3.1 Future possibilities for invertebrate and microbial rewilding
Stepwise restoration of communities by adding individual species is becoming increasingly unattainable, unrealistic, and ineffective in our rapidly changing and dynamic world. This has no doubt influenced the trajectory of restoration and rewilding projects, which have increasingly focussed on reinstating ecosystem function and self-organising communities, rather than compiling set groups of species (Harris, 2014; Perino et al. , 2019). This changing paradigm suits the unique characteristics of invertebrates and microbes, further encouraging their use in future rewilding projects. For example, the astounding diversity of invertebrates and microbes, the lack of knowledge of their functional roles, and their high spatial turn-over rates means that in any given community we are often unsure of which species are functionally critical (Setälä, Berg and Jones, 2005). Thus, targeted rewilding of single species may not lead to desired changes in ecosystem function efficiency. However, invertebrates and microbes are miniscule and thus easily manipulated, meaning we can readily move whole communities, and the functions they provision, from one place to another (given the habitat is appropriate and enough species establish). This is already how a majority of invertebrate and microbial rewilding projects operate. For instance, soil inoculation is a common form of invertebrate and microbial rewilding which consists of moving soil from target sites (with invertebrate and microbe communities in situ ) into restoration sites (Wubs et al. , 2016). We term this practice “whole-of-community” rewilding and although it is evident within soil inoculation studies, it is highly under-researched outside of soil transplants and thus rarely considered during restoration (Table 1). Whole-of-community rewilding consists of transporting small subsets of whole habitats, complete with invertebrate and microbe communities, from desired sites into restoration areas. The desired sites are at the practitioner’s discretion; thus, they can tailor the constructed community based on whichever site they choose. However, a summary of whole-of-community rewilding for restoration purposes highlights that nearby remnant sites are most frequently chosen (86% of projects), which conforms to mainstream restoration paradigms (i.e., choosing a nearby “intact” site as the desired endpoint community) (Table 1) (Mcdonald et al. , 2016).
Even within the limited examples of whole-of-community rewilding, there are clear knowledge gaps and missed opportunities. For example, very few studies monitor invertebrate and microbe communities post-inoculation (Table 1), making it difficult to quantify the efficacy of whole-of-community rewilding and its effect on community dynamics. Post-reintroduction changes were only recorded in 24% of transplant projects for invertebrates and 29% of projects for microbes, highlighting that monitoring post-reintroduction is crucial for greater understanding of the successes and failures of this holistic form of rewilding. Changes in ecosystem function post-reintroduction were recorded more frequently (67% of projects), but it is difficult to link the effect of rewilded invertebrates and microbes to changes in function when they are not monitored. Further, invertebrates and microbes are ubiquitous, meaning there may be many more instances outside of those documented (Table 1) where whole-of-community rewilding may be applied. For example, litter communities are critical for efficient nutrient cycling and can be easily transported within their habitat (Silvaet al. , 2020). However, litter transplants with the purpose of improving decomposition rates during restoration have not been attempted before (Box 1).
Although the direct mechanisms by which whole-of-community rewilding improves ecosystem function is likely highly contextual, this practice can influence a broad range of functions, including; nutrient dynamics (Lance et al. , 2019), plant growth (Emam, 2016), and community trajectories (Wubs et al. , 2016). One possible link between ecosystem function and this rewilding practice is the associated rapid increase in biodiversity. This relationship is known as the Biodiversity – Ecosystem Function (BEF) hypothesis and posits that increases in biological diversity (number of species, genotype varieties etc.) will see similar increases in the efficiency of ecosystem functions (Srivastava and Vellend, 2005). Although debate surrounds the generality of patterns between biodiversity and ecosystem function (e.g., how the effect varies over spatial scales (Thompson et al. , 2018)) it may be of particular use to restorationists as post-disturbance communities are biologically depauperate and their diversity can be easily and directly manipulated through practices such as rewilding (Srivastava and Vellend, 2005).