Environmental correlates for tree occurrences, species distribution and richness on a high-elevation tropical island.

Philippe Birnbaum,Robin Pouteau,Vanessa Hequet,Thomas Ibanez,Tanguy Jaffré,Hervé Vandrot,Elodie Blanchard

doi:10.1093/aobpla/plv075

Abstract

High-elevation tropical islands are ideally suited for examining the factors that determine species distribution, given the complex topographies and climatic gradients that create a wide variety of habitats within relatively small areas. New Caledonia, a megadiverse Pacific archipelago, has long focussed the attention of botanists working on the spatial and environmental ranges of specific groups, but few studies have embraced the entire tree flora of the archipelago. In this study we analyse the distribution of 702 native species of rainforest trees of New Caledonia, belonging to 195 genera and 80 families, along elevation and rainfall gradients on ultramafic (UM) and non-ultramafic (non-UM) substrates. We compiled four complementary data sources: (i) herbarium specimens, (ii) plots, (iii) photographs and (iv) observations, totalling 38 936 unique occurrences distributed across the main island. Compiled into a regular 1-min grid (1.852 × 1.852 km), this dataset covered ∼22 % of the island. The studied rainforest species exhibited high environmental tolerance; 56 % of them were not affiliated to a substrate type and they exhibited wide elevation (average 891 ± 332 m) and rainfall (average 2.2 ± 0.8 m year(-1)) ranges. Conversely their spatial distribution was highly aggregated, which suggests dispersal limitation. The observed species richness was driven mainly by the density of occurrences. However, at the highest elevations or rainfalls, and particularly on UM, the observed richness tends to be lower, independently of the sampling effort. The study highlights the imbalance of the dataset in favour of higher values of rainfall and of elevation. Projected onto a map, under-represented areas are a guide as to where future sampling efforts are most required to complete our understanding of rainforest tree species distribution.

Highlights

Despite an ever increasing amount of data, the geographical distribution of most plant species remains incomplete and biased, for the most diverse taxonomic groups and regions (Whittaker et al 2005)
We examined whether the spatial distribution of rainforest tree species is more driven by the environment than by dispersal ability
Tree occurrences were distributed into 1213 cells of a regular 1-min grid, which covers 22 % of New Caledonia’s main island (Fig. 1)

Summary

Introduction

Despite an ever increasing amount of data, the geographical distribution of most plant species remains incomplete and biased, for the most diverse taxonomic groups and regions (Whittaker et al 2005). This so-called Wallacean shortfall frequently originates from the difficulty of evaluating the distribution of species diversity across large, heterogeneous areas where biodiversity is high but collecting efforts have been insufficient or inadequately planned (Schmidt-Lebuhn et al 2012). The relationship between species richness and elevation is more complex and highly dependent on the organism being considered (Lomolino 2001; Nogues-Bravo et al 2008; McCain and Grytnes 2010; Kessler et al 2011)

Methods

Results

Discussion

Conclusion