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?