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When does diversity matter? Species functional diversity and ecosystem functioning across habitats and seasons in a field experiment

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Despite ample experimental evidence indicating that biodiversity might be an important driver of ecosystem processes, its role in the functioning of real ecosystems remains unclear. In particular, the understanding of which aspects of biodiversity
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   " #$%& %& '$( )**(+'(, -(.&%/0 /1 '$( 1/22/3%04 ).'%*2(5 " 6.)%0(.7 897 :*;%(7 <9 =97 )0, :)2>?-%&'7 <9 @AB"CD E$(0 ,/(& ,%-(.&%'F >)''(.G A H+(*%(& 1I0*'%/0)2 ,%-(.&%'F )0, (*/&F&'(> 1I0*'%/0%04 )*./&& $)J%')'& )0, &()&/0& K %0 ) 1%(2, (L+(.%>(0'9  !"#$%&' ") *%+,&' -."'"/0  7 !" 7 CMBNCMO7 C 3$%*$ $)& J((0 +IJ2%&$(, %0 1%0)2 1/.> )' P $''+5QQ/02%0(2%J.).F93%2(F9*/>Q,/%Q"B9""""Q"KMPNAMPM9"A"CAQ)J&'.)*'Q   M R When does diversity matter? Species functional diversity and ecosystem S functioning across habitats and seasons in a field experiment O "B André Frainer  1* , Brendan G. McKie 2  and Björn Malmqvist 1   "" "A 1. Department of Ecology and Environmental Science, Umeå University, Umeå, "K Sweden, SE 901 87 "C 2. Department of Aquatic Sciences and Assessment, Swedish University of Agricultural "P Sciences, Uppsala, Sweden, SE 750 07 "M * Current address: Department of Arctic and Marine Biology, University of Tromsø, "R Tromsø, Norway, 9037 "S Corresponding author:  André Frainer, Department of Arctic and Marine Biology, "O Faculty of BioSciences, Fisheries and Economics, University of Tromsø, Tromsø, AB  Norway, 9037. andre.frainer@emg.umu.se A" Running headline: Functional diversity and ecosystem functioning AA AK   A Summary AC 1.   Despite ample experimental evidence indicating that biodiversity might be an AP important driver of ecosystem processes, its role in the functioning of real ecosystems AM remains unclear. In particular, the understanding of which aspects of biodiversity are AR most important for ecosystem functioning, their importance relative to other biotic and AS abiotic drivers, and the circumstances under which biodiversity is most likely to AO influence functioning in nature, is limited. KB 2.   We conducted a field study that focussed on a guild of insect detritivores in K" streams, in which we quantified variation in the process of leaf decomposition across KA two habitats (riffles and pools) and two seasons (autumn and spring). The study was KK conducted in six streams, and the same locations were sampled in the two seasons. KC 3.   With the aid of structural equations modelling, we assessed spatio-temporal KP variation in the roles of three key biotic drivers in this process: functional diversity, KM quantified based on a species trait matrix, consumer density and biomass. Our models KR also accounted for variability related to different litter resources, and other sources of KS  biotic and abiotic variability among streams. KO 4.   All three of our focal biotic drivers influenced leaf decomposition, but none was CB important in all habitats and seasons. Functional diversity had contrasting effects on C" decomposition between habitats and seasons. A positive relationship was observed in CA  pool habitats in spring, associated with high trait dispersion, whereas a negative CK relationship was observed in riffle habitats during autumn. CC 5.   Our results demonstrate that functional biodiversity can be as significant for CP functioning in natural ecosystems as other important biotic drivers. In particular, CM variation in the role of functional diversity between seasons highlights the importance CR   K of fluctuations in the relative abundances of traits for ecosystem process rates in real CS ecosystems. CO Key-words:  stream ecosystems, path analyses,   spatial-temporal variability, species PB evenness, species traits, litter decomposition P"   C PA Introduction PK Despite evidence from controlled experiments demonstrating the potential for species PC diversity to influence ecosystem processes (Balvanera et al.  2006; Cardinale et al.   PP 2006), there is still much uncertainty about the relevance of these results for PM understanding the functioning of real ecosystems (Reiss et al.  2009). In particular, the PR understanding of when and where  changes in species diversity are most likely to PS influence functioning in real world ecosystems, relative to other biotic variables, PO remains limited (Reiss et al.  2009; Duffy 2009; Hooper et al.  2012). In part, this is a MB consequence of the dominance of short-term, small scale manipulative settings used in M" testing biodiversity-ecosystem functioning (B-EF) theory, where the spatial and MA temporal variability characteristic of real ecosystems, and its implications for ecosystem MK functioning, may have been overlooked (Stachowicz et al.  2008; Reiss et al.  2009). MC Spatio-temporal variation in species composition and diversity can influence MP ecosystem functioning by affecting the distribution of functional traits present in local MM communities (Mouillot et al.  2013). These are traits that directly influence habitat use MR and organismal performance, particularly in relation to resource use and processing, and MS  biomass production. Increases in functional diversity may often be a consequence of MO increases in species richness (Petchey & Gaston 2002), but the two components are not RB always linearly correlated (Botta-Dukát 2005; Ricotta 2005; Laliberté & Legendre R" 2010). An increase in species richness may have little impact on ecosystem functioning RA if the new species are functionally very similar to those already present in a community RK (Fonseca & Ganade 2001; Joner et al.  2011). In contrast, the addition of specific, novel RC traits which have direct positive or negative effects on key ecosystem processes RP (Burkepile & Hay 2008; Jousset et al.  2011) may have stronger functional RM   P consequences. Traits which affect species interactions within functional guilds can also RR alter ecosystem processes. This is seen both when the activities of a new species RS facilitates feeding by other consumers, enhancing ecosystem functioning (Jonsson & RO Malmqvist 2003), and when particular species increase antagonistic interactions (Polley, SB Wilsey & Derner 2003; McKie et al.  2009; Jousset et al.  2011), driving down resource S" consumption overall. SA  Not only the occurrence of specific functional traits, but also the relative SK distribution of those traits, can vary across habitats and seasons (Beche, Mcelravy & SC Resh 2006; McGill, Sutton-Grier & Wright 2009). Decreasing trait evenness may be SP associated with enhanced ecosystem functioning in cases where the dominant trait (or SM group of traits) are also the most productive (Hector et al.  2002; Dangles & Malmqvist SR 2004; McKie et al.  2008). Alternatively, if the effects of the traits on a process are SS complementary to one another, then a more even distribution of traits may favour SO enhanced ecosystem functioning (Kirwan et al.  2007). OB A further key biological determinant of ecosystem functioning is the biomass of O" individuals within functional guilds (Reiss et al.  2011). Well-known relationships OA  between biomass and consumer metabolic requirements have strong effects on OK consumer-resource interactions (Gruner et al.  2008), and rates of energy flow at an OC ecosystem level are positively related to bulk producer and consumer biomass (Brown OP et al.  2004). However, these basic relationships between resource consumption and OM  biomass can be altered by density-dependent variation in inter- and intraspecific OR interactions and resource use efficiency. In systems characterised by pulses of OS allocthonous resource, progressively decreasing resource availability may increase OO consumer densities relative to that resource, increasing the potential for strong species "BB interactions (Presa Abos et al.  2006; Tiegs et al.  2008). At high densities, negative "B"
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