Environmental niche divergence among three dune shrub sister species with parapatric distributions

Abstract

Background and aims: The geographical distributions of species are constrained by their ecological requirements. The aim of this work was to analyse the effects of environmental conditions, historical events and biogeographical constraints on the diversification of the three species of the western Mediterranean shrub genus Stauracanthus , which have a parapatric distribution in the Iberian Peninsula.

Methods: Ecological niche factor analysis and generalized linear models were used to measure the response of all Stauracanthus species to the environmental gradients and map their potential distributions in the Iberian Peninsula. The bioclimatic niche overlap between the three species was determined by using Schoener's index. The genetic differentiation of the Iberian and northern African populations of Stauracanthus species was characterized with GenalEx. The effects on genetic distances of the most important environmental drivers were assessed through Mantel tests and non-metric multidimensional scaling.

Key results: The three Stauracanthus species show remarkably similar responses to climatic conditions. This supports the idea that all members of this recently diversified clade retain common adaptations to climate and consequently high levels of climatic niche overlap. This contrasts with the diverse edaphic requirements of Stauracanthus species. The populations of the S. genistoides-spectabilis clade grow on Miocene and Pliocene fine-textured sedimentary soils, whereas S. boivinii , the more genetically distant species, occurs on older and more coarse-textured sedimentary substrates. These patterns of diversification are largely consistent with a stochastic process of geographical range expansion and fragmentation coupled with niche evolution in the context of spatially complex environmental fluctuations.

Conclusions: : The combined analysis of the distribution, realized environmental niche and phylogeographical relationships of parapatric species proposed in this work allows integration of the biogeographical, ecological and evolutionary processes driving the evolution of species adaptations and how they determine their current geographical ranges.

Keywords: Biogeography; Messinian salinity crisis; Stauracanthus; diversification; ecological niche factor analysis (ENFA); niche overlap; phylogeography; species distribution modelling.

Fig. 1

(A) Geographical range of Stauracanthus species in the Iberian Peninsula. (B) Total extent of occurrence of the genus. In (A) each symbol represents a 10×10 km UTM cell, according to the data compiled in this study; the shaded areas represent soils suitable for the genus, namely sedimentary soils.

Fig. 2

Responses of the three Stauracanthus species to the most significant bioclimatic variables determining their geographical distribution, according to ecological niche factor analysis (ENFA) and generalized linear model (GLM) analysis. Circles, cells without presences; triangles, S. boivinii; squares, S. genistoides; crosses, S. spectabilis.

Fig. 3

Overlap of habitat suitability (HS) maps obtained with ecological niche factor analysis (ENFA) using the set of four variables identified as the most relevant for the distribution of the three species (see text). (A–D) Overlap of HS maps between S. boivinii and S. genistoides (A), S. boivinii and S. spectabilis (B), S. genistoides and S. spectabilis (C) and the three species (D), after reclassification using the continuous Boyce index.

Fig. 4

Realized niches of the three Stauracanthus species in the climatic space available in the Iberian Peninsula. (A–C) Niches along the two first axes of the PCA in the Iberian Peninsula. (A) S. boivinii. (B) S. genistoides. (C) S. spectabilis. Grey shading shows the density of occurrences of each species by grid cell. Solid and dashed contour lines illustrate, respectively, 100 and 50 % of the available (background) environment. (D) Contribution of the climatic variables on the two axes of the PCA. PC1 (x axis) reflects a gradient between Mediterranean and Atlantic conditions in the Iberian Peninsula; PC2 (y axis) reproduces a gradient of continentality.

Fig. 5

Axes 1 and 2 of the two-dimensional non-metric multidimensional scaling ordinations of populations based on their pairwise genetic distances. The final stress value for the two-dimensional configuration was 0·08. Vectors fitted represent the substrate classes dominating the Stauracanthus populations: Miocene and Pliocene substrates (r² = 0·51, P< 0·01) and older and coarse-textured sedimentary substrates (r² = 0·66, P< 0·001). Triangles, S. boivinii; squares, S. genistoides and S. spectabilis; dark colours, Moroccan populations; pale colours, Iberian populations.