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 property (ball) can remain in a steady state prior to being shifted by a restoration activity past a barrier (peaks) towards a higher functioning state. Notes: (1) Cessation or mitigation of the original drivers of degradation at the site (e.g. land clearing, mining, grazing, etc.) is assumed to have already occurred as the first stage of the restoration; (ii) A site’s pre-restoration condition may start at any point along the trajectory; (iii) Biological barriers can be complex (not necessarily involving lack of propagules); (iv) In some cases overcoming one barrier type (e.g. physical-chemical barriers) may be sufficient alone; and, (v) small barriers of any type may occur in any sequence along the degradation/recovery trajectory.

Box 2 Identifying the appropriate ecological restoration approach

Correctly assessing the capacity of various parts of a site to naturally regenerate, with or without assistance, aids the selection of appropriate approaches and treatments, avoiding costly errors and increase the likelihood of attaining similarity between the restoration outcome and the appropriately identified reference ecosystem.

Regeneration potential can be hidden in dormant in-situ propagules or nearby sources. A useful initial rule of thumb is to undertake skilled assessment of the site and its surrounds to identify any scope for harnessing this potential through one of the regeneration approaches described in this box. Reintroductions of species (or reinforcements of populations) should only be carried out if and when potential for regeneration has been tested or is known to be not possible or sufficient (Figure 2).

Four approaches are typically applied, informed by ecological ground-based site assessment to determine level of in-situ resilience and colonisation potential (Figure 3, Table 1). All will require ongoing adaptive management until recovery is secured. A mosaic of approaches is common where there is a diversity of different conditions across a site. That is, some parts of a site may require one of the natural regeneration approaches, while others require a combined regeneration/reintroduction or a reconstruction approach.

  1. Spontaneous regeneration. Pre-existing biota are very often able to recover unaided after cessation of recent or relatively low levels of predation or competition from invasive species and cessation of degrading practices such as native vegetation clearing, overgrazing, over-harvesting, over-fishing, restriction of water flows or inappropriate fire regimes. Animal species may be able to migrate back to the site if connectivity is in place, while plant species may recover through resprouting or germination from remnant soil seed banks or seeds that naturally disperse from nearby sites. Examples of spontaneous regeneration.

  2. Facilitated regeneration (In Australia and elsewhere this is often referred to as 'assisted regeneration'). Where the removal of causes of degradation is insufficient to allow spontaneous regeneration, active interventions can often foster natural regeneration from in-situ propagules (e.g. soil seed banks or buds) or from recolonisation. Examples of lower-level abiotic interventions include removing competition from invasive species, 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 to attract fauna. Higher level abiotic interventions include remediating contamination or substrate chemistry, reshaping substrates, watercourses and landforms, and building habitat features such as shell reefs. Examples of facilitated regeneration.

  3. Combined regeneration/reintroduction. Varying responses by individual species to the same impact type can cause some plant or animal species drop out of an ecosystem earlier than others. This is often reflected in one or other vegetation stratum or faunal trophic level being impacted more than others, resulting in variable recovery. In such cases, after addressing the causal factors, and fostering as much natural regeneration as possible, the less resilient plants and/or animals may require reintroduction. This approach can also be appropriate where existing populations of plants and/or animals are low and supplementary genetic reintroductions may be needed (reinforcement). Care should be taken to ensure that decisions to reintroduce or reinforce are strictly evidence-based. Example of Combined regeneration/reintroduction.
  4. Reconstruction Where damage is very high, not only will all causes of degradation need to be addressed, and abiotic damage corrected, but also all (or at least a high proportion) of a site’s desirable biota will need to be reintroduced using current best practice methods. Works required to prepare substrates prior to reintroductions may be nil, low or high depending on the extent of damage at the site. As recovery after reconstruction commences, regeneration approaches can be applied superimposed, further improving outcomes. Examples of reconstruction.

Figure 2 It is useful to conceptually separate restoration approaches that enable natural regeneration from those that depend upon reintroduction of biota or reinforcement of populations. The arrows in this diagram indicate the appropriate sequence of approaches. This sequence not only ensures that reintroductions or reinforcements do not suppress regeneration but helps us to better design reintroductions to meet the needs of missing species and inadequate populations.

Figure 3 Schematic diagram of four restoration approaches that align with the degree of natural regeneration potential present at a degraded site at any scale from individual organism to whole of ecosystem.

Table 1 Various synonyms are used for restoration approaches (and methods) around the world, reflecting the development of similar ideas in different regions.

Term used in this document to describe the overall approach Synonyms for approach (or methods) used in Australia Synonyms for approach (or methods) used elsewhere
Natural regeneration Spontaneous regeneration. Natural regeneration. Unassisted regeneration
Facilitated regeneration Assisted regeneration. Bush regeneration. Rainforest regeneration Assisted regeneration, Assisted Natural Regeneration (ANR), Farmer Managed Natural, Regeneration (FMNR)
Combined regeneration/reintroduction Revegetation. Partial reconstruction. Faunal reintroduction Assisted Natural Regeneration (ANR). Rewilding. Framework Species Method. Applied Nucleation. Partial planting
Reconstruction Revegetation Full planting. Reforestation