Testing the Water–Energy Theory on American Palms (Arecaceae) Using Geographically Weighted Regression

Wolf L. Eiserhardt,Jens-Christian Svenning,Stine Bjorholm,Henrik Balslev,Thiago F. Rangel,Nathan G. Swenson

doi:10.1371/journal.pone.0027027

Wolf L. Eiserhardt, Jens-Christian Svenning + Show 4 more

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https://doi.org/10.1371/journal.pone.0027027

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Journal: PloS one	Publication Date: Nov 3, 2011
Citations: 84	License type: CC BY 4.0

Affiliation: Aarhus University, Universidade Federal de Goiás

Abstract

Water and energy have emerged as the best contemporary environmental correlates of broad-scale species richness patterns. A corollary hypothesis of water–energy dynamics theory is that the influence of water decreases and the influence of energy increases with absolute latitude. We report the first use of geographically weighted regression for testing this hypothesis on a continuous species richness gradient that is entirely located within the tropics and subtropics. The dataset was divided into northern and southern hemispheric portions to test whether predictor shifts are more pronounced in the less oceanic northern hemisphere. American palms (Arecaceae, n = 547 spp.), whose species richness and distributions are known to respond strongly to water and energy, were used as a model group. The ability of water and energy to explain palm species richness was quantified locally at different spatial scales and regressed on latitude. Clear latitudinal trends in agreement with water–energy dynamics theory were found, but the results did not differ qualitatively between hemispheres. Strong inherent spatial autocorrelation in local modeling results and collinearity of water and energy variables were identified as important methodological challenges. We overcame these problems by using simultaneous autoregressive models and variation partitioning. Our results show that the ability of water and energy to explain species richness changes not only across large climatic gradients spanning tropical to temperate or arctic zones but also within megathermal climates, at least for strictly tropical taxa such as palms. This finding suggests that the predictor shifts are related to gradual latitudinal changes in ambient energy (related to solar flux input) rather than to abrupt transitions at specific latitudes, such as the occurrence of frost.

Highlights

Among the contemporary environmental factors that are correlated with species richness on broad scales, water and energy have emerged as key influences [1,2,3,4]
Latitudinal trends in the amount of variation of palm richness that is locally explained by water or energy (Fig. 3) were largely robust to the choice of models, i.e., the spatial kernel used in Geographically weighted regression (GWR) and the use of ordinary least squares (OLS) vs. simultaneous autoregressive (SAR) models for evaluating the GWR results against latitude (Tables 1 and 2)
Water–energy dynamics theory predicts that species richness is primarily controlled by the availability of water and ambient energy, with water being most influential at low latitudes and energy being most influential at high latitudes [2]

Summary

Introduction

Among the contemporary environmental factors that are correlated with species richness on broad scales, water and energy have emerged as key influences [1,2,3,4]. These two factors do not appear to be important worldwide. Based on a review of 85 studies of broad-scale richness gradients, Hawkins et al [2] hypothesized that ‘the relative importance of the two components of water–energy dynamics shifts latitudinally’ A dynamic relationship between energy and water may result from life’s dependence on both liquid water and ambient energy [2,4,20]

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