Discussion
Despite the productivity of the native grassland communities tended to
be lower than the baseline during dry events and higher than the
baseline during wet events (Figures S3-5), our results showed that the
effects of biodiversity on both resistance and resilience depend on the
community functional structure, on the biodiversity component evaluated
and the direction and intensity of anomalous climatic events.
Although the effects of biodiversity on the ecosystem resistance were
mostly positive, they were closely related to the community type defined
in terms of functional traits, irrespective of the direction or
intensity of climatic events. To examine this pattern, we grouped the
studied grassland communities by their functional relatedness (Figure 2)
based on traits that are relevant for biomass production and water and
nutrient balance (Lundgren et al. 2014; Engel 2017; Bruelheideet al. 2018; Testolin et al. 2021). Hence, along the first
axis (Axis I; Figure 2), positive scores can be interpreted as
communities dominated by acquisitive species (high SLA, Leaf N, C3), and
therefore, it is expected that they would be predominantly drought
sensitive (Griffin-Nolan et al. 2018). Along the same first axis,
communities with negative scores indicate dominance by conservative
species (high LDMC) and thus, predominantly drought resistant (Shiet al. 2016; García-Palacios et al. 2018). Our results
showed that the effects of biodiversity on resistance were predominantly
positive and stronger in resource-conservative communities, while they
were predominantly negative and weaker in resource-acquisitive
communities (Table S1). Considering biodiversity effects on the
ecosystem resilience, they were more pronounced for resource-acquisitive
communities and mainly positive under wet events (Table S2).
From a taxonomic perspective, species richness and species diversity
were negatively correlated with species dominance in all groups. Species
richness affects resistance, as we predicted and has already been shown
by others (Hautier et al. 2015; Isbell et al. 2015).
However, regardless of the direction or intensity of the climatic
events, species richness increased ecosystem resistance in
resource-conservative communities’, while it decreased resistance in
resource-acquisitive communities. Considering resource-acquisitive
communities under wet events, species richness negative effects on
resistance may indicate an increased biomass production given
water/resource inputs (Fischer et al. 2016) (Figures 4, S2 and
S4). Interestingly, positive effects of species richness on resilience
were only observed in communities recovering from wet events, while the
effects on the recovery from dry events were negative. According to the
portfolio effect (Doak et al. 1998; Tilman et al. 1998),
ecosystem stability may be ensured by maintaining high levels of species
richness, but our results show that this may be consistently observed
only in resource-conservative communities.
Species diversity also increased the resistance of grassland
communities. However, its effect was evidenced under extreme dry and
extreme wet climatic events, increasing the ecosystem resistance of
resource-conservative communities, thus partially supporting our
hypothesis. For resource-acquisitive communities under moderate and
extreme wet events, an increased species diversity had a negative effect
on resistance and a positive effect on resilience. Therefore, given
resource-acquisitive communities facing wet events, species diversity
may induce a compensatory stabilizing mechanism of biomass production
(Lehman & Tilman 2000; Grman et al. 2010; Mackie et al.2019).
From the functional perspective, we observed a consonant result between
species diversity and functional redundancy effects on ecosystems’
resistance, as we predicted, according to the insurance hypothesis
(Yachi & Loreau 1999). Their high positive correlation values indicate
that the functional redundancy found in the communities is promoted by
species diversity more than by the number of species per se. Functional
redundancy had a strong positive effect on the resistance of the most
resource-conservative communities facing extreme dry and wet events. In
turn, ecosystems’ resilience of resource-acquisitive communities was
increased by functional redundancy while recovering from moderate dry
and extreme wet events.
The effect of functional response diversity on resistance was dependent
on the community functional structure, as it was mainly observed in
resource-conservative communities and was independent from the anomalous
climatic events. However, given the resource-conservative communities,
we observed contrasting effects. While in communities where species
richness and functional response diversity were positively related, in
general functional response diversity had a positive effect on ecosystem
resistance, and in communities where the relationship was negative, its
effect was negative. Thus, we may conclude that a positive relationship
between species richness and traits reflecting functional response
diversity (Lavorel & Garnier 2002; Elmqvist et al. 2003) plays
an important role on the resistance, according to the insurance
hypothesis (Yachi & Loreau 1999).
As already highlighted (Isbell et al. 2015; Wright et al.2015), when analyzing empirical data, it may be difficult to completely
decouple resistance and resilience, especially when disturbances are
recurrent. This implies a challenge to distinguish resistance and
resilience over a temporal time series once both may be skewing each
other. However, it should be further explored in future studies, for
example, by looking for periods when productivity no longer depends on
the previous anomalous event. Additionally, this is an important issue
considering the already known dangerous effects of climate change on
ecosystem stability (Hautier et al. 2015; Craven et al.2018; García-Palacios et al. 2018; IPCC 2021).
Here we have shown that the ecosystem stability measured as resistance
and resilience of native grasslands depend on the biodiversity metric
evaluated in the communities and the direction and intensity of the
climatic anomaly. In addition, more than a single taxonomic or
functional component, communities’ functional structure plays an
important role in driving the effects of biodiversity on ecosystem
resistance and resilience under different anomalous climatic events.
This emphasizes the importance to consider different components of
biodiversity when investigating ecosystem services under climate change.
There is no unified measure able to synthesize all the possible answers
of the ecosystems under climate change. Moreover, our results stressed
the importance of biodiversity to guarantee grassland biomass
productivity under disturbance events generated by climate change. As
climate change models indicate that the intensity and frequency of wet
events are likely to increase in the Campos Sulinos region (IPCC 2021),
there is a pressing need for further studies to help understand
ecosystem-level response to such disturbances.