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).