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).