The dimensionality of niche space allows bounded and unbounded processes to jointly influence diversification - PubMed (original) (raw)

The dimensionality of niche space allows bounded and unbounded processes to jointly influence diversification

Matthew J Larcombe et al. Nat Commun. 2018.

Abstract

There are two prominent and competing hypotheses that disagree about the effect of competition on diversification processes. The first, the bounded hypothesis, suggests that species diversity is limited (bounded) by competition between species for finite ecological niche space. The second, the unbounded hypothesis, proposes that innovations associated with evolution render competition unimportant over macroevolutionary timescales. Here we use phylogenetically structured niche modelling to show that processes consistent with both of these diversification models drive species accumulation in conifers. In agreement with the bounded hypothesis, niche competition constrained diversification, and in line with the unbounded hypothesis, niche evolution and partitioning promoted diversification. We then analyse niche traits to show that these diversification enhancing and inhibiting processes can occur simultaneously on different niche dimensions. Together these results suggest a new hypothesis for lineage diversification based on the multi-dimensional nature of ecological niches that can accommodate both bounded and unbounded evolutionary processes.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1

Fig. 1

Conifer species richness. Global species richness patterns in 455 conifer species based on cleaned empirical distribution data used here to analyse diversification processes. See Supplementary Fig. 1 for equivalent map of all 600 recognised living conifer species

Fig. 2

Fig. 2

Associations between species richness and diversification metrics. Scatter plots between clade species richness and selected clade metrics for two divisions of the conifer phylogeny into a 10 large clades and b 42 smaller clades. Straight lines indicate significant linear effects detected using phylogenetic generalized least squares (PGLS) regressions. The presence of multiple correlations made interpretation difficult; for this reason, we performed a path analysis (Fig. 3)

Fig. 3

Fig. 3

Path analysis of variation in conifer species richness. Bayesian 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)

Fig. 4

Fig. 4

Extinction driven reduction in clade niche size. Example of how niche partitioning combined with extinction associated with allee effects and/or competition, can result in a negative relationship between clade niche size and species richness as found in Fig. 3a. Different coloured curves represent species

Fig. 5

Fig. 5

Phylogenetic independent contrasts of niche evolution rate. Left: clade-level conifer phylogeny showing terminal nodes (4–9) in bold black, internal nodes (1–3) in grey (not consider in the contrasts). Taxonomic information about the groups, as well as their clade number and species richness (Clade #, n = species richness) are given at the tips. Right: Contrasts between the terminal nodes. The vertical bars correspond to the terminal nodes (labelled once in bold to match those on the tree), and the coloured arcs show the difference in mean niche evolution rate between the nodes (calculated by subtracting the mean of the top node from the mean of the bottom node). For example the red line joining nodes 5 and 4 indicates that the niche evolution rate at node 5 is 1.3 lower than node 4. Significance is indicated by line thickness and type. The pattern that emerges is that the nodes which give rise to clades with high species richness (e.g., nodes 4 and 9) have significantly higher niche evolution rates than nodes giving rise to clades with low species richness (with the exception of node 6, see Results and Discussion). For further details on the contrasts see Supplementary Table 1

Fig. 6

Fig. 6

Niche dimensionality in Pinus. Phylogenies of Clade 7 (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 (see Results and Discussion). The filled circle on trapezoid and step 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

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