Network motifs involving both competition and facilitation predict biodiversity in alpine plant communities

Abstract

Biological diversity depends on multiple, cooccurring ecological interactions. However, most studies focus on one interaction type at a time, leaving community ecologists unsure of how positive and negative associations among species combine to influence biodiversity patterns. Using surveys of plant populations in alpine communities worldwide, we explore patterns of positive and negative associations among triads of species (modules) and their relationship to local biodiversity. Three modules, each incorporating both positive and negative associations, were overrepresented, thus acting as "network motifs." Furthermore, the overrepresentation of these network motifs is positively linked to species diversity globally. A theoretical model illustrates that these network motifs, based on competition between facilitated species or facilitation between inferior competitors, increase local persistence. Our findings suggest that the interplay of competition and facilitation is crucial for maintaining biodiversity.

Keywords: biodiversity change; community ecology; ecological networks; mountain ecosystems; plant interaction networks.

Fig. 1.

Global map of alpine plant networks studied here. Red dots on the map indicate the spatial location of the networks, with a few networks plotted for reference. In the networks, green dots represent plant species, and blue and red arrows represent negative- and positive species associations, respectively. Dot size is proportional to species abundance. The four network modules analyzed here are represented at the bottom of the figure, from left to right: intransitive competition, facilitation-driven competition, and competition-driven facilitation 1 and 2.

Fig. 2.

Positive and negative species associations and overrepresentation of network modules. (A) The density distribution (y axis) of pairwise species associations ${\alpha }_{ij}$ (x axis) across the networks. (B) Overrepresentation (z score, y axis) of the four modules studied here (x axis). Data points, median lines, and quartiles are shown. While intransitive competition does not occur more often than expected by chance, the other three modules involving both facilitation and competition are overrepresented and can therefore be considered as network motifs.

Fig. 3.

Biodiversity is associated with network motifs across alpine plant communities worldwide. (A–C) Relationship between biodiversity (average number of species per plot over a community, y axis) and overrepresentation of the three network motifs combining both positive and negative associations (z score, x axis). The figure shows estimated trends and 95% CI

Fig. 4.

Effects of network modules on local persistence. Mean persistence values (y axis) over time (x axis) are shown for each module (blue for intransitive competition, red for facilitation-driven competition, green for competition-driven facilitation 1, and yellow for competition-driven facilitation 2).