Little consensus has emerged regarding how various drivers (e.g., productivity, disturbance, temperature) influence species richness. This is due in part to the fact that the majority of studies examine richness at a single scale and ignore how the underlying components of richness vary. We propose a new framework for the measurement of biodiversity (MoB) which decomposes scale-specific changes in richness into components attributed to: 1) the number of sampled individuals, 2) the degree of evenness, and 3) the degree of intraspecific spatial aggregation. MoB accomplishes the richness decomposition using a nested comparison of individual-based, sample-based, and spatially explicit sample-based rarefaction curves. Each curve provides some unique scale-specific information on the underlying components of richness. We demonstrate our new framework by examining plant richness in invaded and uninvaded grasslands.
Results/Conclusions
The traditional single-scale analysis of average species richness indicated that invaded sites had fewer species, but rarefying richness to an equivalent number of individuals between the treatments suggests that the invaded sites actually have more species. In contrast to these contradictory, single scale results our MoB analysis uncovered scale-consistent signatures that invasion resulted in higher evenness (i.e., a positive effect on biodiversity) but lower individual density (i.e., a negative effect on biodiversity). Invasion does not appear to influence richness via changes to intraspecific spatial aggregation in our study sites. The application of the MoB approach demonstrates how species accumulation curves can be used to decompose changes in richness into its underlying components which provides greater ecological insight. Additionally, MoB can be used to decipher seemly contradicting conclusions from single scale analyses. This general approach can also be applied to other metrics of biodiversity (e.g., phylogenetic and functional diversity) that can be examined using accumulation curves. We believe that it is critical for biodiversity analyses to explicitly incorporate scale, and to recognize that changes in richness can be driven by different components of biodiversity.