Disentangling distance decay of similarity from richness gradients: response to Baselga (2007)

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Baselga (2007) responded to our recent review-paper on the distance decay of similarity in ecological communities (Soininen et al. 2007) and stressed the importance of choosing the right metric to examine how community similarity decays with distance. It is widely known that ecological literature is replete with different beta diversity measures (Koleff et al. 2003) of which some are based on richness gradients, some on changes in community composition and some are based on measuring the variation in both richness and species identity such as Sorensen or Jaccard indices. We acknowledge Baselga for raising this important methodological point. Here, we clarify some main points of our analysis and respond to criticism by Baselga. 1) Baselga states that ‘‘ . . . one of their most remarkable results is the finding of a faster distance decay of similarity at higher latitudes, contradicting all current data and disagreeing Rapoport’s rule . . .’’. Here, he misinterprets our findings, as we actually found higher beta diversity at small spatial scales (i.e. initial similarity) in the tropics (Fig. 2 in Soininen et al. 2007), agreeing with the main tenets of macroecological literature (Qian and Ricklefs 2007, Qian et al. 2007). However, what we additionally found is that the halving distance is shorter at high latitudes. Thus, the initially low beta diversity at high latitudes shows accelerating turnover at higher spatial scales, indicating strong scale-dependence, whereas the initially high beta diversity in low latitudes is more constant at different scales (Fig. 1). This is the same as saying that low latitudes are characterized by small mosaics of community composition with high turnover (low initial similarity), but these mosaics are repetitive at higher spatial scales, resulting in patterns where turnover at one scale is not much larger than at the next higher scale. By contrast, higher latitudes are characterized by patches of similar composition (high initial similarity), which however turnover a higher proportion of species at larger spatial scales (short halving distance). We agree with Baselga that this result is surprising, but it is not ‘‘ . . . contradicting all current data . . .’’, as the distance decay has to our knowledge not been analyzed before simultaneously at multiple spatial scales (i.e. both initial similarity and halving distance) and across such large number of different organism groups and environments. Most importantly, we were able to sustain this outcome by the re-analysis of an independent data set on species area-relationships (SAR), which showed the same pattern (data extracted from Drakare et al. 2006, see Fig. 4 in Soininen et al. 2007). We showed in our paper that small-scale SARs produce steeper slopes in the tropics, which corresponds to higher tropical beta diversity. Slopes of large-scale SAR, however, do not reproduce this pattern, i.e. the slope is not consistently related to latitude. Thus, the increase in species richness with increasing area is more rapid at small scales in the tropics, but this latitudinal difference levels off at larger spatial scales (Fig. 1). This result corresponds to finding many new species at small distances (low initial similarity in the tropics), which then decelerates at larger distances (large halving distances). The SAR data we used represent relative (not absolute) increases in richness with nested areas (i.e. hierarchical beta diversity) and thus do not underlie the same caveats as the Sorensen index. Moreover, the SAR data set shows virtually no overlap with the distance decay data set used in Soininen et al. (2007). Therefore, it is possible that the pattern observed for halving distances truly reflects latitudinal gradient in the scale-dependence of beta diversity. However, we would Ecography 30: 842 844, 2007 doi: 10.1111/j.2007.0906-7590.05387.x # 2007 The Authors. Journal compilation # 2007 Ecography