3.2 Effects on co-occurring plants
It is commonly believed that toxic weeds have negative effects on the quantity of forage via allelopathy, thereby decreasing grassland productivity (Pan et al. 2015). However, toxic weeds actually provide biotic refuges and keep surrounding herbaceous species away from livestock in overgrazed grasslands (Fig. 4). The number of species and the coverage of neighbouring plants are noticeably higher in plots with toxic weeds than in those in open grasslands (Cheng et al.2014b). There are two principal means by which toxic weeds can facilitate the proliferation of neighbouring plants in overgrazed pastures. First, the toxic smell could repel livestock and thus reduce the ingestion and trampling of edible forage surrounded by toxic weeds (Oesterheld & Oyarzabal 2004). Second, toxic weeds alter the surrounding micro-environmental conditions. For example, toxic weeds can redistribute soil nutrients, form fertility islands (Sun et al.2009) and create a cool environment that promotes soil moisture retention via the height of the plant canopy (Rebollo, Milchunas & Chapman 2002). All of these micro-environmental changes provide better soil conditions and microclimates for plant growth. Additionally, the niche overlap between toxic weeds and fine herbage is smaller than that between toxic weeds and unpalatable weeds, reflecting the lower degree of competition between toxic weeds and edible forage (Ren, Zhao & An 2013).