The relative roles of bounded and unbounded processes in conifer diversification
This project tries to demystify diversification dynamics in conifers, and evolved out of a chance meeting at small research group presentation. Araucaria araucana in Huerquehue National Park Chili. Photo Greg Jordan.
This paper evolved out of a project looking at the relationship between niche area and species richness in conifers. We wanted to use a process-based niche modelling tool developed by Steve1 in a phylogenetic context to look for relationships between niche size and diversification. We used GIBIF data and a phylogeny of the conifers and started looking at the relationships between clade species richness, clade niche area and clade age.
After a few months of playing around with this data I presented it at the University of Tasmania while on a research visit there in mid-2016. It was a meeting of the Eucalyptus Genetics Group, and was also attended by a few others who are interested in conifer evolution, including Greg Jordan. The talk generated quite a bit of discussion and Greg quickly identified a conceptual problem and also suggested a path analysis would be better than the independent regression analyses I had presented. This was immediately appealing because it would allow us to partition the relative importance of the different sources of variation. Greg also said “while you’re at it, I have better distribution data and a more complete phylogeny if you like?” (Figure 1). And that was the start of a productive collaboration.
Around this time I’d become interested in the ideas of Howard Cornell2 and Charles Marshall and Tiago Quental3 who were articulating clear hypotheses explaining how bounded (diversity-dependent) and unbounded (diversity-independent) processes might work together to shape diversification. And, during the process of developing our path model (as suggested by Greg), it dawned on us that we had all the elements necessary to test the importance of bounded/unbounded processes in conifers. Specifically, for 455 species we had data on: the niche size; niche and geographic overlap between species (expected competition); estimates of niche evolution rate for multiple physiological niche parameters; and clade ages. So we used this data to ask: is competition negatively associated with species richness, as predicted by the bounded diversification model; is niche evolution positively associated with clade species richness, as predicted by the unbounded hypothesis; and what effects do clade niche size and age have on species richness (Figure 2).
Figure 1. Species diversity based on cleaned species distribution data used to model physiological niche of 455 conifer species.
We found evidence for both of bounded and unbounded processes, and they were of similar importance in explaining the species richness patterns (Figure 2). We wanted to drill into this result and understand what ecological processes might facilitate these seemingly opposing forces, and found that niche dimensionality is a likely candidate. We showed that in subclades, one niche dimension, for example the minimum temperature needed for photosynthesis, could be actively evolving so as to promote diversification, while another niche dimension, such as the soil nitrogen level required for growth, is constrained by competition and potentially limiting diversification (Figure 3).
Figure 2. Path analysis showing the relative effects of niche and phylogenetic parameters on clade species richness for 455 conifer species in a) 10 large clades, and b) 42 smaller clades. Total effect size is shown in bold, while direct effects and their standard deviation are shown along the vertices. Solid lines indicate significant effects (95\% credible intervals not including zero)
We think that variation in niche dimensionality in space and time will result in periods when competitive process prevail and others when evolutionary process are shaping the accumulation of biodiversity.
Figure 3. Phylogenies of Pinus showing ancestral state reconstructions of the 11 most important niche dimensions in order of importance (a-k). The bottom right panel shows the same phylogeny with species names. Sub clades within Pinus with conservative (solid ellipse) and labile (dashed ellipse) niche dimensions are highlighted and discussed in the text. The filled circle on trapezoid and logistic diagrams beside the trait names, show how the trait relates to the modelled growth or resource acquisition function. For example, (a) is the point at which soil moisture causes a reduction in N uptake, that is, when waterlogging reduces N uptake.
The paper in Nature Communications is here: go.nature.com/2Ch7k55
1. Higgins, S.I., et al., A physiological analogy of the niche for projecting the potential distribution of plants. Journal of Biogeography, 2012. 39(12): p. 2132-2145.
2. Cornell, H.V., Is regional species diversity bounded or unbounded? Biological Reviews, 2013. 88(1): p. 140-165.
3. Marshall, C.R. and Quental, T.B., The uncertain role of diversity dependence in species diversification and the need to incorporate time-varying carrying capacities. Phil. Trans. R. Soc. B, 2016. 371(1691): p. 20150217.