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

Abstract

Pollination is essential for ecosystem functioning, yet our understanding of the empirical consequences of species loss for plant–pollinator interactions remains limited. It is hypothesized that the loss of abundant and generalized (well‐connected) species from a pollination network will have a large effect on the remaining species and their interactions. However, to date, relatively few studies have experimentally removed species from their natural setting to address this hypothesis. We investigated the consequences of losing an abundant, generalist native species from a series of plant–pollinator networks by experimentally removing the flowers of Helianthella quinquenervis (Asteraceae) from half of a series of 10 paired plots (15 m diameter) within a subalpine ecosystem. We then asked how the localized loss of this species influenced patterns of pollinator visitation, floral visitor composition, and interaction network structure. The experimental removal of Helianthella flowers led to an overall decline in plot‐level pollinator visitation rates and shifts in pollinator composition. Species‐level responses to floral removal differed between the two other abundant, co‐flowering plants in our experiment: Potentilla pulcherrima received higher visitation rates, whereas Erigeron speciosus visitation rates did not change. Experimental floral removal altered the structural properties of the localized plant–pollinator networks such that they were more specialized, less nested, and less robust to further species loss. Such changes to interaction network structure were consistently driven more by species turnover than by interaction rewiring. Our findings suggest that the local loss of an abundant, well‐linked, generalist plant can bring about diverse responses within intact pollination networks, including potential competitive and facilitative effects for individual species, changes to network structure that may render them more sensitive to future change, but also numerous changes to interactions that may also suggest flexibility in response to species loss.

Plant–hummingbird pollination networks exhibit limited rewiring after experimental removal of a locally abundant plant species

“…Nevertheless, similar experiments have induced changes in foraging patterns and/or network structure after temporarily removing a single plant species from relatively small areas (≤0.05 ha) that likely represent a fraction of a pollinator's home range (e.g. Bain et al, 2022; Ferrero et al, 2013; Lopezaraiza–Mikel et al, 2007). For instance, Bain et al (2022) observed that bumblebees ( Bombus spp.)…”

“…Bain et al, 2022; Ferrero et al, 2013; Lopezaraiza–Mikel et al, 2007). For instance, Bain et al (2022) observed that bumblebees ( Bombus spp.) avoided the 15‐m diameter treatment plots where an abundant flower species had been removed—likely contributing to changes in network specialization within those plots—even though these exclusion areas were smaller than the average bumblebee foraging distance (generally 25–100 m in similar habitats: Elliott, 2009; Geib et al, 2015).…”

“…Species exclusions are logistically difficult to implement at appropriate spatial scales, and experimental manipulations of mutualistic networks have thus far yielded variable behavioural responses. In some studies, pollinators emigrated from the exclusion area(s) after removal of the most abundant plant species, leading to reduced floral visitation rates (Bain et al, 2022; Biella et al, 2019; Ferrero et al, 2013; Lopezaraiza–Mikel et al, 2007). In other studies, mutualists have persisted after losing an abundant plant resource by changing their dietary preferences and/or becoming more ecologically generalized (Costa et al, 2018; Goldstein & Zych, 2016; Kaiser‐Bunbury et al, 2017), perhaps reflecting a ‘stay and rewire’ strategy.…”

“…Pollinator observations at a small number of focal plants may not detect interactions occurring at broader spatial scales, but pollen networks can characterize the perspective of highly mobile pollinators visiting spatially dispersed flowers (Jordano, 2016; Ramírez‐Burbano et al, 2017). Our study is one of relatively few field experiments testing how mutualistic networks respond to plant species loss (<10 studies: Bain et al, 2022; Biella et al, 2019; Costa et al, 2018; Ferrero et al, 2013; Goldstein & Zych, 2016; Kaiser‐Bunbury et al, 2017; Lopezaraiza–Mikel et al, 2007) and the only one that combines a relatively high amount of replication (>10 replicates), a large spatial scale (~1 ha on average), and two different network sampling approaches.…”

Indirect interactions between pollinators drive interaction rewiring through space

“…Indeed, competition between pollinators for floral resources has been shown to affect pollinator behavior both under experimental (Brosi & Briggs, 2013) and field conditions (Magrach et al, 2017). This research has shown that the absence (Brosi & Briggs, 2013) or presence (Magrach et al, 2017) of a highly competitive pollinator species modifies the behavior and visitation patterns of other pollinator species in the community (Bain et al, 2022). However, a larger community‐level analysis of pollinator competitive interactions and how they influence pollinator preference patterns is currently lacking (Bartomeus et al, 2021).…”