owing to the high costs of procuring native propagules and/or the high likelihood of
failed introductions owing to the highly modified biophysical conditions. However,
if re-invasion is likely, there may be a case for revegetating modified areas with
alternative, non-invasive species to pre-empt re-invasion. In highly modified sites,
there may be examples where replacing the invasive alien with resilient native
species could improve landscape functions, such as reconnecting fragmented areas
of natural habitat. In less modified habitats where there is potential for autogenic
recovery through spontaneous regeneration, alien control may be sufficient to
re-instate a structurally and functionally representative ecosystem (Gaertner et al.
2011; Mostert et al. 2017). However, where key structural or functional guilds have
been severely depleted, active restoration through native propagule re-introduction,
and possibly other actions, may be required. Where the conservation status of an
ecosystem is a high priority, active restoration may be justified to further improve
community composition and restore viable populations of threatened species (Morgan 1999; Hitchcock et al. 2012). Ensuring that key ecosystem processes (e.g. fire
and herbivory) are initiated and maintained are important for optimising recovery.
23.3.2 Ecological Restoration Following Invasion: A
Conceptual Framework
Gaertner et al. (2012b) modified models for dryland ecosystem degradation and
repair (Milton and Siegfried 1994; Whisenant 1999, 2002) to develop a conceptual
framework for restoring ecosystems degraded by invasive alien plants. They
outlined a three-threshold conceptual model (Fig. 23.1) and linked the concept of
ecosystem resilience to degradation thresholds. Resilience in this context is the
ability of an ecosystem to spontaneously regenerate following invasive alien plant
controls. A threshold is the point beyond which the ecosystem cannot self-repair
following alien control alone, has lost resilience, and has formed an alternative state.
The first threshold along the invasion-degradation gradient is usually biotic, and is
indicative of a shift in structural biotic interactions within the invaded community for
which recovery requires manipulation of vegetation components. In ecosystems
where the biotic threshold has been reached, restoration interventions should include
the re-introduction of missing plant guilds in addition to invasive alien vegetation
control.
The second transitional threshold is reached after a longer duration of invasion
and is harder to reverse. It is controlled by abiotic limitations that result from
amplified biotic interactions and restoration requires manipulation of the physical
environment in addition to the biotic components. Where alien species incur legacy
effects such as N enrichment (Nsikani et al. 2018), this may be a barrier to restoration
and may require the physical removal of nutrient-rich biomass (Marchante et al.
2009), the manipulation of soil nutrients (Zink and Allen 1998; Török et al. 2000), or
the use of fire to volatilise excess nutrients. In many cases, biotic and abiotic
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failed introductions owing to the highly modified biophysical conditions. However,
if re-invasion is likely, there may be a case for revegetating modified areas with
alternative, non-invasive species to pre-empt re-invasion. In highly modified sites,
there may be examples where replacing the invasive alien with resilient native
species could improve landscape functions, such as reconnecting fragmented areas
of natural habitat. In less modified habitats where there is potential for autogenic
recovery through spontaneous regeneration, alien control may be sufficient to
re-instate a structurally and functionally representative ecosystem (Gaertner et al.
2011; Mostert et al. 2017). However, where key structural or functional guilds have
been severely depleted, active restoration through native propagule re-introduction,
and possibly other actions, may be required. Where the conservation status of an
ecosystem is a high priority, active restoration may be justified to further improve
community composition and restore viable populations of threatened species (Morgan 1999; Hitchcock et al. 2012). Ensuring that key ecosystem processes (e.g. fire
and herbivory) are initiated and maintained are important for optimising recovery.
23.3.2 Ecological Restoration Following Invasion: A
Conceptual Framework
Gaertner et al. (2012b) modified models for dryland ecosystem degradation and
repair (Milton and Siegfried 1994; Whisenant 1999, 2002) to develop a conceptual
framework for restoring ecosystems degraded by invasive alien plants. They
outlined a three-threshold conceptual model (Fig. 23.1) and linked the concept of
ecosystem resilience to degradation thresholds. Resilience in this context is the
ability of an ecosystem to spontaneously regenerate following invasive alien plant
controls. A threshold is the point beyond which the ecosystem cannot self-repair
following alien control alone, has lost resilience, and has formed an alternative state.
The first threshold along the invasion-degradation gradient is usually biotic, and is
indicative of a shift in structural biotic interactions within the invaded community for
which recovery requires manipulation of vegetation components. In ecosystems
where the biotic threshold has been reached, restoration interventions should include
the re-introduction of missing plant guilds in addition to invasive alien vegetation
control.
The second transitional threshold is reached after a longer duration of invasion
and is harder to reverse. It is controlled by abiotic limitations that result from
amplified biotic interactions and restoration requires manipulation of the physical
environment in addition to the biotic components. Where alien species incur legacy
effects such as N enrichment (Nsikani et al. 2018), this may be a barrier to restoration
and may require the physical removal of nutrient-rich biomass (Marchante et al.
2009), the manipulation of soil nutrients (Zink and Allen 1998; Török et al. 2000), or
the use of fire to volatilise excess nutrients. In many cases, biotic and abiotic
672
P. M. Holmes et al.