Introduction
Many grassland ecosystems are subjected to livestock grazing, and these
grazing systems provide up to a third of global food consumption and
support the livelihoods of more than one billion people in the
world(Suttie et al. 2005;
Kemp et al. 2013). Increasing
demand for livestock products has driven a global increase in grazing
(Tilman et al. 2011;
Kemp et al. 2013;
Fetzel et al. 2017), which is
threatening grassland biodiversity and ecosystem functioning
(Schönbach et al. 2011;
Eldridge et al. 2016). Yet,
understanding how grazing impacts biodiversity, ecosystem functioning,
and their relationship is still a major challenge in the context of
climate change at broader spatial and temporal scales.
Biodiversity at multiple trophic levels (i.e., plants, animals, or
microbes)and dimensions (i.e., taxonomic and functional diversity)
respond differently to grazing due to their differential biological
response mechanism(Fischer et al.2019; Wang & Tang 2019;
Filazzola et al. 2020), which
makes it difficult to estimate the overall grazing effects on
biodiversity and its relationship with ecosystem functioning. Therefore,
recent studies have acknowledged using multi-diversity by integrating
different dimensions of diversity across multiple trophic levels
simultaneously to reflect the overall effects of grazing on total local
biodiversity(Allan et al. 2014;
Wang et al. 2019). In addition,
biodiversity becomes more important in maintaining multiple ecosystem
functions (hereafter ecosystem multifunctionality, EMF)
(Byrnes et al. 2013;
Lefcheck et al. 2015;
Gamfeldt & Roger 2017) than individual
functions of productivity (Ma & Chen
2016; Zhang et al. 2018),
stability (Loreau & Mazancourt 2013;
Hautier et al. 2015), carbon
storage (Yang et al. 2019), or
nutrient availabilities(Komarov et
al. 2018; Wang et al. 2020)
based on a common positive biodiversity–ecosystem functioning (BEF)
relationship in natural grasslands (Tilmanet al. 2001; Zhang et al.2018). Furthermore, evidence is also mounting that the positive BEF
relationship can be facilitated by intermediate environmental stress
(Baert et al. 2018;
Guo et al. 2019). However, how
grazing affects multi-diversity, EMF, and their relationships in managed
ecosystems still remain poorly understood and a systematic assessment at
the global scale is lacking.
Grazing effects on biodiversity and ecosystem functioning are highly
dependent on intensity quantified by stocking rate or density
(Schönbach et al. 2011;
Herrero-Jáuregui & Oesterheld 2018), and
grazing duration (Porensky et al.2017). For example, previous studies suggested that species diversity
and aboveground net primary productivity (ANPP) is maximized at moderate
grazing intensity due to reduced competitive exclusion and increased
compensatory growth in the plant community compared with non-grazing
condition(McNaughton 1983;
Milchunas et al. 1988), whereas
high-intensity grazing with long duration decreases plant diversity and
community productivity by reducing abundance of annuals and several
weedy species in the plant community
(Porensky et al. 2017;
Zhang et al. 2018), as well as
soil water-holding capacity and nutrient availability
(Zhang et al. 2017;
Sitters et al. 2020). Moreover,
the magnitude and direction of grazing intensity effects on biodiversity
vary across the wide range of ecological
contexts(Olff & Ritchie 1998;
Gao & Carmel 2020). In relatively humid
and productive grasslands, moderate grazing may increase plant species
diversity through reducing the dominance of palatable species and
improving the availability of limited resources (i.e., light and
nutrients) to rare species colonization
(Olff & Ritchie 1998;
Gao & Carmel 2020). In relatively arid
and low productive grasslands, intermediate grazing may reduce species
diversity by increasing dominance of grazing-tolerant species and
aggravating resources stress (i.e., water and nutrients) to rare
palatable species (Herrero‐Jáuregui &
Oesterheld 2018; Zhang et al.2018). Collectively, these disparities of grazing effect not only
depend on grazing intensity, but also are driven by environmental
gradients.
In addition, different domestic herbivores (e.g., cattle and sheep) may
have differential impacts on biodiversity due to their different grazing
behavior (i.e., distinct diet of selectivity)
(Grant et al. 1985;
Dumont et al. 2011;
Bremm et al. 2012;
Tóth et al. 2016). For example,
compared to cattle, sheep are more likely to reduce taxonomic and
functional diversity by reducing the amount of forbs at light or
intermediate grazing intensities, but differences in selectivity between
sheep and cattle may decrease with increasing grazing
intensity(Tóth et al. 2016).
Overgrazing and the use of inappropriate livestock may lead to grassland
degradation and desertification (Tóthet al. 2016; Gao & Carmel 2020).
Therefore, understanding how grazing intensity interacts with livestock
type in changing grassland multi-diversity and EMF are essential for
determining sustainable grazing management strategies.
Here, we conducted a global meta-analysis from 138 grazing intensity
studies to evaluate grazing effects on the multiple biodiversity and
ecosystem functioning in grassland ecosystems worldwide. Together, 16
biodiversity metrics across 5 groups (i.e., plant species diversity,
functional group diversity, functional diversity, insect and soil
microbial diversity) and 12 individual ecosystem functions (i.e., above-
and belowground biomass, temporal stability of plant community, soil
nutrients and moisture, net ecosystem productivity, ecosystem
respiration and gross ecosystem productivity) were aggregated to
estimate the multi-diversity and EMF, respectively, by
weighted-averaging the natural log-transformed response ratio (lnRR) of
each variable. In particular, we examine how the relationship between
multi-diversity and EMF changes with intensifying grazing disturbance by
incorporating the regulation of livestock types and grazing duration
across a wide range of the aridity index.