How does variation in total and relative abundance contribute to
gradients of species diversity?
Patterns of biodiversity provide insights into the processes that shape
biological communities around the world. Variation in species diversity
along biogeographical or ecological gradients, such as latitude or
precipitation, can be attributed to variation in different components of
biodiversity: changes in the total abundance (i.e. more-individual
effects) and changes in the regional species abundance distribution
(SAD). Rarefaction curves can provide a tool to partition these sources
of variation on diversity, but first must be converted to a common unit
of measurement. Here, we partition species diversity gradients into
components of the SAD and abundance using the effective number of
species (ENS) transformation of the individual-based rarefaction curve.
Because the ENS curve is unconstrained by sample size, it can act as a
standardized unit of measurement when comparing effect sizes among
different components of biodiversity change. We illustrate the utility
of the approach using two datasets spanning latitudinal diversity
gradients in trees and marine reef fish, and find contrasting results.
Whereas the diversity gradient of fish was mostly associated with
variation in abundance (86%), the tree diversity gradient was mostly
associated with variation in the SAD (59%). These results suggest that
local fish diversity may be limited by energy through the
more-individuals effect, while species pool effects are the larger
determinant of tree diversity. We suggest that the framework of the
ENS-curve has the potential to quantify the underlying factors
influencing most aspects of diversity change.