Results and Discussion
Our synthesis provides global-scale evidence that both multi-diversity
and ecosystem multifunctionality (EMF) are significantly altered by
intensified grazing disturbance in grasslands. However, multi-diversity
and EMF responded differently to grazing intensity. Specifically, light
and moderate grazing increased multi-diversity, but heavy grazing
reduced multi-diversity (Fig. 1). The similar results were found for the
grazing effects on individual biodiversity, where light and moderate
grazing intensity had slightly positive effects on diversity of plant
species and soil microbes, and heavy grazing decreased the diversity of
plant species and functional groups as well as functional diversity
(Fig.S3). This result is different from the previous meta-analysis that
included a much smaller sample size than ours and showed little effect
of grazing intensity on plant species
diversity(Herrero-Jáuregui & Oesterheld
2018).
The EMF decreased consistently with increasing grazing intensity, and
the reduction of EMF response under heavy grazing (-19.3%) was
significantly stronger than that under light (-5.8%) and moderate
grazing (-11.1%) (Figs. 1, 2). Moreover, increasing grazing intensity
had significant negative effects on individual ecosystem functions (Fig.
S3). Compared to no grazing, all grazing intensities significantly
reduced above- and belowground biomass, net ecosystem productivity
(NEP), ecosystem respiration (ER) and gross ecosystem productivity
(GEP). In addition, heavy grazing further decreased soil moisture,
organic carbon and total phosphorus content, but had a non-significant
effect on soil nitrogen content (Fig. S3). This is because overgrazing
decreases aboveground plant biomass, leading to less plant litters
inputs into the soil and thereby less carbon and phosphorus storage, but
the loss of nitrogen can be mitigated by increases in nitrogen inputs
through N-fixation of legumes and herbivore dung and
urine(He et al. 2011;
Martinsen et al. 2011;
Sitters et al. 2020).
The negative effect of grazing intensity on multi-diversity becomes
stronger over time (grazing duration) (Fig. 2). On the one hand,
long-term grazing disturbance may facilitate a shift in plant community
composition from weedy and annual plants to grazing-tolerant species,
thereby leading to a limited decline in plant species richness and
functional diversity (Marriott et
al. 2009; Lyseng et al. 2018;
Fischer et al. 2019). On the other
hand, the accumulation of herbivores trampling and removal of
aboveground biomass by grazing may reduce carbon and nitrogen inputs
through litters into the belowground processes, and promote the
evaporation of soil moisture by increasing bare land area, leading to
lower soil microbial activity and plant regrowth
rates(Liu et al. 2016;
Sitters et al. 2020). Furthermore,
even low-intensity grazing may also aggravate biodiversity loss and
grassland degradation with the accumulation of grazing years. This
result suggests that the magnitude of grazing effects estimated on
multi-biodiversity and EMF may be underestimated when only short
experimental durations are considered.
The shift in livestock type from small livestock (e.g., sheep and goat)
to large livestock (e.g., cattle and yak) had a significantly positive
effect on multi-diversity (Fig. 2). Moreover, a similar tendency towards
a positive effect was also found in the interaction between grazing
intensity and larger livestock type (Fig. 2). This is likely because of
the distinctive diet selectivity and grazing behavior of large and small
livestock, which leads to differences in vegetation structure and soil
properties (Tóth et al. 2016;
Wang et al. 2019;
Gao & Carmel 2020). Previous research
has found that cattle are likely to select the dominant perennial
grasses that are higher and have greater aboveground
biomass(P et al. 2014) and thereby
alleviate competitive exclusion by alleviating ground-level light
limitation(Hautier et al. 2009;
Borer et al. 2014), leading to
increased abundance of subordinate species and prevent rare species
extinctions(Olff & Ritchie 1998).
Compared with cattle grazing, sheep grazing is more likely to threaten
plant species and functional diversity by increasing extinction of rare
palatable species due to their greater selective preference for forbs
with high abundance in the short-grass community
(Tóth et al. 2016;
Zhang et al. 2018).
