Beta diversity of plant-pollinator networks and the spatial turnover of pairwise interactions

Abstract

Interactions between species form complex networks that vary across space and time. Even without spatial or temporal constraints mutualistic pairwise interactions may vary, or rewire, across space but this variability is not well understood. Here, we quantify the beta diversity of species and interactions and test factors influencing the probability of turnover of pairwise interactions across space. We ask: 1) whether beta diversity of plants, pollinators, and interactions follow a similar trend across space, and 2) which interaction properties and site characteristics are related to the probability of turnover of pairwise interactions. Geographical distance was positively correlated with plant and interaction beta diversity. We find that locally frequent interactions are more consistent across space and that local flower abundance is important for the realization of pairwise interactions. While the identity of pairwise interactions is highly variable across space, some species-pairs form interactions that are locally frequent and spatially consistent. Such interactions represent cornerstones of interacting communities and deserve special attention from ecologists and conservation planners alike.

Figure 1. Site-pair comparison and interaction specific site-pair combinations.

A) Site-pair comparison. Site 1 and 2 each have unique plants and pollinators. The central square represents the interaction-matrix between shared species. Here, six interactions are present in both sites (interaction consistency, filled squares) and six interactions are only observed in one of the two sites (interaction turnover, open squares). Unique species to either site 1 or 2 were discarded and only the central matrix was used for analysing the turnover of pairwise interactions. B) Interaction specific site-pair combinations. This (hypothetical) interaction is observed at sites 1 and 6 (filled squares) while the species pair is also present at site 3, however without interacting (open square). One or both species are absent from the remaining sites (in grey) and they are excluded from the analysis for this particular interaction. Three site-pair combinations are possible in this case; 1↔3 and 6↔3: interaction turnover and 1↔6: no interaction turnover (interaction consistency).

Figure 2. Beta diversity of species and interactions and geographical distance.

Total interaction beta diversity β_WN (black), plant beta diversity β_Plants (green), beta diversity of interactions between shared species β_OS (red), and pollinator beta diversity β_Pollinators (blue) as a function of geographical distance between sites. All measures relate positively to geographical distance. Only for β_Pollinators the correlation was non-significant (β_WN: P = 0.002, R ² = 0.40, α = 0.02; β_OS: P = 0.047, R ² = 0.19, α = 0.03; β_Plants: P = 0.004, R ² = 0.37, α = 0.04; β_Pollinators: P = 0.086, R ² = 0.15, α = 0.01). Shaded areas delimit corresponding 95% confidence intervals.

Figure 3. Interaction frequency and interaction turnover.

Average interaction frequency is negatively related to the probability of interaction turnover. The more frequent interactions show lower probabilities of turnover between sites. Superimposed points result in darker marks.

Figure 4. Flower abundance and the probability of losing an interaction.

Whether pairwise interactions are realized or not is highly dependent on local flower abundance of the plant species. The figure shows the increasing probability of losing an interaction when going from a site with higher local flower abundance of the plant species to a site with lower local flower abundance. The larger the difference in local flower abundance, the stronger the effect. Superimposed points result in darker marks.