Introduction
Most research on biodiversity-ecosystem functioning (BEF) relationships
has focused on effects of varying horizontal diversity (i.e., diversity
within a single trophic level), most commonly of plants in controlled
experimental communities (e.g., Isbell et al. 2015). However, natural
communities are characterized by complex interaction networks that
integrate diversity and its effects across trophic levels (Duffy et al.
2007; Brose et al. 2019), with their BEF relationships varying
substantially in strength (Barnes et al. 2014; Duffy et al. 2017; van
der Plas 2019). Recent research has aimed at resolving this separation
between within-trophic level and multi-trophic approaches to BEF
relationships (Loreau 2010; Brose & Hillebrand 2016). For example, the
vertical diversity hypothesis links ecosystem functions of primary
producers, and hence their diversity effects, to variance in vertical
diversity (i.e., diversity across trophic levels), specifically the
maximum trophic levels and body-masses of multi-trophic ecosystems (Wang
& Brose 2018). It also points to other related aspects such as food-web
structure (Thompson et al. 2012; Montoya et al. 2015; Brose et al. 2017)
or animal diversity (Naeem et al. 1994; Schneider et al. 2016; Zhao et
al. 2019) that influence ecosystem functions at the producer trophic
level. Despite ample evidence for such top-down effects on producer BEF
relationships, the underlying mechanisms have remained elusive.
The many biological mechanisms involved in creating positive diversity
effects in producer communities can be broadly categorized into two
classes (Loreau & Hector 2001; Loreau 2010). First, complementarity
mechanisms occur when functionally different species use dissimilar
niches, hence have a low interspecific competition. This low competition
fosters coexistence, which simultaneously increases the ecosystem
functioning of the whole community. Second, selection mechanisms favor
species with competitive advantages. If such advantages are associated
with particular functional traits such as a higher growth rate,
selection can affect ecosystem functioning. Complementarity and
selection are both enhanced by a larger species-pool that may provide
more complementary species and strong competitors alike (i.e., sampling
effect). However, they have opposite implications for realized
diversity, which is maintained by complementarity but reduced by
selection mechanisms. Even though the functional identity of the
dominating species can be important depending on the ecosystem function
in question (Loreau 2004; Hooper et al. 2005), most evidence points
towards complementarity mechanisms as the dominant driver of BEF
relationships (Hooper et al. 2005; Cardinale et al. 2007; Barry et al.
2018).
Complementarity between co-occurring producer species is most commonly
associated with resource-use complementarity (synonymous with resource
partitioning; Barry et al. 2018), expressing fundamental differences in
resource-access of coexisting species. These differences can arise from
varying aspects of resource-use such as differences in the chemical
forms of resources used (McKane et al. 2002; Von Felten et al. 2009;
Ashton et al. 2010), phenological asynchrony (Henry et al. 2001;
Sapijanskas et al. 2014), or spatial separation, both above- (e.g.,
crown packing in Sapijanskas et al. 2014) and belowground (e.g., rooting
depth in Mueller et al. 2013). Additional resource-based mechanisms such
as facilitation (Wright et al. 2017) and niche plasticity (Von Felten et
al. 2009; Mueller et al. 2013) can modify resource niches to decrease
competition and increase complementarity among producers further.
In the presence of animal consumers, however, competition is not only
resource-based (exploitative competition) but can be mediated by
multi-trophic interactions (apparent competition; Holt 1977; Loreau
2010). When herbivorous feeding is complementary (i.e. herbivores have
different resource-species), apparent competition between producer
species is low, which fosters coexistence as it creates complementarity
at the producer trophic level (Thébault & Loreau 2003; Brose 2008;
Poisot et al. 2013; Wang & Brose 2018). As a result, simple herbivore
communities alone are sufficient to create positive diversity effects on
standing biomass and resource uptake (i.e., primary production) of
producers, even without resource-use complementarity (Thébault & Loreau
2003). Increasing the vertical diversity in complex trophic networks can
further enhance coexistence, indicating that complementarity scales with
the diversity of the multi-trophic animal community (Wang & Brose
2018). Additionally, herbivorous feeding can amplify differences in the
competitive abilities of some producer species and thereby introduce
selection mechanisms that can affect ecosystem functioning both
positively or negatively (Thébault & Loreau 2003). For example, large
producers that have low mass-specific metabolic rates are more suited to
cope with herbivory, thus are more competitive and maintain higher
biomasses (Schneider et al. 2016). Investigating complementarity
mechanisms without considering selection is therefore impossible when
trying to understand what drives BEF relationships in multi-trophic
ecosystems.
It is evident that resource-use complementarity and multi-trophic
interactions can both shape BEF relationships at the producer trophic
level. Complementarity from either source will favor a positive
relationship between biodiversity and ecosystem functioning, while
selection may interact in more complex ways, potentially having opposing
effects. It is therefore important to investigate how these mechanisms
most likely combine in realistic complex food-webs. Our study addresses
this issue by integrating multi-trophic interactions and resource-use
complementarity into a complex allometric food-web model to examine how
they create and shape the positive effects of producer species richness
on primary production (hereafter: net diversity effects). First, we
investigate how resource-use complementarity amongst producers creates
positive net diversity effects across levels of producer richness. The
subsequent inclusion of multi-trophic interactions allows us to
investigate how such effects are modified through changes to the
producer community’s functional composition, potentially driving both
selection and complementarity mechanisms. By varying the species
richness of the multi-trophic animal community, we investigate how
diversity across trophic levels influences the mechanistic interaction
with resource-use complementarity and thus determines net diversity
effects.