2.1 Active restoration of invertebrates
The return of invertebrates to revegetated areas is crucial for restoration goals as they are critical components of functioning ecosystems. Invertebrates may fail to actively recolonise due to inadequate habitat within the restoration site or characteristics that limit dispersal, such as a lack of wings (Haase and Pilotto, 2019). Regardless of the cause, proactive solutions are rarely implemented when monitoring reveals that important trophic groups have failed to recolonise revegetated sites. As such, there are few examples of active rewilding of invertebrates. The limited examples centre on rewilding earthworms (usually a single species) into degraded areas to improve decomposition rates (Snyder and Hendrix, 2008). Jouquet, Blanchart and Capowiez, (2014) reviewed the role of earthworms (and termites) in restoration so far and highlighted their limited use (only three field studies from 1999 - 2014) and how projects could be expanded e.g., using earthworms to alter reduce erosion. Further, although the practice of rewilding to improve ecosystem function and biodiversity may be informed by the much larger literature on invertebrate translocations, the intention of this practice is very different. Species translocations are usually conducted for species conservation and the functional role of the species is rarely considered, let alone assessed (Bellis et al. , 2019). Invertebrates targeted for translocations are often large, charismatic endangered species (such as Wetas and butterflies), with smaller, functionally important, taxa ignored. Similarly, there are emerging studies noting the effect of trophic rewilding on invertebrates and microbes which differ from the points raised in this review (Andriuzzi and Wall, 2018; van Klink and WallisDeVries, 2018; Gibb et al. , 2021). These studies examine the effect of rewilding other biota on invertebrate and microbe communities, rather than directly manipulating invertebrates and microbes via rewilding.
Entire communities of invertebrates have been reintroduced in multiple studies, although the practice is in its infancy. Topsoil inoculum contains whole communities of invertebrates (and microbes), potentially offering an avenue for community restoration. Several studies test the impacts of inoculating restoration sites with soil taken from target areas (Brown and Bedford, 1997; Wubs et al. , 2016; Lance et al. , 2019). Although the focus is often on changes in ecosystem function, the process of soil transplantation is in effect rewilding the whole soil invertebrate community. Yet, quantification of soil invertebrate responses to these treatments is rare, with only 29% of studies monitoring post-transplant changes in invertebrate communities (Table 1). Those studies that have quantified invertebrate responses have shown that transplants of whole soil communities can improve the biodiversity and density of mites and springtails (Wubs et al. , 2016; van der Bij et al. , 2018), soil nematode abundance (Benetková et al. , 2020), and soil macrofauna abundance (Moradiet al. , 2018).
The paucity of invertebrate rewilding projects demonstrates that there are significant knowledge gaps regarding if, how and when invertebrates should be used to restore ecosystem function. However, the diversity of ecosystem functions provisioned by invertebrates may be matched by an equally diverse range of situations which call for active rewilding efforts.
Table 1. We found 21 published examples where whole communities of invertebrates and microbes were reintroduced during restoration projects (ignoring mesocosm and glasshouse experiments). This is excluding single species reintroductions of earthworms and termites (which are reviewed in Jouquet, Blanchart and Capowiez, (2014)), single species reintroduction of Arbuscular Mycorrhizal Fungi (which are reviewed in Asmelash, Bekele and Birhane, (2016)), and single species reintroductions of cyanobacteria (which are reviewed in Rossi et al. , (2017)).