Additionally, increasing aridity index had a significantly negative
effect on the grazing-induced responses of multi-diversity and EMF (Fig.
2). This result suggests that increasing grazing intensity reduces the
biodiversity of arid grasslands more strongly than that of humid
grasslands, as reported previously for plant species diversity
(Herrero-Jáuregui & Oesterheld 2018;
Gao & Carmel 2020). Importantly, we
found a significant negative interaction effect between grazing
intensity and aridity on multi-diversity index (Fig. 2), indicating a
negative synergistic effect of grazing intensity and aridity. This may
be attributed to the idea that herbivores may further aggravate water
stress and limitation of nutrient resources of dry grasslands, causing
decreases of plant and soil microbial diversity
(Ren et al. 2018;
Gao & Carmel 2020;
Zhang et al. 2020). Therefore, our
results suggest that the predicted increasing drought events might
aggravate the negative effect of grazing disturbance in thefuture.
Compared with the single biodiversity-ecosystem function relationship,
there was a stronger positive relationship between multi-diversity and
EMF responses when integrated a great number of biodiversity groups and
functions (Fig. S10). Further, we found that the slope of the
multi-diversity and EMF relationship increased significantly by 66.6%
from light grazing to heavy grazing (P =0.005) (Fig. 3). The
underlying mechanism may be attributable to increased abiotic
facilitation under a certain extent of increased stress of disturbances,
as predicted by the “stress gradient
hypothesis”(Baert et al. 2018). On
the one hand, the environmental stress caused by increasing grazing
intensity may strengthen the positive BEF relationship by promoting the
dominance effects that high functional adaptive species increasingly
replace low functional vulnerable
species(Baert et al. 2018;
Guo et al. 2019). On the other
hand, although extremely high environmental stress was predicted to
weaken the BEF relationship by inhibiting the functioning of all
species(Baert et al. 2018), we did
not detect this under any grazing intensities, which may be due to that
the levels of heavy grazing in most studies are not sufficient to cause
extreme environmental stress. Overall, our results suggest that
increasing grazing intensity strengthens the dependence of EMF on
multi-diversity globally.
The structural equation modeling reveals that an increase in grazing
intensity decreased EMF not only directly, but also indirectly via
reducing multi-diversity (Fig. 4). Grazing duration had a direct
negative effect on the response ratio of EMF, and the changing livestock
type from small to large size mainly had an indirect positive effect on
EMF by increasing multi-diversity. In contrast, rising aridity index had
an indirect negative effect on the response ratio of EMF via decreasing
multi-diversity (Fig. 4). These findings established a unique and
critical role of multi-diversity in mediating EMF under the integrative
impacts of grazing intensity, duration, livestock type and climatic
factors at the global scale, which expands our understanding based on
the regulation of single-trophic biodiversity on ecosystem functions
under land use and environmental changes at a local scale, as shown by
previous studies(Allan et al. 2015;
Hautier et al. 2015).
In conclusion, this study provides new insights into the effects of
grazing intensity interacting with livestock type, grazing duration and
aridity on the multi-diversity, EMF, and their relationship. We found
that intensifying grazing disturbance strengthens the dependence of EMF
on multi-diversity, and reduces ecosystem functioning through
extensively decreasing biodiversity. Moreover, the negative grazing
effects are stronger in drier grasslands with smaller livestock and
longer grazing duration. The findings extend our current understanding
on the grazing management practices in promoting biodiversity
conservation and sustainability of ecosystem services
(Schönbach et al. 2011;
Kemp et al. 2013;
DeLonge & Basche 2018). We strongly
suggest to carefully modify the grazing duration and select proper
livestock types while optimizing the grazing intensity. Establishing
optimal adaptive management strategy based on biodiversity conservation
is critical for preventing grassland degradation and promoting
sustainable development of global pastoral areas, especially when
extreme drought events are predicted to occur more intensively and
frequently under global climate change.