Grazing reduces the temporal stability of temperate grasslands in northern China
Introduction
Temporal stability, defined as the inverse of variability over time (Pimm, 1984), is a basic attribute of any ecological system. A stable ecosystem is key to providing sustainable ecological goods and services to humanity (Tilman et al., 2014), so understanding the determinants of ecological stability has received much attention in the context of intensive anthropogenic disturbances. The temporal stability of community biomass production has been linked to species diversity (Tilman et al., 2006; Hautier et al., 2015), the degree of species asynchrony (Hautier et al., 2014) and the stability properties of dominant species (Hillebrand et al., 2008). First, increased species diversity may enhance community temporal stability by increasing species compensatory effects (Gonzalez and Loreau, 2009), over-yielding effects (i.e. an increase in ecosystem functions with increasing diversity) (Hector et al., 2010) and the portfolio effect (Thibaut and Connolly, 2013). Second, variations in the degree of species asynchrony are likely to regulate the responses of community stability to environmental changes (Grman et al., 2010). Finally, the population stability of dominant species could lead to changes in temporal stability at the community level, in particular when a community is dominated by a small number of species (Smith and Knapp, 2003; Hillebrand et al., 2008; Sasaki and Lauenroth, 2011). The relative contribution of these drivers may vary between ecosystems.
Inner Mongolian temperate grassland in northern China, including meadow steppe, typical steppe and desert steppe, is one of the most important components of the Eurasian steppes which have always been exposed to periodical or stochastic disturbances (e.g. grazing and fire) and thus are rather dynamic ecosystems (Nandintsetseg et al., 2018). Temperate grassland is typical in arid and semi-arid regions of the Eurasian steppe and stretches across the Eurasian continent. Grasslands not only provide essential goods and services for humans, such as water, medicinal plants and cultural recreation, but also maintain ecosystem services, such as hydrological functions and carbon sequestration, which are reported to be sensitive to environmental disturbances (Kang et al., 2007). However, there are still disputes about the effects of grazing. Household land use rights that are the norm in Inner Mongolia reduce flexibility and force herders to adopt unsustainable grazing practices on their limited land (Liang et al., 2012; Conte and Tilt, 2014). However, complete grazing exclusion is a questionable recommendation and may be seen as a form of “disturbance” in view of the evolutionary grazing history of steppes in Central Asia (Wesche et al., 2016). In Palaearctic grasslands, large areas of former pastures and meadows have been abandoned, leading to succession towards secondary scrublands or forest and the encroachment of competitor grass species, a process associated with a decrease in biodiversity (Valkó et al., 2018). Grazing has been demonstrated to influence the structure and dynamics of vegetation communities (Milchunas and Lauenroth, 1993) and species interactions (Riginos and Young, 2007) that may translate into changes in community stability. Grazing is expected to have positive or negative effects on diversity (Cingolani et al., 2005), depending on resource availability (Proulx and Mazumder, 1998) and grazing intensity, and consequently could lead to shifts in stability. In addition, grazing could promote species level responses that are asynchronous with increasing community stability (Sasaki and Lauenroth, 2011; Wilcox et al., 2017). The effect of grazing on stability can also depend on grazing intensity (Li et al., 2017). Some studies have found that grazing intensity has a significant effect only on species richness (Tóth et al., 2018). Heavy grazing may support ruderals and cause a decline in steppe specialists (Deák et al., 2017). Nico et al. (2016) found that land-use intensity imperils plant and animal community stability through changes in asynchrony rather than diversity in forests and grasslands. Nevertheless, there is a lack of data on whether and how different grazing intensities affect community stability in different types of temperate grasslands. Studies manipulating grazing intensity are vital for assessing grazing effects on community biomass stability and predicting the dynamics of ecosystems under environmental change.
Grazing is the main land-use of natural grasslands across the world (Squires et al., 2018). Nearly a quarter of the global land mass is now under livestock grazing (Asner et al., 2004). Over the past three centuries, the area of land dedicated to grazing has reached 3451 million hectares, a six-fold increase, due to population growth and colonization (Goldewijk, 2001, 2005). With more than 800,000 km2 of steppes and drylands, Mongolia hosts one of the world’s largest intact rangeland systems (Batsaikhan et al., 2014). Here, we report on a 5-year grazing investigation conducted to explore the temporal stability of plant biomass production in response to different grazing intensities in the world’s largest remaining temperate grassland in northern China. Structural equation modelling (SEM), an advanced and robust multivariate statistical method, allows hypothesis testing of complex path-relation networks (Grace et al., 2007), which has been widely used to understand the response of grassland ecosystems to environmental changes (e.g. Xu et al., 2015). In the present study, SEM was used to elucidate complex relationships in the grazing-plant-stability system. We aimed to address two questions: 1) How does grazing intensity affect community stability in meadow steppe, typical steppe and desert steppe? 2) What controls the response of the temporal stability of plant biomass production to grazing intensity? We hypothesized that grazing would influence community stability by altering species richness, species asynchrony and population stability (Ren et al., 2018).
Section snippets
Study sites
We conducted field investigations in three temperate grasslands in Northern China, including a meadow steppe, a typical steppe and a desert steppe (Fig. 1). The meadow steppe, the typical steppe and the desert steppe are located in Xiwuzhumuqin Banner (43°40′N, 117°45′E; 1090 m), Keshikten Banner (43°26′N, 116°33′E; 1370 m) and Siziwang Banner (41°47′N, 111°53′E; 1450 m) in Inner Mongolia, respectively. There are clear differences in climatic variables between these regions, with mean annual
Effects of grazing intensity on plant community properties
Over the 5-year investigation period, heavy grazing intensity reduced plant community biomass, with a decrease compared to light grazing intensity of 108 g m−2 (57%) in meadow steppe (P < 0.01; Tables S1; Fig. 2a). The reduction in community biomass induced by heavy grazing was not significant in the typical steppe or desert steppe (P < 0.01; Fig. 2b, c). Compared to light grazing, species dominance increased by 56% under moderate grazing and heavy grazing in the meadow steppe (P < 0.001; Fig. 2
Discussion
We set out to explore the responses of community stability to grazing intensity through a 5-year grazing investigation in three temperate grasslands. Three main results emerged: (1) grazing reduced the temporal stability of all types of grassland; (2) decreased stability was associated with decreased species asynchrony; and (3) grazing reduced community stability mainly through altering species asynchrony.
Research findings on the effects of grazing on community stability have been inconsistent.
Conclusion
This study, to our knowledge, is the first to report the negative effect of grazing on the temporal stability of plant community biomass production across three types of grasslands. We found that grazing reduced community temporal stability mainly by altering species asynchrony. In addition, the responses of functional group stability to grazing may be dependent on ecosystem. These findings suggest that grazing practices can alter the stability properties of communities, and point to the
Acknowledgements
We thank the anonymous reviewers for their comments. This research was financially supported by the National Key Project (2016YFC0500504), the National Natural Science Foundation of China (31770500,31760146, 41877343), the Scientific Research Foundation for Advanced Talents by Inner Mongolia Agricultural University (NDGCC2016-19), and the Innovative Team of Grassland Resources of the Ministry of Education of China (IRT_17R59).
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