BOX 1: Biodiversity-ecosystem function relationships (BEFs)
across biodiversity facets
Ecologists have long sought to understand how changes in community
composition and species loss of species alter the fate of ecosystem
functions. Theoretical works and large-scale experiments using plant
communities have provided the foundation of BEFs. For instance, many
studies have investigated the relationships between plant species
richness and primary production, demonstrating a positive and saturating
relationship between richness and primary production
(e.g., Tilman et
al. 1996; Loreau 1998). The conclusions have then been extended to
multiple ecosystem types (e.g., aquatic ecosystems), functional groups
(e.g., consumers species), and ecosystem functions (e.g., secondary
production, carbon storage or nutrient recycling)
(Hooper et al.2005; Balvanera et al. 2006; Cardinale et al. 2012).
Biodiversity-ecosystem relationships are explained by several
non-exclusive mechanisms, including complementarity, facilitation and
sampling (or selection) effects. Complementarity among species allows
species to use different resources, eventually releasing competitive
interactions; facilitation occurs when species provide resources or
modify habitat that benefit the others in the community; sampling
effects (aka selection or dominance effect) leads to a positive effect
of biodiversity on ecosystem functions because in diverse communities
the probability to include a highly productive (competitive) species is
higher. Interestingly, these mechanisms often led ecosystem functions to
increase at low biodiversity level and then reach a plateau at higher
biodiversity levels according to a saturating relationship. The
probability to include species with similar functional roles is indeed
higher when biodiversity is high, increasing functional redundancy among
species. Contrastingly, in some cases, negative (or neutral) BEF
relationships can arise
(see Hagan et
al. 2021 for further discussions). These particular examples suggest
that in some communities, increasing diversity might actually induce
negative competitive interactions among species. Finally, biodiversity
has also been shown to stabilise ecosystem functions (over space and
time) by buffering ecosystem variation against environmental fluctuation
(the insurance
hypothesis, Yachi & Loreau 1999). Richer communities displayed higher
resilience after a perturbation than poorer communities, because of the
presence of species with high recovery rate.
While BEF relationships have primarily been investigated at the
interspecific level, diversity within species also determines ecosystem
functions. Similar mechanisms are at play -such as complementarity,
redundancy, sampling effects- acting here not among species but among
individuals within species. Importantly, the effects of intraspecific
diversity on ecosystem functioning can be as strong as those of species
diversity (Raffardet al. 2019). Therefore, recent studies plead for the existence
of intraspecific- BEFs. This corroborates some mechanistic models
that did not initially distinguish between intra- and interspecific
diversity in their formulation, and demonstrates that biodiversity lossper se alters ecosystem functions
(Loreau 1998; Norberget al. 2001). These processes (complementarity, redundancy,
sampling effect) can actually be transferred to gene functions, and
hence directly applied to a BEF framework in which PCCGs would be the
inclusive measure of biodiversity.