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).