empirical studies can be tested against a quantitative systems model to iteratively
improve restoration outcomes for a particular ecosystem.
A major complication for restoration is the incorporation of larger-scale ecological processes, such as hydrological regimes and climatic factors that may limit the
feasibility or trajectory of a desired outcome (Richardson et al. 2007). This may
entail relaxing restoration goals based on historically informed reference systems
(Prins et al. 2004; Aronson et al. 2017; Balaguer et al. 2014) to more flexible ones
based on the new combination of environmental factors (Hobbs et al. 2009). This
requires a detailed understanding of how local and regional processes influence
population and community dynamics in the target ecosystem (Holmes and Richardson 1999; Suding and Leger 2012).
23.2.3 Biological Invasions and Restoration Ecology
Studies have shown that invasions of alien plants cause substantial changes to
ecosystems by breaching biotic or abiotic thresholds to recovery (Suding and
Hobbs 2009; Gaertner et al. 2012b). Biological invasions can also be the driver of
ecosystem change during the restorative process (Norton 2009), resulting in altered
ecosystem composition and structure. This implies that removal of the invaders
alone may not lead to recovery, and indeed that such actions may divert successional
processes in unwanted directions that may require additional interventions (active
restoration measures) to fix. Despite the well-documented problem of impacts
caused by invasive alien plants (Vilà et al. 2011; Downey and Richardson 2016;
Le Maitre et al. 2011), a review of ecosystem restoration studies found that only 8%
had control of invasive alien species as their main objective (Gaertner et al. 2012a).
In those 8% of studies, the prevalent cause for degradation was invasive alien species
outcompeting and replacing native species, indicating that a biotic threshold to
recovery had been crossed. Measures other than invasive alien control were
implemented in 65% of those cases (Gaertner et al. 2012a).
Invasive aliens with potential for causing regime shifts should be prioritised for
control, as these species can modify ecosystems to their own benefit and suppress
native species through reinforcing feedback processes that present barriers to recovery (Gaertner et al. 2014). Feedbacks likely to result in regime shifts were related to
processes associated with seed banks, fire and nutrient cycling (Gaertner et al. 2014).
A recent review of soil legacy effects resulting from invasion by alien N 2 -fixing
woody species identified several potential barriers to restoration following alien
control (Nsikani et al. 2018). Biotic barriers included altered soil microbial communities, depleted native seed banks, secondary invasions of alien species, and weedy
native species dominance. Altered soil properties, especially those involving N, C
and moisture, potentially thwart restoration in some ecosystems.
These barriers to restoration point to the need for active restoration to facilitate
recovery. There are numerous potential interventions that can be applied on their
own or in combination. However, for most ecosystems, more experimental work is
23 Biological Invasions and Ecological Restoration in South Africa
669
improve restoration outcomes for a particular ecosystem.
A major complication for restoration is the incorporation of larger-scale ecological processes, such as hydrological regimes and climatic factors that may limit the
feasibility or trajectory of a desired outcome (Richardson et al. 2007). This may
entail relaxing restoration goals based on historically informed reference systems
(Prins et al. 2004; Aronson et al. 2017; Balaguer et al. 2014) to more flexible ones
based on the new combination of environmental factors (Hobbs et al. 2009). This
requires a detailed understanding of how local and regional processes influence
population and community dynamics in the target ecosystem (Holmes and Richardson 1999; Suding and Leger 2012).
23.2.3 Biological Invasions and Restoration Ecology
Studies have shown that invasions of alien plants cause substantial changes to
ecosystems by breaching biotic or abiotic thresholds to recovery (Suding and
Hobbs 2009; Gaertner et al. 2012b). Biological invasions can also be the driver of
ecosystem change during the restorative process (Norton 2009), resulting in altered
ecosystem composition and structure. This implies that removal of the invaders
alone may not lead to recovery, and indeed that such actions may divert successional
processes in unwanted directions that may require additional interventions (active
restoration measures) to fix. Despite the well-documented problem of impacts
caused by invasive alien plants (Vilà et al. 2011; Downey and Richardson 2016;
Le Maitre et al. 2011), a review of ecosystem restoration studies found that only 8%
had control of invasive alien species as their main objective (Gaertner et al. 2012a).
In those 8% of studies, the prevalent cause for degradation was invasive alien species
outcompeting and replacing native species, indicating that a biotic threshold to
recovery had been crossed. Measures other than invasive alien control were
implemented in 65% of those cases (Gaertner et al. 2012a).
Invasive aliens with potential for causing regime shifts should be prioritised for
control, as these species can modify ecosystems to their own benefit and suppress
native species through reinforcing feedback processes that present barriers to recovery (Gaertner et al. 2014). Feedbacks likely to result in regime shifts were related to
processes associated with seed banks, fire and nutrient cycling (Gaertner et al. 2014).
A recent review of soil legacy effects resulting from invasion by alien N 2 -fixing
woody species identified several potential barriers to restoration following alien
control (Nsikani et al. 2018). Biotic barriers included altered soil microbial communities, depleted native seed banks, secondary invasions of alien species, and weedy
native species dominance. Altered soil properties, especially those involving N, C
and moisture, potentially thwart restoration in some ecosystems.
These barriers to restoration point to the need for active restoration to facilitate
recovery. There are numerous potential interventions that can be applied on their
own or in combination. However, for most ecosystems, more experimental work is
23 Biological Invasions and Ecological Restoration in South Africa
669