Ecological Restoration Methods
Ecosystem degradation is the movement of a high functioning and healthy
ecosystem into an altered state whereby ecosystem functions are reduced
or lost as a result of a single or multiple disturbance event, usually
due to human actions (Suding & Hobbs, 2009). Healthy ecosystems are
often resilient enough to withstand moderate disturbances, and these
events can be important for maintain biodiversity and healthy
ecosystems. When the intensity and/or frequency of these disturbance
events change, an ecosystem can undergo hysteresis and pass through
irreversible degradation thresholds (Suding & Hobbs, 2009). An example
of this was observed in the grasslands of California where the combined
factors of weed invasion by Mediterranean species and altered grazing
regimes from the introduction of livestock as well as multiple severe
drought seasons resulted in the sever degradation of this ecosystem,
pushing it into an alternative stable state (Bartolome et al .,
2009). Different levels of degradation can influence the scale of
restoration required, depending on if the biotic factors (weed
invasion), abiotic factors (drought or altered fire regimes) or a
combination of both are affected.
Throughout the world, grasslands are currently subjected to multiple
degrading pressures. The most common of these pressures include; habitat
fragmentation, which has been observed in Australia (Prober et
al ., 2005), New Zealand (Standish et al ., 2009) and Europe
(Kiehl, 2010), altered grazing pressures in the USA (Martin et
al ., 2005; Bartolome et al ., 2009), Africa ( Sankaran &
Anderson, 2009) and Australia (Prober et al ., 2005),
desertification and bush encroachment have been observed in Africa
(Sankaran & Anderson, 2009), and climate change has been linked to the
gradual decline in grasslands throughout China (Zha & Gao, 2011). These
degrading pressures often act to promote the invasion of exotic plants,
which in turn create positive feedback loops that maintain the degraded
altered state. The alternative state theory explains how internal
disturbances and external shocks lead to positive feedback loops which
promote a stable degraded state (Chisholm et al ., 2015). In this
state, the degrading factors have altered the environment to promote
their own development, as observed in south east Australian grasslands
where annual exotic grasses outcompete the perennial native grass,
Themeda by developing new positive feedback loops that increases soil
nitrogen, and unless the available soil nitrogen levels are reduced, the
invasive species will maintain its competitive edge (Prober et
al ., 2009). Further, cross-facilitation of invasive plant feedback
loops has been identified by observing Agropyron cristatum in
northern USA, which alters native soil biota and thus, soil dynamics
(Jordan et al ., 2008). This reduces the competitiveness of the
native vegetation, promoting niche availability for the invasiveBromus inermis (Jordan et al ., 2008). It is in this
aggressive context that ecological restoration needs to reverse and
prevent further degradation, and then assist in the recovery of an
ecosystem. An important element in this task is to more clearly
understand how human behaviour influences different aspects of an
ecosystem (Schroder, 2009). Restoration models have been evolving for
several decades (Suding & Hobbs, 2009), and have proven to be
successful for prioritising large- and small-scale restoration projects.
The restoration of weed-dominated grasslands has received extensive both
popular and academic interests. Native tufted perennial grasses have
been described as keystone species as they resist weed invasion and
maintain ecosystem processes (Stromberg et al ., 2009; Proberet al ., 2005). Human induced disturbance has resulted to the
decline of these native grasses and thus non-native perennial grasses
have established, which significantly reduces the carrying capacity and
biodiversity. This has been observed in grasslands dominated byNassella trichotoma throughout south-eastern Australia (Campbell
& Nicol, 1999; Jacobs & Everitt, 2012), South Africa (Joubert, 1984)
and New Zealand (Lameroux et al ., 2011; Lusk et al .,
2017). Annual weeds are also highly problematic, and outcompete native
perennial grasses in their early life stages, and this is most prevalent
after disturbance (Musil et al ., 2005; Bartolome et al .,
2009; James et al ., 2011). Because invasive plants are generally
one of the main drivers for holding these ecosystems in degraded states,
the main focus of restoration efforts is on reducing the dominant weed
population and promoting competition from native species. Passive and
active restoration techniques have been used in diverse combinations to
achieve this outcome at varying levels of success throughout different
temperate grasslands (Table 2).