Mounting evidence has documented that biodiversity increases community stability in the fluctuating environments, but the underlying mechanisms are complex and remain controversial (Loreau and de Mazancourt 2013). Exploring the biotic drivers of temporal community stability across diverse communities will facilitate the knowledge of biodiversity-ecosystem function relationships. Therefore, unraveling the biotic mechanisms is critical for understanding the biodiversity effects on ecosystem functioning and the stable provisioning of goods and services (Craven et al. 2018; Valencia et al. 2020).
Plant communities are often considered to be stable when they have low temporal variability in primary production. So far, there are several biotic mechanisms to explain how ecosystem stability is maintained under perturbations (Goodman 1975; Grman et al. 2010; Loreau and de Mazancourt 2013; Tilman 1980). First, species diversity has been intensively found to keep communities stable through “portfolio effect” (Chen et al. 2016; Tilman et al. 2012; Wang et al. 2020). Second, species interaction creates a “compensatory effect” among asynchronous species to balance between the increase and decrease of productivity in different species (Grman et al. 2010; Loreau and de Mazancourt 2013; Song and Yu 2015). Third, dominant species can beget a “selection effect” to stabilize communities due to its prominent contribution to the productivity of the overall communities (Donohue et al. 2016; Grman et al. 2010). Species dominance may promote stability if the relative abundances of dominant species are constant (Grman et al. 2010; Sasaki and Lauenroth 2011) or asynchronous (Valencia et al. 2020). These mechanisms may act alone or operate in combination, depending on the environmental context (Grman et al. 2010). The major drivers of stability and their relative importance in respective mechanisms remain elusive in different natural ecosystems (Valencia et al. 2020; Wang et al. 2020). Previous studies have examined extensively the importance of the richness effects and species asynchrony to community stability. However, the effects of dominant species and dominance are less understood. Until more recently their stabilizing effects on stability are emphasized (Hillebrand et al. 2008; Polley et al. 2007; Sasaki and Lauenroth 2011; Valencia et al. 2020).
Dominant species may strongly influence biotic conditions and interact with other species, and thus are important drivers of community dynamics and ecosystem functioning (Grime 1998). Dominant species may affect community stability in at least two pathways (Song et al. 2019; Wayne Polley et al. 2007). First, dominant species play a disproportionate effect than other species on the community stability through selection effect (Hillebrand et al. 2008; Loreau and de Mazancourt 2013). Second, dominant species may enhance the compensatory effect by promoting the asynchrony among dominant species (Valencia et al. 2020; Zhang et al. 2018). In fact, dominant species may be more valuable to community stability in communities with few species than with many species (Wayne Polley et al. 2007). The changes in the abundance of dominant species will influence dominance structure, i.e., dominance (or evenness) in the communities, which may differentially modify species interaction and dominance hierarchies, with uncertain consequences on community dynamics and stability (Hillebrand et al. 2008; Klanderud and Totland 2005; Sasaki and Lauenroth 2011). Given dramatic changes in the dominance structure of communities in different environment, however, few studies have addressed the effects of dominance hierarchies and on maintaining community stability in different communities despite dominance is a measure of biodiversity (Grman et al. 2010; Hillebrand et al. 2008).
Northern Tibetan alpine grasslands are characterized by low temperature and cryophilic vegetation which shifts successively from alpine meadows to meadow steppes, steppes and desert steppes along a precipitation gradient ranging from over 600 mm in the east to less than 100 mm in the west (Zong et al. 2019). These grasslands, with relatively poor species richness but diverse functional groups of sedges, grasses, forbs and legumes, are dominated by sedges in the meadows, forbs in meadows steppe, legumes in the steppes and some grasses in desert steppes (Wu et al. 2013; Zhao et al. 2017; Zong et al. 2019). Dominants often act as nursery (or foundation species) and play a key role in determining functional structure and promoting biodiversity in alpine ecosystems (Cavieres et al. 2014; Elumeeva et al. 2017). Species diversity declines throughout the precipitation gradient from east to west, yet species dominance is greatest in meadows and steppes at both ends (Wu et al. 2014; Zhang et al. 2020; Zong et al. 2019). As a result, the components of species diversity, dominance structure vary from different types of grasslands, which may contribute differently to the community stability. This combination offers a natural framework to evaluate the relative importance of species richness, temporal asynchrony, and dominance as biotic mechanisms that stabilize community stability in a precipitation gradient.
In this study, we aimed to: 1) evaluate the patterns of community stability and the components of plant diversity, i.e. species richness, evenness, dominance, and species asynchrony in different types of grasslands; and 2) unravel biotic mechanisms, and their importance in modulating the temporal stability of aboveground productivity in the overall alpine grasslands on the Northern Tibetan Plateau. We focused on investigating the roles of dominant species and dominance structure as the direct or indirect biotic drivers of community stability. We hypothesized that community stability was primarily determined by species asynchrony in the eastern alpine meadows with diverse species. Whereas, community stability was majorly shaped by the selection effect of the dominant species in the western desert steppes with poor species. In addition, dominant species might play a leading role in shaping community stability through dominance structure. To test the hypotheses, we measured aboveground biomass, species composition and abundance of each species over seven years (2014–2020) in the peak of growing season across the four types of grassland to unravel the biotic mechanisms underpinning diversity-stability relationships.