Evolution of climatic niche in the Pacific Island genus, Coprosma (Rubiaceae)

Background/Question/Methods

Recent studies have used environmental niche models in conjunction with phylogenetic analyses to investigate mechanisms leading to species diversification.  We used this integrative approach to investigate the evolution of the climatic niche within the vegetatively and ecologically diverse New Zealand plant genus, Coprosma (Rubiaceae).  Specifically we aimed to evaluate the following:  (1) What is the ecological niche to which each species is adapted?  (2) Do closely related species differ in relation to their niche?  If so, along what niche axes have they diverged?  (3) What is the rate, and overall pattern of climatic niche evolution within the genus?  Climatic niche breadth was estimated using the GIS-based habitat modeling program (MaxEnt).  Variables included in the model consisted of 18 “BIOCLIM” variables that are indicative of species climatic tolerances.  The evolutionary patterns of diversification were estimated by mapping each of the 18 variables onto a pre-existing phylogeny of the genus.  Relative disparity - measured as the mean squared difference between two daughter lineages extending from each interior node of the clade – was plotted through evolutionary time.  Each of these empirical plots was compared to a rate of change as expected under a null expectation.

Results/Conclusions

Coprosma occurs throughout northern and southern New Zealand and species occupy a diverse array of habitats.  While a few species are widespread, most have somewhat restricted predicted distributions.  Although species often co-occur, they differ along one or more climatic niche axes suggesting that co-existence of congeners may be aided by niche partitioning.  Evaluation of sister species pairs indicates that there has been divergence along at least one ecologically important climatic niche axis.  We observe considerable differences among sister species when comparing mean annual temperature and annual precipitation variables, and it is possible that these two variables have been influential in leading to species divergence and radiation throughout the islands.  Finally, plotting disparity as a function of relative time, we see varying patterns of diversification of our variables of interest.  While some variables (e.g. annual precipitation) exhibit less than expected disparity towards the base of the tree followed by period of considerable divergence, others appear to have diversified early in the radiation with subsequent patterns of stasis and divergence (e.g. mean annual temperature).  These results indicate that climatic variables have been important factors influencing the diversification of the genus, although the relative influences of each variable are not equal.