An empirical attack tolerance test alters the structure and species richness of plant–pollinator networks

Abstract

Ecological network theory hypothesizes that the structuring of species interactions can convey stability to the system. Investigating how these structures react to species loss is fundamental for understanding network disassembly or their robustness. However, this topic has mainly been studied in‐silico so far. Here, in an experimental manipulation, we sequentially removed four generalist plants from real plant–pollinator networks. We explored the effects on, and drivers of, species and interaction disappearance, network structure and interaction rewiring. First, we compared both the local extinctions of species and interactions and the observed network indices with those expected from three co‐extinction models. Second, we investigated the trends in network indices and rewiring rate after plant removal and the pollinator tendency at establishing novel links in relation to their proportional visitation to the removed plants. Furthermore, we explored the underlying drivers of network assembly with probability matrices based on ecological traits. Our results indicate that the cumulative local extinctions of species and interactions increased faster with generalist plant loss than what was expected by co‐extinction models, which predicted the survival or disappearance of many species incorrectly, and the observed network indices were lowly correlated to those predicted by co‐extinction models. Furthermore, the real networks reacted in complex ways to plant removal. First, network nestedness decreased and modularity increased. Second, although species abundance was a main assembly rule, opportunistic random interactions and structural unpredictability emerged as plants were removed. Both these reactions could indicate network instability and fragility. Other results showed network reorganization, as rewiring rate was high and asymmetries between network levels emerged as plants increased their centrality. Moreover, the generalist pollinators that had frequently visited both the plants targeted of removal and the non‐target plants tended to establish novel links more than who either had only visited the removal plants or avoided to do so. With the experimental manipulation of real networks, our study shows that despite their reorganizational ability, plant–pollinator networks changed towards a more fragile state when generalist plants are lost. A free Plain Language Summary can be found within the Supporting Information of this article.

Removing flowers of a generalist plant changes pollinator visitation, composition, and interaction network structure

“…Rare and more specialized species are hypothesized to be the most susceptible to ecological disturbances (Burkle et al, 2013;Mathiasson & Rehan, 2020). However, more generalized species are also susceptible to ecological disturbances (Inouye, 2008;Renjifo, 1999), although less common, and the loss of generalist species from interaction networks may have disproportionately strong effects on the remaining species and their interactions (Biella et al, 2020;Memmott et al, 2004;Palacio et al, 2016). In particular, generalist plant species are often abundant and well connected within pollination networks (Fort et al, 2016;Ollerton et al, 2007;Waser et al, 1996).…”

Patterns of pollination interactions at the community level are related to the type and quantity of floral resources

“…Asterisks indicate values of nectar volume derived from the literature (Petanidou & Smets, 1995). Cases of missing nectar or of uncollectible pollen were indicated with a dot Since descriptors of network structure and robustness can be sensitive to the network size (total number of plant species total number of pollinator species), we included this term in the models to account for its possible effects (Biella et al, 2020;Olesen & Jordano, 2002).…”

“…Plant-pollinator interactions are crucial for maintaining the biodiversity and functionality of terrestrial ecosystems. A fundamental question in community ecology is how the quantity of floral resources affects pollinator activity (Akter et al, 2017;Klumpers et al, 2019) and how this relates to the robustness of pollination interactions (Biella et al, 2020). So far, the issue has been mainly addressed at the species level.…”

Harnessing the Power of Metabarcoding in the Ecological Interpretation of Plant-Pollinator DNA Data: Strategies and Consequences of Filtering Approaches

“…Furthermore, at the level of a single individual pollinator, the Connectivity index was calculated. This index of centrality quantifies the putative central role of an individual or of a species in connecting different parts of the whole network [75]. It could provide information in ranking individuals or species according to their contribution to the stability of the interactions and the cohesion among network participants.…”