Box 1: Rewilding litter invertebrates and microbes to improve nutrient cycling
Litter dwelling detritivore invertebrates and microbes are critical, yet overlooked, components of ecosystems (Bender, Wagg and van der Heijden, 2016; Eisenhauer, 2019). They support the breakdown of leaf litter, which turns organically bound nutrients into nutrients available for uptake by plants. Their full return to revegetated areas is therefore of paramount importance for revegetated plant communities and restoration. Litter invertebrate and microbe communities in revegetated areas sometimes track towards remnant communities (Waterhouse et al. , 2014; Wodika, Klopf and Baer, 2014), but this is not always the case (Wodika and Baer, 2015; Strickland et al. , 2017) as species can have limited dispersal abilities (Peay, Garbelotto and Bruns, 2010). Where they fail to recolonise following restoration, active rewilding may not only improve biodiversity but also the efficiency of litter breakdown and nutrient cycling.
Increasing revegetation of farmland opens many opportunities in which dispersal constraints of litter invertebrates and microbes may justify rewilding (Gibb, Durant and Cunningham, 2012). Revegetated areas often exist as “habitat islands” surrounded by intensively managed pastures or crops. Dispersal-constrained invertebrates and microbes can struggle to recolonise these habitat islands due to unfavourable microclimatic and biotic conditions of pastures (Strickland et al. , 2017; Pompermaier et al. , 2020). Revegetated areas may therefore never become indistinguishable from remnants in terms of species composition, which is critical in driving ecosystem functions such as decomposition (Schuldt et al. , 2018). Active translocations of litter communities from remnant sites into revegetated areas may boost leaf litter breakdown and nutrient cycling by increasing species diversity or introducing dispersal-limited species that are driving community differences.
Species interaction networks in litter communities are notoriously complex. It is therefore difficult to identify keystone drivers of nutrient cycling and litter breakdown. Indeed, efficient breakdown of litter at one stage is often dependent on functions performed by different taxa at previous stages (e.g., microbial conditioning makes leaf litter more palatable for invertebrates) (Peralta-Maraver et al. , 2019). We have limited understanding of the specifics of these inter-dependencies. Whole-of-community reintroductions may therefore be more appropriate to improve ecological function in revegetated areas and would entail transporting leaf litter habitat with its complete biota from remnant sites into revegetated areas (Figure 3). Timing and source of litter transplants is likely to be crucial as ecosystem functions vary spatiotemporally. For instance, litter mass loss and functional diversity of detritivores is reduced during drought conditions as litter invertebrates may enter diapause and move deeper into the litter layer, where they are less likely to be captured (Silva et al. , 2020). Litter transplants will therefore be more effective at the height of detritivore activity which is generally during cool and wet conditions.