Passive restoration
Passive ecological restoration involves removing human induced degrading pressures from a site with minimal remediation. In many cases, it is presumed that non-target species will expand without human intervention, however many passive restorations have observed weeds decline with sufficient native recruitment (Sinkins & Otfinowski, 2012; Valkoet al ., 2017). Notable vegetation shifts often occur within 10 years of rest (Smallbone, 2014), however, some may take several decades to reach similar species richness as the remnant sites, and even then, the composition of vegetation can significantly differ (van de Merwe & van Rooyen, 2011).
Successful passive restoration involves careful grazing management and the ability for target species to recruit and establish. Rapid recovery of a degraded Hungarian alkaline grassland was observed (within one year) in sites directly adjacent a natural grassland, and within six years, all sites where restored regardless of proximity to the remnant site (Valko et al ., 2017). In this example, the dispersal of native plant propagules was promoted by livestock roaming between natural and degraded sites (Valkko et al ., 2017). Grazing animals were also observed to provide an important service in maintaining species richness for highly productive Themeda grassland in south-east Australia (Schultz et al ., 2011). Grasslands often require disturbances such as grazing or fire to maintain species richness and grazing animals remove excess phytomass in order to generate niche space for rarer species.
While grazing plays an important role in maintaining highly productive grasslands, those suffering extensive degradation or of lower productivity often benefit from the complete removal of grazing livestock. Grazing exclusion is a cost-effective tool for passive restoration, particularly if native species are well represented. A long-term (20 and 30 years) grazing exclusion zone was developed in the steppe grasslands of China, which observed an increase in perennial grass cover, as well as higher density bud banks of these favourable grasses when compared to the grazing sites (Zhao et al ., 2019). The long-term (40 years) removal of cattle from northern fescue prairies in Canada was effective for reducing some invasive plants, but not others, including Poa pratensis , which in some areas occupied up to 90% of the canopy (Sinkins & Otfinowski, 2012). Generally, grassland species have short-lived seedbanks and if desirable species are rare, or the site is isolated from remnant patches, the seedbank will continue to diminish (Bossuyt & Hermy, 2003). Further, the recruitment of native species in degraded temperate grasslands is rare, as seedlings often fail to survive as a result of competitive weed interactions (Morgan, 2001; Lenz & Facelli, 2005). This demonstrates that isolated sites dominated by aggressive weeds may not be suitable for passive restoration. While passive restoration has proven successful under specific conditions, in sites where invasive plants have dominated for several decades and biotic and abiotic thresholds have been crossed, active intervention will be required.