Spatial Configuration of Multiple Aspects of Biodiversity Along a Tropical Elevational Gradient

Although taxonomic biodiversity is a multidimensional concept, most research at intermediate to broad spatial scales has solely considered gradients of species richness.  In contrast, we quantified spatial variation in multiple metrics of taxonomic biodiversity (i.e., species richness, evenness, diversity, dominance, and rarity) along an extensive tropical elevational gradient, and did so by decomposing each metric into a number of hierarchical spatial components (e.g., a, ß, and ? components).  We sampled terrestrial gastropods along two parallel transects in the Sonadora River Watershed of the Luquillo Mountains of Puerto Rico: a mixed forest transect passed through multiple vegetation types (i.e., tabonuco, palo colorado, and elfin forests), whereas a palm forest transect passed through a single vegetation type (i.e., palm forest).  Despite considerable differences in the flora along these two transects, the relative contributions of scale (i.e., within plots [a _plots]; among plots within elevational strata [ß _plots]; among elevational strata [ß _strata]) to total biodiversity (? _transect) were quite similar for all five metrics.  Regardless of transect, heterogeneity among elevational strata contributed appreciably to total biodiversity for richness and rarity, whereas mean biodiversity at the plot level contributed appreciably to total biodiversity for metrics that are sensitive to variation in abundance (i.e., species evenness, diversity, and dominance).  Nonetheless, the form and parameterization of elevational gradients of biodiversity for each metric were quite different at the focal scale of strata.  In general, the fit of empirical data to the best model (linear or quadratic) was greater along the mixed forest transect than along the palm forest transect.  Moreover, differences between transects in the form of elevational gradients were most apparent for metrics that were insensitive to abundance (i.e., species richness and rarity) compared to those that weighted estimates of richness by species abundances (i.e., species diversity, evenness, and dominance).  Unweighted measures of taxonomic biodiversity (i.e., species richness or species rarity) at the a-scale and ?-scale often had linear or random relationships with elevation, whereas measures of taxonomic biodiversity that are weighted by species abundances consistently evinced modal patterns with mid-elevational peaks.  Approaches for evaluating the spatial dynamics of biodiversity, such as those used here with respect to elevation, show promise for quantifying how the hierarchical organization of biodiversity changes with respect to other environmental gradients (i.e., productivity, disturbance), or to climate change and land use change.