3.3 Potential ecological roles in degraded grasslands
From a successional perspective, the spread of toxic weeds is a consequence of their high adaptability rather than a cause of grassland degeneration. As an important part of the grassland ecosystem, toxic weeds improve plant community structure in degraded pastures (Tan & Zhou 1995) and play a crucial role in preventing further desertification of degraded grasslands (Wang et al. 2016). Animals usually avoid poisonous toxic weeds, which inherently suppresses excessive disturbance by livestock when overgrazing occurs. The unfounded removal of toxic weeds might lead to ecosystem collapse (Fig. 5) because grazing pressure on pasture is greater without the protection that toxic grasses provide (Wang et al. 2014). This hypothesis is potentially consistent with previous studies that report that the degree of degradation of mowed grasslands was greater than that of grazed grasslands inhabited by toxic weeds (Wang & Gilbert 2007; Li et al. 2008).
Furthermore, the presence of toxic weeds provides an essential means by which the coverage of vegetation can be maintained and the ecological functions of degraded grassland can be preserved (Fig. 5), although these should be considered some of their “better-than-nothing” effects. Toxic weeds provide an important gene pool, and their invasion increases the diversity of insects and invertebrates, facilitating the maintenance of biodiversity (Sun et al. 2013). Consequently, degenerated grassland with toxic weeds do not require any special interventions aside from controlling grazing intensity or limiting the overgrowth of toxic weeds. In support of these effects, the occurrence of toxic weeds is inhibited by the absence of grazing (Ren et al.2016). The potential process and underlying mechanism are as follows: First, residual yak dung deposition accelerates the proportional increase in graminoids and promotes the transformation of grasslands to gramineous communities following the exclusion of grazing (Mou et al. 2013). Moreover, grasses will recolonise and regain prevalence due to the maintenance of local genetic variation and because they can regenerate rapidly through the production of a large number of seeds (Liu & Ma 2010; Cheng et al. 2014b). Finally, degraded grassland ecosystems will eventually be restored and become prosperous again following a long period of self-healing (Fig. 5).