Introduction
In the face of climate change, biodiversity may be an important driver
of ecosystem stability (Eisenhauer et al. 2016; Craven et
al. 2018; Pillar et al. 2018), i.e., the resistance of a given
ecosystem function to change and its resilience to recover from
disturbances or climate extremes (Tilman & Downing 1994; Isbellet al. 2015). Some studies have shown that plant biodiversity can
provide resistance of primary productivity to climate extremes, but this
relationship depends on the direction and intensity of the event (Isbellet al. 2015; Fischer et al. 2016; García-Palacios et
al. 2018; Mackie et al. 2019; Biggs et al. 2020; Valenciaet al. 2020). The biotic mechanisms driving ecosystem stability
under such anomalies have been mainly addressed through experimental
studies based on the manipulation of species composition (Fischeret al. 2016; Craven et al. 2018; Mackie et al.2019; Biggs et al. 2020). Often the available evidence emerges
from communities that were artificially assembled from scratch, which
also entangles the effects of the disturbance involved in the
manipulation (but see Jochum et al. 2020). Despite the importance
of those findings, experimental studies may not fully translate the
natural community assembly effects on the ecosystem stability under
climatic anomalies.
With the predicted changes in temperature and precipitation, extreme
climatic events are about to become more common (IPCC 2021). In fact, we
are already experiencing it. Specifically for southeastern South
America, an increase in frequency and intensity of droughts and rainfall
events has been predicted (Hoover et al. 2014; Souza & Manzi
2014; Gao et al. 2019; NOAA 2020; IPCC 2021). Climate controls
vegetation phenology, while the amount and distribution of annual
precipitation strongly influences the annual net primary productivity of
grasslands (Sala et al. 1988; Paruelo & Lauenroth 1995; Gordo &
Sanz 2010). If water availability is constrained, there is an increasing
influence of limited evapotranspiration for grassland biomass production
(Vicente-Serrano et al. 2010). Thus, as an increase in extreme
climatic events would interfere with the seasonal patterns of rainfall
distribution (IPCC 2021), it is expected that this will also affect the
primary productivity of plant communities. However, plant biodiversity
may buffer ecosystems from the effects of these anomalies, providing
resistance and resilience (Biggs et al. 2020).
According to the insurance hypothesis (Yachi & Loreau 1999),
biodiversity should provide stability due to the functional redundancy
of different species in nature. Functional redundancy in a community can
be operationally defined as the difference between taxonomic and
functional diversity based on traits driving ecosystem functions (Pillaret al. 2013), i.e., the portion of species taxonomic diversity in
a community that plays similar ecosystem functions. Increased functional
redundancy for a given ecosystem function implies that the community
contains species that can replace each other in case of species losses
due to their different environmental sensitivity (Walker et al.1999; Oliver et al. 2015). High taxonomic diversity may also
imply increased functional diversity in terms of responses to drivers
such as extreme climate events, i.e., increased response
diversity sensu Elmqvist et al. (2003) defined by response
traits sensu Lavorel & Garnier (2002). In other words, on the
one hand, ecosystem stability relies on the degree of functional
equivalence regarding effect traits (Lavorel & Garnier 2002) of the
resident species in the community (Elmqvist et al. 2003; Violleet al. 2007; Fischer et al. 2016). On the other hand,
plant functional diversity loss in terms of response traits can be
associated with a decrease in ecosystem functioning stability and hence,
environmental changes that potentially affect biodiversity may induce
long-term changes (Hautier et al. 2015).
Closely related to the insurance hypothesis, increased species diversity
may induce the “portfolio effect” (Doak et al. 1998; Tilmanet al. 1998). Beyond the simple idea of a statistical averaging
of individual species contribution to biomass production, together,
these two hypotheses predict a stabilizing effect of species diversity
on ecosystem properties through species asynchrony. This mechanism
ensures, via species richness and its different environmental responses,
that the more species, the greater the probability of asynchronous
species responses to environmental fluctuations, thus leading to an
increased stability (Yachi & Loreau 1999; Loreau 2010).
Plant traits directly related to environmental conditions (Bruelheideet al. 2018; Testolin et al. 2021) reflect a resource
acquisition and conservation trade-off known as the “leaf economics
spectrum” (Wright et al. 2004; Díaz et al. 2016; Garnieret al. 2016). The conservative side of this spectrum typically
comprises species that are able to store resources and use water more
efficiently. So, conservative species would withstand anomalous events,
providing resistance. In the acquisitive side, species use the resources
to grow faster. Therefore, they would offer less resistance during
anomalous events. A similar spectrum has been observed at the community
level (Bruelheide et al. 2018). It is an open question, however,
if communities that differ in terms of resource-use strategy would also
differ regarding their stability under extreme climatic conditions. It
is plausible to believe that they will. For example, under increased
water availability, communities characterized by acquisitive species
would benefit from resource-inputs ensuring biomass production and
providing resistance. Or they could be more productive than in normal
periods, leading to low resistance (Wright et al. 2015; Fischeret al. 2016). In contrast, communities defined by conservative
species could maintain productivity under decreased water availability
(García-Palacios et al. 2018)
In this study, we address the question whether plant communities with
higher diversity provide increased ecosystem stability under climatic
anomalies, and if the effects of biodiversity differ regarding the
dominant resource-use strategy of the communities. We evaluate the
effects of species richness and diversity, functional diversity and
functional redundancy on the stability (i.e. resistance and resilience)
of biomass production at the ecosystem level. From a taxonomic
perspective, (i) we hypothesize that induced by compensatory dynamics,
species richness (i.e. number of species) will present a positive effect
on resilience, induced by a “portfolio effect” (Doak et al.1998; Tilman et al. 1998; Valencia et al. 2020), (ii)
while species diversity will have a positive effect on resistance,
corroborating the insurance hypothesis (Yachi & Loreau 1999; Loreau
2010). From a functional perspective, (iii) using traits that are
relevant for biomass production, we hypothesize that functional
redundancy (FR) will guarantee biomass production, with a positive
effect on stability (resistance and/or resilience), whereas (iv)
increased functional diversity in terms of response diversity (hereafter
functional response diversity - FRD) will have a positive effect on
stability (resistance and/or resilience).
Overall, high levels of species richness, species diversity, functional
redundancy, and functional response diversity were positively related to
the resistance of biomass productivity in dry and wet events, whereas
resilience of biomass productivity to drought was positively related
mostly to species richness.