Changes in community assembly may shift the relationship between biodiversity and ecosystem function

Can differences in community assemblyalter the relationship between biodiver-sity and ecosystem function? Pholchanet al. (2013) used a variety of manipu-lations to change microbial communityassembly in sludge reactors and exam-ined the subsequent links between diver-sity and a rare function, the removal ofendocrinedisruptingcompounds(EDCs). Interestingly, the authors saw no consis-tent differences between shifts in alphadiversity (e.g., species richness and even-ness) and ecosystem function, observingan increase, decrease and no difference inthe amount of removal of speciﬁc EDCswith increases in diversity. They suggestedthat differences in community assemblymay be driving variation in the rela-tionship between biodiversity and func-tion, a fascinating hypothesis that unitesprocesses in community and ecosystemecology.Combinations of four processes affectcommunity assembly: dispersal and diver-siﬁcation add new taxa to communitieswhile selection and drift affect their rela-tive abundances (Vellend, 2010; Nemergutet al., 2013). Particular research emphasishas been placed on assembly processesthataredriven bydifferences betweentaxa(“niche”) compared to those in whichany such differences are irrelevant to ﬁt-ness(“neutral”)(Hubbell,2001). Likewise,researchers have focused on the roleof stochasticity, where assembly is moreprobabilistic vs. determinism, in whichrandomness does not affect communitydynamics.Nicheandneutralprocessescanoperateinunison(Adler et al., 2007)andboth can be affected by stochastic anddeterministic forces (Fox, 2012). Indeed,extensive data demonstrate that a varietyof factors, including nutrients, produc-tivity, resource availability, successionalstage, and disturbances may affect the rel-ative importance of different communityassembly mechanisms (Chase, 2007, 2010;Ferrenbergetal.,2013;Kardoletal.,2013). However, to our knowledge, no studieshave directly tested how shifts in com-munity assembly may affect the relation-ship between biodiversity and ecosystemfunction.Of course, a great deal of researchhas focused on pairwise combinationsof the interactions between communityassembly, biodiversity and/or function inisolation. First, a large body of workdemonstrates links between biodiversityand ecosystem function (Cardinale et al.,2011; Hooper et al., 2012), even formicrobial systems (Bell et al., 2005; Hsuand Buckley, 2009; Langenheder et al.,2010; Levine et al., 2011; Jousset et al.,2014). However, the nature and strengthof biodiversity ecosystem function (BEF)relationships have been widely debatedand strongly depend on the type of func-tion and ecosystem examined (Grime,1997; Hooper et al., 2005)andthedegree of redundancy within the com-munity (Reich et al., 2012; Jousset et al.,2014). These complexities may be height-ened for microorganisms due to theextraordinary phylogenetic diversity har-bored within microbial communities, andthe fact that a typical microbial commu-nity contains organisms from within avariety of functional guilds.Second, it is known that differentassemblymechanismsdrivebiodiversityindistinct ways. For example, spatial or tem-poral variation in environmental condi-tions increases biodiversity through nicheprocesses while increases in the diver-sity of the metacommunity or in theratio of immigration/emigration rates canincrease biodiversity through neutral pro-cesses (Vellend, 2010). Finally, a relatively new topic in the lit-erature relates community assembly andecosystem function (Fukami et al., 2010;Nemergut et al., 2013). Vital to such aconsideration is the relationship betweenresponse traits, or traits that can interactwithenvironmentalvariationtodeterminespecies distribution and abundance pat-terns, and effect traits, or traits that deter-mine the functional roles of different taxa(Naeem and Wright, 2003). When com-munities are largely structured by nicheprocesses, variation in the environmentcan directly correlate to effect traits thatare linked to selected response traits(Allison, 2012). However, when commu-nities are structured by neutral processes,ecosystem function will primarily dependon effect trait abundances within themetacommunity, dispersal and ecologicaldrift; thus, relationships between varia-tion in the environment and effect traits