Beta diversity and the scale-dependence of the productivity-diversity relationship: a test in the Californian serpentine flora

1 The relationship of productivity to species diversity is usually positive at regional scales, but is often neutral, unimodal or negative at local spatial scales. Recent studies have pointed out that beta diversity, or among-locality and within-region variation in species composition, must therefore tend to increase with productivity. 2 We tested for a positive relationship of productivity to beta diversity in herbaceous plants at 105 widely distributed sites on serpentine soil in California. We also asked whether any such pattern could be explained, as previously proposed, by increased environmental heterogeneity at higher levels of productivity. 3 We found that one measure of beta diversity (the species dissimilarity between paired 500 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 productivity with alpha (1 m(2)) diversity to a positive relationship of productivity with gamma (1000 m(2)) diversity. 4 The positive effect of productivity on beta diversity was not related to increasing heterogeneity in coarse measures of vegetation structure (percentage cover of shade, litter, rocks, moss, bare soil, animal disturbance). We speculate that the effect may instead have a 'top-down' explanation: higher beta diversity may be caused by the positive influence of productivity on the size of the regional species pool. 5 Our study illustrates the principle of a 'scale transition', in which an ecological pattern is qualitatively different at different spatial scales. Careful attention to scale-dependence may help narrow the search for mechanisms for such long-studied ecological puzzles as the latitudinal diversity gradient.