Complex floral traits shape pollinator attraction to ornamental plants

Abstract

Background and aims: Ornamental flowering plant species are often used in managed greenspaces to attract and support pollinator populations. In natural systems, selection by pollinators is hypothesized to result in convergent multimodal floral phenotypes that are more attractive to specific pollinator taxa. In contrast, ornamental cultivars are bred via artificial selection by humans, and exhibit diverse and distinct phenotypes. Despite their prevalence in managed habitats, the influence of cultivar phenotypic variation on plant attractiveness to pollinator taxa is not well resolved.

Methods: We used a combination of field and behavioural assays to evaluate how variation in floral visual, chemical and nutritional traits impacted overall attractiveness and visitation by pollinator taxonomic groups and bee species to 25 cultivars of five herbaceous perennial ornamental plant genera.

Key results: Despite significant phenotypic variation, cultivars tended to attract a broad range of pollinator species. Nonetheless, at the level of insect order (bee, fly, butterfly, beetle), attraction was generally modulated by traits consistent with the pollination syndrome hypothesis. At the level of bee species, the relative influence of traits on visitation varied across plant genera, with some floral phenotypes leading to a broadening of the visitor community, and others leading to exclusion of visitation by certain bee species.

Conclusions: Our results demonstrate how pollinator choice is mediated by complex multimodal floral signals. Importantly, the traits that had the greatest and most consistent effect on regulating pollinator attraction were those that are commonly selected for in cultivar development. Though variation among cultivars in floral traits may limit the pollinator community by excluding certain species, it may also encourage interactions with generalist taxa to support pollinator diversity in managed landscapes.

Keywords: Agastache spp; Echinacea spp; Nepeta spp; Rudbeckia spp; Salvia nemorosa; bees; floral traits; ornamentals; plant–pollinator interactions; pollination syndromes; pollinators.

Fig. 1.

Diagram of trait collections for floral area, plant height, flower colour and corolla morphology.

Fig. 2.

Means of trait measurements for each cultivar based on emmeans from LM and LMM regression analyses. Letters denote significant differences among cultivars within plant genera based on Tukey post-hoc test results at P < 0.05. Cultivars within all plant genera vary in some traits, with the greatest variation in flower colour, plant size and corolla morphology. Cultivars with a star over the colour point are those where a UV patten was detected in reflectance measures. ‘Sym’ refers to flower symmetry (either zygomorphic or actinomorphic). Nectar F + G:S refers to the nectar monosaccharide to disaccharide ratios. Pollen amount is measured by weighing dry tissue mass of collected samples before analysis.

Fig. 3.

Simpson’s diversity, log emissions rate of each VOC compound, and total volatile log total emissions rate for each cultivar. Letters denote significant variation among cultivars within genera. Cultivars within some genera differ significantly in emissions rate and diversity, but most genera do not. Cultivars within many plant genera share similar VOC profiles, but some genera emit compounds unique from the rest.

Fig. 4.

Abundance of pollinator visitors by taxonomic group and by bee species to cultivars across sites. Bee morphological traits are listed next to the species name. To avoid biasing results with ‘rare’ taxa, any taxonomic group with fewer than five total visitors to a cultivar across the whole season were removed and any bee species with fewer than four visitors to a cultivar across the whole season were removed. Bees are the primary visitors to cultivars, but cultivars vary in the bee taxa attracted and the functional traits of those species.

Fig. 5.

Biotic associations between pollinator taxonomic groups, bee species and floral phenotypic traits based on interpretation of linear regression and NMDS analyses. Across all plant genera, floral area acts as a general and long-distance attractant to all pollinator taxa, while traits such as height, colour, corolla morphology, nutrition and scent operate across diverse spatial scales to shape the visiting pollinator community based on insect natural history, morphology and learned associations. While these associations can be broadly used as guidelines for creating pollinator habitat in gardens, there was a high degree of generalization across pollinator taxa, particularly among bee visitors. At the level of bee species, variation in cultivar phenotype led to novel associations with some bee species and excluded others, suggesting that cultivar development can influence the attractiveness or accessibility of these varieties to a pollinator community. Furthermore, pollinator species’ response to trait variation was specific to each plant genus, supporting previous studies and demonstrating the multimodal nature of floral visual and chemical traits.

Fig. 6.

Emmeans of probability of Bombus impatiens visitation to cultivars in the field and in choice assays. Forager preferences in choice assays are consistent with those in the field, suggesting that variation in cultivar phenotype is relevant to species-level attraction even in the absence of community and landscape variables.