophic cascades and their widespread losses and resulting trophic downgrading, it often focuses on restoring functional megafaunas. Trophic rewilding is increasingly being implemented for conservation, but remains controversial. Here, we provide a synthesis of its current scientific basis, highlighting trophic cascades as the key conceptual framework, discussing the main lessons learned from ongoing rewilding projects, systematically reviewing the current literature, and highlighting unintentional rewilding and spontaneous wildlife comebacks as underused sources of information. Together, these lines of evidence show that trophic cascades may be restored via species reintroductions and ecological replacements. It is clear, however, that megafauna effects may be affected by poorly understood trophic complexity effects and interactions with landscape settings, human activities, and other factors. Unfortunately, empirical research on trophic rewilding is still rare, fragmented, and geographically biased, with the literature dominated by essays and opinion pieces. We highlight the need for applied programs to include hypothesis testing and science-based monitoring, and outline priorities for future research, notably assessing the role of trophic complexity, interplay with landscape settings, land use, and climate change, as well as developing the global scope for rewilding and tools to optimize benefits and reduce human-wildlife conflicts. Finally, we recommend developing a decision framework for species selection, building on functional and phylogenetic information and with attention to the potential contribution from synthetic biology.

Keywords: conservation; megafauna; reintroduction; restoration; trophic cascades.

Fig. 1.

Current and estimated present-natural diversity patterns for (A and B) megaherbivores (≥1,000 kg), (C and D) large herbivores (45–999 kg), and (E and F) large carnivores (>21.5 kg). The term “present-natural” refers to the state that a phenomenon would be in today in the complete absence of human influence through time (111). For this mapping, omnivores were classified as carnivores when meat constitutes a major part of their diet and as herbivores otherwise.

Fig. 2.

Examples of reintroductions or extant functional counterparts to replace extinct species in ongoing or proposed rewilding projects on islands and continents: Island examples include (Bottom to Top): Snares Island snipe (Coenocorypha huegeli) replacing the South Island snipe (Coenocorypha iredalei) on Putauhinu Island, New Zealand; giant Aldabra tortoise (Aldabrachelys gigantea) replacing giant Cylindraspis tortoises on Rodrigues Island, Mauritius; African sulcata tortoises (Centrochelys sulcata) replacing a flightless bird, the moa-nalo (Chelychelynechen quassus) on Kaua’i, Hawai’i. Continental examples include (Bottom to Top): Wild boar (Sus scrofa), experimental reintroduction to a fenced rewilding site in the Scottish Highlands; European Bison (Bison bonasus), an increasing number of rewilding reintroductions of this species are being implemented across Europe, here in Vorup Enge, Denmark (all ongoing projects); Asian elephant (Elephas maximus) replacing straight-tusked elephant (Elephas antiquus) in Eskebjerg Vesterlyng, Denmark (3-d pilot experiment, 2008). Approximate times because local or regional extinction of the original taxa are given. Images courtesy of (Top Right) J. Kunstmann and (Bottom Left) C. Miskelly.

Fig. 3.

Characteristics of international scientific publications focused on trophic rewilding (n = 91) (see Methods for further details on the systematic review): (A) number published per year, (B) literature categories, (C) attitude toward rewilding using reintroduction of species extirpated <5,000 y, reintroduction of species extirpated >5,000 y ago, or ecological replacements, (D) geographical focus.

Fig. 4.

Characteristics of species mentioned for rewilding introductions in international scientific publications focused on trophic rewilding (n = 91) (see Methods for further details on the systematic review). (A) Organism group and body mass. (B) Introduction geography [range-restricted species (species extirpated locally, but still extant within the focal zoogeographic region, with reduced range), species extirpated <5,000 y ago (species completely extirpated from the focal region within the last 5,000 y, but still extant elsewhere), species extinct >5,000 y ago (species completely extirpated from the focal region more than 5,000 y ago, but still extant elsewhere), ecological replacement for a globally extinct species, novel function (introduction to achieve novel ecological function without a past analog)], and body mass.