Abstract

Our aims were to quantify and map the plant sub regions of the the Caatinga, that covers 844,453 km2 and is the largest block of seasonally dry forest in South America. We performed spatial analyses of the largest dataset of woody plant distributions in this region assembled to date (of 2,666 shrub and tree species; 260 localities), compared these distributions with the current phytogeographic regionalizations, and investigated the potential environmental drivers of the floristic patterns in these sub regions. Phytogeographical regions were identified using quantitative analyses of species turnover calculated as Simpson dissimilarity index. We applied an interpolation method to map NMDS axes of compositional variation over the entire extent of the Caatinga, and then classified the compositional dissimilarity according to the number of biogeographical sub regions identified a priori using k-means analysis. We used multinomial logistic regression models to investigate the influence of contemporary climatic productivity, topographic complexity, soil characteristics, climate stability since the last glacial maximum, and the human footprint in explaining the identified sub regions. We identified nine spatially cohesive biogeographical sub regions. Current productivity, as indicated by an aridity index, was the only explanatory variable retained in the best model, explaining nearly half of the floristic variability between sub regions. The highest rates of endemism within the Caatinga were in the Core and Periphery Chapada Diamantina sub regions. Our findings suggest that the topographic complexity, soil variation, and human footprint in the Caatinga act on woody plant distributions at local scales and not as determinants of broad floristic patterns. The lack of effect of climatic stability since the last glacial maximum probably results from the fact that a single measure of climatic stability does not adequately capture the highly dynamic climatic shifts the region suffered during the Pleistocene. There was limited overlap between our results and previous Caatinga classifications.

Highlights

  • Delineating biogeographical regions is an important step in understanding spatial organization of biological diversity and has been involved in both generating and testing hypotheses since the 19th century [1,2,3]

  • Since we aimed to identify and map cohesive biogeographical regions, we followed the analytical framework proposed by Rueda et al [13] and Moura et al [3] to produce a spatially contiguous estimation of floristic dissimilarity and performed regionalization based on this contiguous surface, as summarized below

  • Aridity is a proxy to local productivity because it reflects the balance between heat load and hydrological dynamics [38,79]

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Summary

Introduction

Delineating biogeographical regions is an important step in understanding spatial organization of biological diversity and has been involved in both generating and testing hypotheses since the 19th century [1,2,3]. Identifying spatially distinct assemblages bound by environmental conditions has led to novel ecological interpretations of the fossil record [5], including greater determinism or inertia of long-term community dynamics than predicted by neutral expectations [5,6]. Bioregionalization can be used to choose priority areas for conservation within delineated biogeographical regions, which could optimize the conservation of unique biotas, and assess the effects of prioritizing species richness or endemism in conservation planning [11]. The WWF Ecoregions [9] have been regarded as prominent global schemes based on the principle of biogeographic representation [11], but the internal structure of most of their units have not yet been established for different taxa

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