2.3 Defensive strategies against animals
Toxic weeds are more resistant to grazing than grasses favoured by
herbivores, especially when available forage is limited (Ren et
al. 2016). Indeed, the niche breadth of toxic weeds is enhanced under
grazing (Mou et al. 2013), but they also exhibit superior
tolerance to physical breakdown because of their tenacious capacity to
regenerate once damaged (Li et al. 2008). Endophytic fungi can
protect plants from nematodes, insect pests and fungal pathogens
(Barillas et al. 2007; Jin et al. 2013). Furthermore, the
toxic components of these weeds are capable of poisoning or killing
small rodents and play a vital role in protecting toxic weeds from
animals and pathogens (Yan et al. 2015). The content of toxic
substances is highest in leaves, which is the vegetative organ most
likely to be consumed by herbivores. Furthermore, the content of toxic
substances dramatically increases in response to trampling and
consumption by livestock, which reduces the grazing intensity on toxic
weeds (Zheng & Hu 2006). The texture and colour of toxic weeds are also
striking (Fig. 2), which likely aid the identification, recognition and
classification of toxic weeds by animals as distasteful and indigestible
food items.
In response to long-term overgrazing and selective foraging, palatable
grasses become miniaturised, restricting their ability to utilize
natural resources. However, the ecological niche of toxic weeds also
widens through increases in the number of reproductive branches and
individual florets (Han et al. 2006). The grazing-induced
reduction of interspecific competition also contributes to the dominance
of toxic weeds in plant communities (Ren et al. 2016). In
addition to grazing duration, grazing intensity also affects the
distribution of toxic weeds, which often aggregate when grazing is
intense but are randomly distributed when grazing is especially intense
(Xing & Song 2002; Zhao et al. 2011). Thus, the intraspecific
relationship shifts from being mutualistic to competitive depending on
the intensity of grazing (Ren & Zhao 2013).
Reproductive strategies of toxic weeds with high survival rates include
floral traits, such as the brilliant terminal flower head (Fig. 2D),
which increases reproductive success by attracting pollinators (Jameset al. 2005; Zhang, Zhang & Sun 2011). Additionally, the seeds
are hard and durable and the seedlings are capable of exploiting grazed
areas with reduced competition from palatable grasses (Zhao et
al. 2013). The proportion of old plants in grasslands increases with
grazing intensity. In addition, old individuals have a higher fecundity
and produce larger quantities of seeds compared with younger plants
(Xing, Gou & Wei 2004). Thus, the breadth and density of the soil seed
bank increases as the intensity of grassland degradation rises,
enhancing the ability of the population to regenerate (Zhao & Zhang
2010; Du et al. 2015).