How differences in plant abundance measures produce different species-abundance distributions

1. Species-abundance distributions (SADs) are a convenient and common method for describing ecological communities. Despite their long history and the cornucopia of theoretical models, which have been suggested to describe them, no agreement has been reached as to which models are best. 2. This lack of agreement is in part owing to the inherent differences in the abundance measure used. Discrete measures such as density and point quadrat cover produce a distinct veil line (positive skewness) when compared to continuous measures such as biomass or basal area. We compared two different sets of discrete and continuous abundance measures commonly used to estimate plant abundance, (i) cover (estimated from point quadrats) vs. biomass for 35 quadrats in garigue vegetation on serpentine soil in Tuscany, Italy; and (ii) density vs. basal area for the 2005 50 ha BCI (Panama) tree data. We used marginal plots (ordinary scatter plots with a dotplot of each variable along its own axis) to compare the shape of the SADbased on the two abundance measures. 3. The average of all 35 garigue plots gave a reasonably consistent description of the data. In contrast, when all 35 plots were concatenated, or when an individual plot was investigated, the discrete cover marginal plot, but not the continuous biomass plot, was truncated. The discrete density marginal plot, but not the continuous basal area plot was also truncated. 4. We highlighted the substantial effect that the species-abundance measure selected has on the shape of the SADby comparing measures of skewness and kurtosis. This suggests that communities sampled using different abundance methods may fit different theoretical models best, not because they are fundamentally different but because the abundance measure is fundamentally different. Averaging over all the quadrats produced a better correspondence between the two abundance methods. Aside from the theoretical aspects of model fitting, our results clearly show that comparisons of communities or meta-analyses using SADs based on different measures of abundance should be treated with caution.