Exotic plants are ubiquitous in gardens, where they are usually deliberately planted, for example to achieve a continuous flowering over the entire growing season. As at least in Central Europe, relatively few native plants flower in the end of the season, exotic plants often dominate gardens in late summer and autumn.
In a study freshly published in Oecologia (Open Access Link), we (Michael Staab, Helena Maria Pereira-Peixoto, Alexandra-Maria Klein) investigated if seasonal changes in the availability of flowers from native vs. exotic plant species affect flower visits, visitor diversity and plant-pollinator interaction networks.
While flower visitor species richness decreased over the course of the season, gardens with higher proportions of flowering exotic plants relative to natives partly compensated the seasonal decrease in flower-visitor species richness. Plant-pollinator interaction networks were not influenced by exotic species. Thus, later in the season when only few native plants were flowering, exotic garden plants have at least partly substituted native flower resources without apparent influence on plant-pollinator network structure.
This research suggests that as long as exotic plants are appropriately managed and risk of naturalization is minimized, late-flowering exotic garden plants may provide floral resources to support native pollinators when native plants are scarce.

Seasonal changes in plant-pollinator networks and related quantitative network indices. (a) Exemplary bipartite plant-pollinator interaction networks from a single garden. Width of bars corresponds to flower cover per plant species in the lower and to the number of visits by each flower-visiting species in the higher level (species identities can be resolved with the numerical codes in Table S1 of the publication); width of arrows corresponds to the number of interactions between two species, with the most narrow bars and arrows indicating single interactions each (note that the number of interactions varied: April=229, June=20, August=33, October=13). Arrows narrowing from top to bottom indicate that a plant species was more often visited than expected solely from the cover of this plant among all plants. In turn, arrows that widen from top to bottom indicate relatively less visited plant species. For plants, light grey bars and arrows indicate interactions of native plant species and red of exotic species, respectively. Non-visited plant species are included but do not have any interactions. While in April, most flowers were from native plants, this changed over the season and in October all interactions in the exemplary garden were with exotic plants. Both, (b) the Shannon diversity of species interactions and (c) the linkage density of networks peaked in summer. Regression lines in (b) and (c) indicate the bootstrapped (n=1000) predictions of quadratic lmms (significant at p<0.01) with 95% CI (dashed lines).
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