Biogeographical network analysis of plant species distribution in the Mediterranean region

Abstract

The delimitation of bioregions helps to understand historical and ecological drivers of species distribution. In this work, we performed a network analysis of the spatial distribution patterns of plants in south of France (Languedoc-Roussillon and Provence-Alpes-Côte d'Azur) to analyze the biogeographical structure of the French Mediterranean flora at different scales. We used a network approach to identify and characterize biogeographical regions, based on a large database containing 2.5 million of geolocalized plant records corresponding to more than 3,500 plant species. This methodology is performed following five steps, from the biogeographical bipartite network construction to the identification of biogeographical regions under the form of spatial network communities, the analysis of their interactions, and the identification of clusters of plant species based on the species contribution to the biogeographical regions. First, we identified two sub-networks that distinguish Mediterranean and temperate biota. Then, we separated eight statistically significant bioregions that present a complex spatial structure. Some of them are spatially well delimited and match with particular geological entities. On the other hand, fuzzy transitions arise between adjacent bioregions that share a common geological setting, but are spread along a climatic gradient. The proposed network approach illustrates the biogeographical structure of the flora in southern France and provides precise insights into the relationships between bioregions. This approach sheds light on ecological drivers shaping the distribution of Mediterranean biota: The interplay between a climatic gradient and geological substrate shapes biodiversity patterns. Finally, this work exemplifies why fragmented distributions are common in the Mediterranean region, isolating groups of species that share a similar eco-evolutionary history.

Keywords: Mediterranean France; biogeographical regions; plant species; regionalization; spatial network; species network.

Figure 1

Distribution of the number of species per grid cell (l = 5 km). The inset shows a map of France including the studied area colored in red. An altitude map of the studied area is available in Supporting Information

Figure 2

Steps of the biogeographical network analysis. 1. Biogeographical bipartite network where grid cells and species are linked by the presence of a species (or a group of species) in a given grid cell during a certain time window. Note that there is no link between nodes belonging to the same set. 2. The bipartite network is then spatially projected by using a similarity measure of species composition between grid cells. Bioregions are then identified with a network community detection algorithm. 3. The test value matrix based on the contribution of species to bioregions is computed. 4. Then, a network of similarity between species is built, based on the test value matrix. Groups of species sharing similar spatial features are identified using a community detection algorithm. 5. Finally, a coarse‐grained biogeographical network unveiling the biogeographical structure of the studied area and the relationship between bioregions is obtained

Figure 3

Degree distributions of the biogeographical bipartite network. Probability density functions of the number of plant species per grid cell (in blue) and the number of cells covered per plant species (in red). Similar figures showing histograms instead of densities are available in Supporting Information Figure S13

Figure 4

Bioregions based on similarity in plant species (l = 5 km). Eight bioregions have been identified. 1. Gulf of Lion coast in red. 2. Cork oak zone in orange. 3. Mediterranean lowlands in light green. 4. Mediterranean border in dark green. 5. Cévennes sensu lato in purple. 6. Subatlantic mountains in pink. 7. Pre‐Alps and other medium mountains in yellow. 8. High mountains in brown. The inset shows a map of France including the studied areas colored in red. An altitude map of the studied area is available in Supporting Information Figure S14

Figure 5

Fraction of species contributing positively and significantly to a given number of bioregions (from 0 to 5 or more) as a function of the significance threshold. The vertical line represents the significance threshold δ = 1.96

Figure 6

Description of the groups of plant species. Box plot of test values according to the bioregions and the plant species groups. The horizontal line represents the significance threshold δ = 1.96. The number of plant species per group is available in Supporting Information Table S3

Figure 7

Network of interactions between bioregions. ${\lambda }_{j{j}^{\prime }}$, expressed herein percentage, represents the average fraction of contribution to cluster j′ of species that contribute significantly to cluster j. Only links with a value ${\lambda }_{j{j}^{\prime }}$ higher than 10% are shown