1.1 Introduction
Globally, ecosystems have suffered extensive, largely negative change through human activity. In efforts to ameliorate our impact, we invest billions into ecological restoration each year to repair environments (Palmer, Zedler and Falk, 2016). Although there has been considerable discussion concerning the goals of such large monetary investments (including debate around embracing novel communities or aiming for a pre-disturbance remnant site (Hobbs, Higgs and Harris, 2009), see section 2.3), there are clear trends in how we have approached restoration so far. For example, although ecological restoration is the process of whole-ecosystem recovery, plant-only restoration dominates current practices (67% of projects) with only 24% of projects restoring both plants and animals simultaneously (McAlpineet al. , 2016) (9% of projects were animal-only restoration and this likely occurs when the plant community is already in good condition). This focus on plants suggests that ecosystems are expected to conform with the “Field of Dreams” paradigm that is embedded within restoration ecology (Palmer, Zedler and Falk, 1997; Prach et al. , 2019), i.e., if you build the habitat, other organisms will recolonise passively.
Plants also receive considerably more attention than non-plants in post-restoration monitoring: plants are surveyed in 54% of projects, whereas less visible groups such as invertebrates and microbes are monitored in only 32% of projects (27% and 5% respectively) (Kollmannet al. , 2016). Studies of passive recolonisation suggest that, although many species do recolonise without additional effort (Wodika, Klopf and Baer, 2014; Barber et al. , 2017), there are many factors that restrict fauna passively recolonising restoration sites, most notably the suitability of the restored habitat, the proximity to source populations, and dispersal limitations of some fauna (Parkyn and Smith, 2011; Kitto et al. , 2015). Dispersal limitations may be especially pertinent in reconstructing communities post-disturbance for smaller organisms such as invertebrates and microbes which are often dispersal constrained (Peay, Garbelotto and Bruns, 2010; Brederveldet al. , 2011; Jourdan et al. , 2019; Chen et al. , 2020).
Invertebrates and microbes are immensely important for restoration processes as they are key drivers of landscape-scale ecosystem functions such as nutrient cycling (Eisenhauer, 2019) and carbon sequestration (Anthony et al. , 2020). However, it is often assumed that they colonize independently following restoration of plant species (Strickland et al. , 2017). Although some invertebrates and microbes passively recolonise revegetated areas (Wodika, Klopf and Baer, 2014; Barber et al. , 2017), not all species can disperse to, colonize or establish successfully. Indeed, invertebrate and microbe communities in revegetated areas do not often become indistinguishable from those in remnant sites. Some macroinvertebrate communities in restoration sites are only ~35% similar to reference sites 20 years post-restoration, whereas the relative abundance of key bacterial Phyla were only half recovered as compared to nearby target sites 16 years post-restoration (Wodika and Baer, 2015; Stricklandet al. , 2017). Although some of this difference is likely related to the complex interaction between temporal changes in habitat suitability and the movement of metacommunities, a significant proportion may be due to dispersal limitations (Kitto et al. , 2015). For example, dispersal constraints have been suggested as a limiting factor in recolonisation of restored streams by macroinvertebrates (Brederveld et al. , 2011), restored meadows by snails (Knop, Herzog and Schmid, 2011), and restored arable land by microbes (Chen et al. , 2020).
Where passive recolonization fails, more proactive attempts to improve ecosystem function and biodiversity in revegetated areas include actively reintroducing or ‘rewilding’ missing biota. Rewilding is an increasingly popular conservation tool whereby select fauna are reintroduced to reinstate ecosystem function and restore degraded areas (Corlett, 2016). Although a relatively new term, the concept of rewilding can be seen as a subset of restoration (Hayward et al. , 2019). As such, rewilding is similarly biased towards highly visible groups (vertebrates in this instance), with comparatively few published examples of rewilding with less obvious groups such as invertebrates and microbes. In the related field of reintroduction biology, invertebrates make up as little as 3% of reintroduction studies, despite their roughly 95% contribution to species diversity (Bajomi et al. , 2010). Rewilding projects have therefore tended to ignore the “unseen majority”: functionally important yet overlooked groups such as invertebrates and microbes. Examples of invertebrate and microbial rewilding are however common in soil inoculation studies, which often rewild whole communities during soil transplants. There are significant knowledge gaps within these studies as few monitor changes in invertebrates and microbes post-soil inoculation. The effect of rewilding was thus difficult to quantify in these instances (see section 2.1 and Table 1). The potential for rewilding dispersal constrained invertebrates and microbes into areas they fail to recolonise naturally is under-researched outside of soil inoculation studies and is therefore rarely considered during restoration. However, rewilding may increase the likelihood of achieving restoration goals, particularly where the aim is to restore to a state of biodiversity and ecosystem function that is similar to the source area.