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.