Principle 2 Restoration inputs will be dictated by level of resilience and degradation
All species (and ecosystems) possess an evolved but variable level of resilience: that is, a capacity to recover naturally from external stresses or shocks as long as those stresses are similar in type and degree to those previously experienced during the evolution of the species. This means that where human-induced impacts are low (or where sufficient time frames and nearby populations exist for effective recolonisation) recovery may be able to occur without assistance, but in sites of somewhat higher impact, at least some intervention is likely to be needed to initiate recovery. Where impacts are substantially higher or sufficient recovery time or populations are not available, correspondingly higher levels of restoration inputs and interventions are likely to be needed (see Figure 1). These may include remediation of the physical and chemical properties of the site, supplementing populations or reintroducing missing species or ecological processes. At extremely damaged sites, intransigent barriers to recovery may occur, in which case adaptive management and/or active research will be needed to identify specific solutions for restoration.
Skillful assessment of capacity for natural recovery should be done prior to prescribing whether regeneration-based or reconstruction-based approaches are needed (Box 2). This is essential to optimise success but is also important to assist prioritisation. That is, variation in the resilience of sites (and the higher cost of assisting recovery where the potential is lower) highlights the strategic advantage that can be gained by investing scarce resources into areas where resilience and potential for connectivity is higher.
Figure 1 Conceptual model of ecosystem degradation and restoration. (Adapted from Keenleyside et al 2012, after Whisenant 1999, and Hobbs & Harris 2001). The troughs in the diagram represent basins of stability in which an ecosystem can remain in a steady state prior to being shifted by a restoration or a degradation event past a threshold (represented by peaks in the diagram) towards a higher functioning state or a lower functioning state. [note: Not all sites in need of physical/chemical amendment depend upon reintroduction for the return of biota—e.g. if colonisation potential in that ecosystem is high.]
Box 2 Identifying the appropriate ecological restoration approach
Correctly assessing the capacity of various parts of a site to recover facilitates the selection of appropriate approaches and treatments—avoiding inefficient use of natural resources or restoration inputs. A useful initial rule of thumb is to identify any potential for harnessing the natural regeneration capacity of plants, animals or other biota and to use one of the ‘regeneration’ approaches outlined below where such potential exists. Introductions can then be focused on areas (or for species) where natural or assisted recovery is low or not possible.
Three approaches are usually used alone or combined if appropriate. All will require ongoing adaptive management until recovery is secured.
- Natural regeneration approach. Where damage is relatively low, pre-existing biota should be able to recover after cessation of the degrading practices. Examples of degrading practices include removal of native vegetation, over-grazing, over- fishing, restriction of water flows or inappropriate fire regimes etc. Animal species may be able to migrate back to the site if connectivity is in place. Plant species may recover through resprouting or germination from remnant soil seed banks or seeds that naturally disperse from nearby sites. Examples of natural regeneration.
- Assisted regeneration approach. Recovery at sites of intermediate (or even high) degradation need both the removal of causes of degradation and further active interventions to correct biotic or abiotic damage. Examples of biotic interventions include controlling invasive plants and animals and reintroducing dispersal attracting species. Lower level abiotic interventions include e.g. reinstating environmental flows and fish passage (in aquatic sites), and (in terrestrial sites) applying disturbances such as fire to break seed dormancy or installing habitat features such as hollow logs, rocks, woody debris piles and perch trees. Higher level abiotic interventions include remediating contamination or substrate chemistry, reshaping watercourses and landforms and building habitat features such as shell reefs. Examples of assisted regeneration.
- Reconstruction approach. Where damage is high, not only may all causes of degradation need to be removed or reversed and all biotic and abiotic damage corrected to suit the identified local indigenous ecosystem, but also all or a major proportion of its desirable biota need to be reintroduced. Examples of reconstruction.
Combined approaches are sometimes warranted. Varying responses by individual species to the same impact type can mean that some species drop out of an ecosystem earlier than others. In such cases less resilient species may require reintroduction in an area where a natural or assisted regeneration approach is generally applicable. In addition, plant species may require reintroduction, while all or some animal species may recover without the need for reintroduction (or vice versa). Reintroductions of plants or animals may also be justified where genetic diversity requires supplementation. Examples of fauna reintroduction.
A mosaic of approaches can be warranted where there is a diversity of different conditions across a site. That is, some parts of a site may require a regeneration approach, while others require a reconstruction approach or combinations as appropriate.
Responding to site conditions in this way will ensure optimal levels of similarity between the restoration outcome and conditions observed in the appropriately identified reference ecosystem