Figure 2. Conceptual framework of trajectories and restoration
options for degraded communities modified from Bradshaw, (1996) and
Hobbs and Norton, (1996) (A) . Each step of restoration is
associated with key questions practitioners need to answer to justify
active interventions or to evaluate restoration goals (B) .
Following these stages, the degraded community (S1) is replanted with
vegetation (S2). Fauna from the reference remnant community (S5) then
passively recolonise the new restoration habitat. Where biodiversity and
function are exceedingly slow or unlikely to reach remnant levels,
active intervention via rewilding (S3) may push the restoration
community closer to the reference community. Over time, biodiversity and
function in the restoration community may sit within the natural
variation (wavy lines) of the target reference community (S4).
Given a practitioner has chosen to rewild, the next critical question
is: when is it appropriate to intervene? One of the main advantages of
choosing a desired target community endpoint is that you can track the
trajectory of post-disturbance communities towards that of the target.
The difficulty with this approach is that realistic timelines need to be
set as to when these targets should be met. Some restored wetlands are
over 50 years old and have only recovered 53% of their biogeochemical
function as compared to remnant states (Moreno-Mateos et al. ,
2012). Ecosystem recovery can take much longer than 50 years (100s –
1000s years), however restoration projects are often under pressure to
demonstrate success through attainment of predefined goals (Wortley,
Hero and Howes, 2013). Whether or not a project is failing, or just
exceedingly slow to reach its goals, is a key question for restoration
ecologists. Addressing this question, Parkyn and Smith, (2011)
hypothesised when intervention is needed and how this interacts with the
dispersal capabilities of target fauna. They estimated that well
connected restored streams in New Zealand would often reach their
desired invertebrate community reference state in between 10 – 50 years
whereas poorly connected streams may never reach this state, regardless
of improving environmental conditions. The latter scenario might be
common in highly disturbed systems and may have stimulated emerging
studies that examine the feasibility of rewilding whole communities of
invertebrates into streams undergoing restoration (Haase and Pilotto,
2019; Dumeier, Lorenz and Kiel, 2020).