Journal of Ecology 2006 94 , 110–117 © 2006 The AuthorsJournal compilation© 2006 British Ecological Society Blackwell Publishing, Ltd. Beta diversity and the scale-dependence of the productivity-diversity relationship: a test in the Californian serpentine flora SUSAN HARRISON, KENDI F. DAVIES, HUGH D. SAFFORD* and JOSHUA H. VIERS Department of Environmental Science and Policy, University of California – Davis, Davis, California, USA, and * USDA Forest Service, Pacific Southwest Region, 1323 Club Drive, Vallejo, CA, USA Summary1 The relationship of productivity to species diversity is usually positive at regionalscales, but is often neutral, unimodal or negative at local spatial scales. Recent studieshave pointed out that beta diversity, or among-locality and within-region variation inspecies composition, must therefore tend to increase with productivity. 2 We tested for a positive relationship of productivity to beta diversity in herbaceousplants at 105 widely distributed sites on serpentine soil in California. We also askedwhether any such pattern could be explained, as previously proposed, by increasedenvironmental heterogeneity at higher levels of productivity. 3 We found that one measure of beta diversity (the species dissimilarity between paired500 m 2 plots on adjacent north and south slopes) was positively related to productivity(as measured by the normalized difference vegetation index, a remotely sensed index).However, this effect was not strong enough to transform the neutral relationship of pro-ductivity with alpha (1 m 2 ) diversity to a positive relationship of productivity withgamma (1000 m 2 ) diversity. 4 The positive effect of productivity on beta diversity was not related to increasingheterogeneity in coarse measures of vegetation structure (percentage cover of shade,litter, rocks, moss, bare soil, animal disturbance). We speculate that the effect may insteadhave a ‘top-down’ explanation: higher beta diversity may be caused by the positive influ-ence of productivity on the size of the regional species pool. 5 Our study illustrates the principle of a ‘scale transition’, in which an ecological patternis qualitatively different at different spatial scales. Careful attention to scale-dependencemay help narrow the search for mechanisms for such long-studied ecological puzzles asthe latitudinal diversity gradient. Key-words : beta diversity, diversity, heterogeneity, productivity, regional species pool,scale transition, serpentine, spatial scale, species richness Journal of Ecology (2006) 94 , 110–117doi: 10.1111/j.1365-2745.2005.01078.x Introduction Mechanisms to explain the relationship between pro-ductivity and species richness remain highly elusive,and recent studies of this problem have emphasized theneed for careful consideration of appropriate spatialscales (Huston 1999; Waide et al . 1999; Mittelbach et al . 2001; Safford et al . 2001). Productivity is nearlyalways found to be positively correlated with diversityat large scales, such as among biomes or geographicalregions (Francis & Currie 2003; Hawkins et al . 2003),but the relationship is often neutral, weakly unimodal,or negative at smaller scales, such as among fieldsampling plots (Grime 1973, 2001; Tilman 1982; Grace1999; Waide et al . 1999; Gross et al . 2000; Safford et al .2001). The productivity-diversity relationship thusprovides a potential example of a ‘scale transition’ Correspondence: Susan Harrison (fax +1 530 752 3350; e-mail [email protected]).111 Beta diversity and productivity © 2006 The AuthorsJournal compilation © 2006 British Ecological Society, Journal of Ecology , 94 , 110–117 (Chesson et al . 2005), in which an ecological pattern ata larger scale is qualitatively different to that at a smallerone, and an additional mechanism is required to explainsuch non-linear scale-dependence.Chase & Leibold (2002) demonstrated that the scaletransition from a non-positive to a positive productivity-diversity relationship must involve an increase in betadiversity, or among-site dissimilarity in species com-position. They found that zooplankton diversity withinponds (i.e. local, or alpha, diversity) followed a unimodalrelationship to productivity. However, ponds withinhigh-productivity watersheds were more dissimilarfrom one another in composition (i.e. had higher betadiversity) than ponds within low-productivity water-sheds, thus producing a positive relationship betweenproductivity and watershed-scale diversity (i.e. regional,or gamma, diversity). These authors considered, butfound no evidence for, one possible mechanism, namelythat environmental heterogeneity increases at higherlevels of productivity. They speculated that the mech-anism instead involved the existence of alternativestable states at high productivity, an idea that laterreceived a modest amount of support from microcosmexperiments (Fukami & Morin 2003). Subsequently,lack of connectivity among ponds was implicated as anecessary condition for the increase in beta diversitywith higher productivity (Chase & Ryberg 2004).Higher beta diversity at higher productivity is likelyto be a phenomenon of global generality and impor-tance. In a recent review and meta-analysis, Hillebrand(2004) found that the tropical-temperate diversity gra-dient is virtually always stronger at the regional scale,e.g. among countries or latitudinal bands, than at thelocal (field plot) scale, even when total latitudinalextent is held constant. In addition, multiple studieshave found that the regional latitudinal diversity gradientis very strongly correlated with measures of energy andwater availability that control primary productivity(Francis & Currie 2003; Hawkins et al . 2003; Hillebrand2004). Taken together, these results mean that global-scale patterns in productivity are positively associatedwith beta diversity, i.e. with the ratio of regionalrichness to local richness. Thus, although we are stillfar from having a satisfactory mechanistic explanationfor either the latitudinal richness gradient or theproductivity–diversity relationship, further study ofbeta diversity is clearly needed.In this study we ask whether beta diversity increaseswith productivity in a terrestrial herbaceous plant sys-tem. We further ask whether such a relationship can befound at the relatively small scale of tens to hundreds ofmetres, where beta diversity may be caused by hetero-geneity in readily observable microhabitat conditionssuch