1. Foreword
Toxic weeds refer to plants of secondary compounds which are toxic to livestock, wild herbivore, and human (James et al. 2005). Some toxic weeds accumulate toxins at high levels whose concentration can be influence by the inhabiting conditions (Zhao et al. 2013). The toxic principles mainly include toxic proteins, terpenoids, glycosides, alkaloids, polyphenols and photosensitive substances (Zhao et al.2013), which can be extracted and used as pesticides with remarkable pesticidal and antimicrobial activities (Zhang et al. 2011; Gaoet al. 2013; Chen et al. 2017). As important indicators of grassland degradation, toxic weeds have become increasingly global in their distribution in recent decades due to widespread grassland degradation (Sun et al. 2009; Zhao et al. 2010; Zhaoet al. 2013; Wu et al. 2016). Furthermore, a longer growing season and warming induced by climate change will intensify the increases in the occurrence and production of toxic weeds (Klein, Harte & Zhao 2007; Ziska, Epstein & Schlesinger 2009).
Statistically, there are approximately 1300 toxic species covering approximately 33.3 m ha in China’s natural grasslands (Shi & Wang 2004; Zhao et al. 2010). They have been traditionally thought that the wide distribution of toxic weeds leads to pasture degeneration and thereby reductions of grassland forage availability (Zhao et al.2013; Wu et al. 2016). Additionally, poisonous weeds not only damage livestock breeding (Panter et al. 1999) but also poison—or even kill—domestic animals if they are ingested by accident or if the pollen is inadvertently inhaled (Braun et al.2003; Zhao et al. 2013), potentially resulting in substantial economic losses and hindering the sustainable development of the livestock industry. It is estimated that toxic-weed poisoning results in direct or indirect economic losses of billions of CNY in China each year (Shi 2001). The reduced grazing capacity and economic losses induced by toxic weed lead to lower resilience and increase in vulnerability of livelihoods that depend on livestock. Therefore, numerous approaches have been employed to control the spread of toxic weeds (Lu et al. 2012; Stokstad 2013). However, most techniques have done little to eradicate established plants, and some approaches may even have negative environmental effects (Stokstad 2013; Boutin et al. 2014).
In fact, the spread of toxic weeds is not the reason for grassland degradation but a consequence of their strong adaptive capacity. Toxic weeds often have long and well-developed root systems to facilitate the capture of water and nutrients from deep soil profiles (Sun et al. 2014), inhibit the growth of co-occurring plants via allelopathy (Yan et al. 2016), form intraspecific aggregations that enhance their ability to compete with heterospecific competitors (Ren, Zhao & An 2015), and are not exposed to selection by livestock and small rodents (Zhao et al. 2013). From an ecological perspective, the colonization of toxic weeds can be more beneficial than harmful by promoting the process of succession in degraded grasslands by excluding excessive disturbance from livestock (Cheng et al. 2014b). An improved understanding of the potential role of toxic weeds in grassland conservation will challenge the traditional view that toxic weeds are uniformly deleterious and will enable pasture managers and policy makers to modify and design more flexible strategies for addressing global change and promoting sustainability. Here, we conduct a review of the literature to detail the fitness and potential effects of toxic weeds. These findings provide novel insight into the adaptive management of weed-dominated grasslands.