Review b-Diversity, Community Assembly, and Ecosystem Functioning 1, 2 3 Akira S. Mori, * Forest Isbell, and Rupert Seidl Evidence is increasing for positive effects of a-diversity on ecosystem function- Highlights ing. We highlight here the crucial role of b-diversity – a hitherto underexplored A rich body of evidence shows how biodiversity can help to sustain pools facet of biodiversity – for a better process-level understanding of biodiversity and fluxes of matter and energy in eco- b change and its consequences for ecosystems. A focus on -diversity has the systems. Understanding such diversity effects on ecosystem functioning is cru- potential to improve predictions of natural and anthropogenic influences on cial to predicting the potential conse- diversity and ecosystem functioning. However, linking the causes and conse- quences of biodiversity loss. quences of biodiversity change is complex because species assemblages in Although a-diversity has received nature are shaped by many factors simultaneously, including disturbance, envi- great attention in the literature, there ronmental heterogeneity, deterministic niche factors, and stochasticity. Because is a serious knowledge gap for the variability and change are ubiquitous in ecosystems, acknowledging these inher- roles and functions of b-diversity. ent properties of nature is an essential step for further advancing scientific b-Diversity provides insights into the knowledge of biodiversity–ecosystem functioning in theory and practice. mechanisms driving biodiversity changes and their consequences for The Importance of b-Diversity for Understanding the Causes and multiple ecosystem functions. Focus- Consequences ing on b-diversity is especially impor- tant in ecological communities that are There is a growing body of evidence showing that biodiversity is important for generating and subject to large environmental fluctua- stabilizing ecosystem functions, and thus ensures the provisioning of numerous ecosystem tions and disturbances. services to society [1]. Theoretical, experimental, and observational studies across different Considering the increasing importance types of ecosystems and biomes [2,3] confirm positive effects of local-scale biodiversity on of variability in ecological systems, par- ecosystem functions. The largest body of evidence exists for the linkages between local ticularly in the context of global – species richness (a-diversity the number and abundance of species within local communities change, insights gained from studying b-diversity are of importance for both of interacting species) of plants and biomass productivity. Studies on species richness– theoretical and applied ecology. productivity relationships have substantially advanced our understanding of the mechanisms underlying the functional roles of biodiversity [4–7]. Recently, research on the functional role of biodiversity has broadened its view beyond a strong focus on productivity, simultaneously considering the effects of diversity on multiple ecosystem functions [8–15]. A series of studies have demonstrated that high levels of species richness are important for sustaining multiple functions and services, and thus a loss of species can adversely affect the functionality of ecosystems. However, another facet of biodiversity, which – as we posit here – is essential in 1 Graduate School of Environment and the context of ecosystem multifunctionality, has been considered relatively scantily in inves- Information Sciences, Yokohama National University, Yokohama tigations of biodiversity–ecosystem functioning (see Glossary) relationships to date. The 240-8501, Japan multiscale nature of biodiversity, and specifically b-diversity – the variation in the identities and 2 Department of Ecology, Evolution – abundances of species among local assemblages has received much less attention com- and Behavior, University of Minnesota, St Paul, MN 55108, USA pared to a-diversity. Our aim here is to highlight the crucial role of b-diversity, by synthesizing its 3 Institute of Silviculture, University of mechanistic effect on biodiversity organization (and its responses to natural and anthropogenic Natural Resources and Life Sciences drivers), and by describing its association with the provisioning of multiple ecosystem functions. (BOKU) Vienna, Peter Jordan Straße 82, 1190 Wien, Austria Biodiversity–Ecosystem Multifunctionality There is increasing concern regarding the causes and consequences of human-induced *Correspondence: b-diversity change [16,17], including biotic homogenization [18,19]. Homogenization of [email protected] (A.S. Mori). Trends in Ecology & Evolution, July 2018, Vol. 33, No. 7 https://doi.org/10.1016/j.tree.2018.04.012 549 © 2018 Elsevier Ltd. All rights reserved. ecological communities could affect ecosystem functioning as strongly as, or even more Glossary strongly than, the effects of local species losses or gains (changes in a-diversity; cf the spatial b-Diversity: the variation in the insurance hypothesis [20]). While potential degradation of ecosystem functions and services identities and abundances of species among local species assemblages. It has been reported in response to a decline of a-diversity, for instance, similar investigations are can be quantified in different ways, still widely lacking for b-diversity (Box 1). A perspective based on b-diversity is especially including taxonomic, functional, and important in the context of multifunctionality. This is because there is no ubiquitous species phylogenetic dissimilarity, either assemblage that can simultaneously support all functions at high levels. Consequently, sus- weighted by relative abundances or not. Biotic homogenization is the taining multiple functions requires different sets of local species assemblages (i.e., b-diversity) outcome of a human-induced in a heterogeneous environment [21]. Given that people depend on multiple, rather than reduction in b-diversity. individual, ecosystem services simultaneously for human well-being, the growing theoretical Biodiversity–ecosystem functioning: the study framework and empirical evidence for a positive contribution of biodiversity to ecosystem multifunctionality that investigates possible is of high practical importance. consequences of biodiversity change on ecosystem functions. In With increasing dimensionality of the functional context, any species could become funda- experimental studies, species diversity is manipulated to quantify mentally irreplaceable [10]. This and related notions (i.e., low multifunctional redundancy the net effects of biodiversity loss on [9,14,22]) are increasingly recognized in ecology, and underline the imperative to conserve ecosystem functioning. With the help high levels of local diversity. While studies of biodiversity–multifunctionality have substantially of advanced statistical methods, contributed to understanding why and how biodiversity is important, remaining uncertainties non-manipulative studies are also increasingly feasible for the include the inevitable trade-offs between different functions [12,14]. Sustaining all functions at evaluation of the relationships high levels in a single locality is unrealistic because ecosystems are heterogeneous in nature between biodiversity and ecosystem (with spatial differences in species richness, identity, and composition), and because some functioning in real-world settings. functions might be mutually exclusive. Recognizing the heterogeneous distribution of species Biotic homogenization: an anthropogenic impact on biodiversity. and functions across space and time calls for a more dynamic appraisal of diversity as a factor Because of human-induced that is not static but changes over space and time (e.g., the course of patch dynamics and decreases in environmental variability succession in a forest ecosystem). (environmental homogenization), species assemblages could increase in similarity in terms of their The Causes and Consequences of Biodiversity Changes taxonomic, functional, and Focusing on spatial attributes and levels of diversity is not necessarily new in the study of phylogenetic composition across – biodiversity ecosystem functioning [20,23]. Previous work showed that spatial and temporal locations. The term was originally used to describe the replacement of turnover in species can contribute towards simultaneously supporting different functions native by non-native species that can [8–10]. New evidence for the effects of b-diversity (Table 1) is becoming available for different result in a decline in community groups of organisms from experimental [11], theoretical [24], and observational studies dissimilarity over spatial and temporal [13–15]. Nevertheless, the observed patterns are not always consistent, likely resulting from scales. Community assembly: considers different definitions and metrics being used to define b-diversity, as well as a possible the mechanisms by which local dependence between a- and b-diversity (Table 1) (also see [25]). We thus cannot yet deduce species assemblages are organized, a generalized theory on the role of b-diversity in ecosystem functioning. Nonetheless, some and describes the final outcome of important implications have emerged that should be considered in future research. First, it is these organization processes. There is debate about whether the indeed important to consider the effect of diversity at multiple spatial scales [26]. The role of outcomes of community assembly b-diversity and spatial scale in general in mediating the functional