Pollination networks from natural and anthropogenic-novel communities show high structural similarity

Insect pollination services in actively and spontaneously restored quarries converge differently to natural reference ecosystem

Ecological restoration has the potential to accelerate the recovery of biodiversity and ecosystem services in degraded ecosystems. However, current research queries whether active restoration is necessary. We evaluated plant-pollinator networks during spring at replicated sites within an actively restored quarry, at abandoned quarries undergoing spontaneous restoration, and within a natural reference area, to compare pollinator community composition and function. Overall, we aimed to assess which approach is more effective in rehabilitating pollination networks. We found that while both approaches allowed for the restoration of pollination function, active restoration provided faster recovery: pollination network structure was more similar to the reference ecosystem after 20-30 years of active restoration, than 40 years of natural succession in spontaneously restored areas. Different restoration approaches sustained distinct pollinator communities providing a similar service in different areas: honey bees played an important role in the natural area, bumblebees in the abandoned quarries and wild bees in the restored sites, suggesting a possible conflict between diverse wild bee communities and honey bee homogenized pollinator communities. In quarries, flower resource availability and diversity influenced networks' structural properties by constraining species interactions and composition. In spontaneously restored areas a rich herbaceous layer of ruderal species from early successional stages buffered against the shortage of flower resources at critical periods. Active restoration, though effective, should include practices that consider wild bee communities and mitigate flower resource scarcity. The use of "bridging" plants that flower in different periods, should be considered in active restoration programs to enhance the pollinator community.

Whole community invasions and the integration of novel ecosystems

The impact of invasion by a single non-native species on the function and structure of ecological communities can be significant, and the effects can become more drastic–and harder to predict–when multiple species invade as a group. Here we modify a dynamic Boolean model of plant-pollinator community assembly to consider the invasion of native communities by multiple invasive species that are selected either randomly or such that the invaders constitute a stable community. We show that, compared to random invasion, whole community invasion leads to final stable communities (where the initial process of species turnover has given way to a static or near-static set of species in the community) including both native and non-native species that are larger, more likely to retain native species, and which experience smaller changes to the topological measures of nestedness and connectance. We consider the relationship between the prevalence of mutualistic interactions among native and invasive species in the final stable communities and demonstrate that mutualistic interactions may act as a buffer against significant disruptions to the native community.

The variable effects of global change on insect mutualisms

Insect mutualisms are essential for reproduction of many plants, protection of plants and other insects, and provisioning of nutrients for insects. Disruption of these mutualisms by global change can have important implications for ecosystem processes. Here, we assess the general effects of global change on insect mutualisms, including the possible impacts on mutualistic networks. We find that the effects of global change on mutualisms are extremely variable, making broad patterns difficult to detect. We require studies focusing on changes in cost-benefit ratios, effects of partner dependency, and degree of specialization to further understand how global change will influence insect mutualism dynamics. We propose that rapid coevolution is one avenue by which mutualists can ameliorate the effects of global change.

Exotic garden plants partly substitute for native plants as resources for pollinators when native plants become seasonally scarce

Urban green spaces such as gardens often consist of native and exotic plant species, which provide pollen and nectar for flower-visiting insects. Although some exotic plants are readily visited by pollinators, it is unknown if and at which time of the season exotic garden plants may supplement or substitute for flower resources provided by native plants. To investigate if seasonal changes in flower availability from native vs. exotic plants affect flower visits, diversity and particularly plant–pollinator interaction networks, we studied flower-visiting insects over a whole growing season in 20 urban residential gardens in Germany. Over the course of the season, visits to native plants decreased, the proportion of flower visits to exotics increased, and flower-visitor species richness decreased. Yet, the decline in flower-visitor richness over the season was slowed in gardens with a relatively higher proportion of flowering exotic plants. This compensation was more positively linked to the proportion of exotic plant species than to the proportion of exotic flower cover. Plant–pollinator interaction networks were moderately specialized. Interactions were more complex in high summer, but interaction diversity, linkage density, and specialisation were not influenced by the proportion of exotic species. Thus, later in the season when few native plants flowered, exotic garden plants partly substituted for native flower resources without apparent influence on plant–pollinator network structure. Late-flowering garden plants support pollinator diversity in cities. If appropriately managed, and risk of naturalisation is minimized, late-flowering exotic plants may provide floral resources to support native pollinators when native plants are scarce.

Native and alien flower visitors differ in partner fidelity and network integration

Globalisation persistently fuels the establishment of non-native species outside their natural ranges. While alien plants have been intensively studied, little is known about alien flower visitors, and especially, how they integrate into natural communities. Here, we focus on mutualistic networks from five Galápagos islands to quantify whether alien and native flower visitors differ consistently in their pairwise interactions. We find that (1) alien flower visitors have more interaction partners and larger species strengths (i.e. plants are more connected to alien insects), (2) native insects tend to have higher partner fidelity as they deviate more from random partner utilisation, and iii) the difference between native and alien flower visitors in network integration intensifies with island degradation. Thus, native and alien flower visitors are not interchangeable, and alien establishment might have yet unforeseen consequences for the pairwise dynamics between plants and flower visitors on the Galápagos - especially on the heavily disturbed islands.