Background
Due to the interference of natural and human factors, the global environment is undergoing changes (Scales, 2005). Understanding how these current and future environmental changes influence plant diversity and community composition is critically important (Moritz and Agudo, 2013; Zhang et al., 2019). There are two common views on this topic. The first is that biodiversity promotes the stability of ecosystems (Cadotte et al., 2012; Weigelt et al., 2008), while the second view generally accepts that the stability of plant communities depends more on changes in inter-specific relationships, and this change in biodiversity depends on environmental changes (Wayne Polley et al., 2007).
Inter-specific relationships have been widely studied since the 1990s (Aiba et al., 2012; Perkins and Wilson, 2005). Inter-specific relationship indicates the nature of mutual attraction or exclusion between species. Species that are interdependent, competitive and co-evolutionary, and ultimately make the community relatively more stable (Greig-Smith, 1983). Due to external environmental disturbances such as habitat conditions, spatial scales, and plant traits, including grasslands, deserts, and forests, plant inter-specific relationships exhibit different ecological adaptations. The studies of Callaway and Armas showed that the interaction of plants affects population dynamics (Armas and Pugnaire, 2005; Callaway and Walker, 1997). Dominant species control most of the resources. When an ecosystem is disturbed, dominant species will provide short-term stability to the ecosystem (Avolio et al., 2019; Geider et al., 2001). Therefore, if the dominant species are resistant to interference, their communities would have higher resistance (Hillebrand et al., 2008). The stability of plant communities is the manifestation of the interaction between populations in competition or symbiosis, so as to achieve dynamic equilibrium (Levine and HilleRisLambers, 2009). Some studies suggested that the temporal stability of the community increases with the increase of inter-specific competition (Tilman, 1999). In our system, the introduction of an external pressure (grazing) led to extremely unbalanced population development, enhanced dominant position of major species, reduced species diversity, and simplified community composition and function (Armas et al., 2011). But studies have also shown that inter-specific competition makes communities more unstable (Loreau and De Mazancourt, 2013).
Grazing is one example of a selective pressure. Generally, the competitiveness of grassland plants is affected by livestock trampling and selective feeding. Competition among plants heavily grazed by livestock becomes weak as there is increasing top-down pressure from the grazing ‘predator’. When grazing intensity remains constant, competition between plants will eventually stabilize at a specific level (Muller-Scharer, 1991). Studies have shown that grazing leads to changes in relationships between populations, mainly due to changes in the ability of plants to intercept light (Ford and Diggle, 1981; Hay and Hunt, 1989). Many studies have shown that grazing can reduce the competition between plants, and the importance of competition will decrease (McCanny et al., 1990). Other work suggests that despite grazing altering the outcome of competition , it does not influence the importance of competition (Tilman, 2020). Taylor et al. indicated that high-intensity grazing disturbance could change the competition and coexistence among species (Taylor et al., 1997). Therefore, it is of great significance to explore the effect of grazing intensity on the inter-specific relationship of plant populations (Brooker et al., 2008; Yin et al., 2019).
Due to the harsh environment of arid climate and poor soil, the composition and function of desert grassland ecosystem are relatively fragile (Briske et al., 2015). Stipa breviflora andCleistogenes songorica are perennial clumped grasses and dominant species in desert steppe, with the advantages of cold resistance, drought resistance and trampling resistance. Despite observing that bothS. breviflora and C. songorica can break up under the disturbance of grazing (Lv et al., 2020), there remains little research on the inter-specific relationship of dominant grass populations in desert steppe. Therefore, we leveraged an existing long-term grazing experiment to answer three main questions. The first, how does grazing influence the density of each species? The second, how does the increase of grazing intensity change the inter-specific competition between these two species, and does this change vary depending on scale? Finally, we asked, as grazing intensity increases, how does the competitive ability of different dominant species change?