2. Adaptation of toxic weeds
In addition to the effects of natural factors, such as soil physiochemical properties and topographical conditions (Hou et al. 2013; Li et al. 2013), toxic weeds are most commonly a product of overgrazing and grassland degeneration. The population gradually increases and becomes dominant in plant communities as grassland degradation and grazing intensity increases (Zhang, Yue & Qin 2004; Zhang et al. 2004; Li, Jia & Dong 2006; Wang et al.2016; Ricciardi et al. 2017). This pattern is likely mostly due to that toxic weeds have various adaptive strategies for environmental stress and anthropogenic disturbance including higher genetic variation, well-developed roots, allelopathy effect, and poisonous for herbivores.