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.