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.