Flowers are visited by various kinds of organisms. Bees are the most commonly known pollinators, but they aren’t the only ones. Different kinds of animals, like flies, butterflies, moths, birds, and bats, with differing pollinating capabilities, may visit the same flower. Even bees of the same species may have different flower preferences according to their experience level. How do plants cope with such morphological and behavioral diversity in their pollinators?
This question has long intrigued floral biologists. The prevailing paradigm on this matter, referred to as “pollinator-mediated trade-off”, is that conflicting selection pressures from different organisms promote the specialization of floral traits that favor the most mutually beneficial interactions, leading to floral diversity. However, this implies that each flower species should only appeal to a particular pollinator group, whereas, in reality, flowers can be commonly seen to attract diverse groups of organisms.
In a new study, we address this anomaly by providing a conceptual framework for floral adaptation under trade-offs caused by different animals. We argue that there are overlooked possibilities that natural selection on floral traits could counter trade-offs and allow flowers to adapt simultaneously to diverse pollinators.
A prominent example of this is floral color change, or the alteration in color of old flowers that are unrewarding to foragers. Retaining old flowers could enhance floral display and lure opportunistic foragers like flies and inexperienced bees, while changing their color would favor the return of more experienced bees by making it easier for them to detect genuine signals from young, rewarding flowers. A field observation with two species of Weigela combined with a laboratory experiment with bumble bees and artificial flowersprovides validation of this hypothesis. It is, thus, possible for flowers to evolve traits that decrease trade-offs, leading to adaptive generalization for multiple groups of pollinators rather than specialization towards a specific pollinator group.
Our conceptual framework also suggests that specialization by exclusion, e.g., hummingbird flowers, could simply be a special case of trade-off mitigation. It occurs when less-efficient pollinators exploit pollen or ovules that are otherwise used for pollination by more efficient agents. This type of trade-off could also be mitigated, but only when pollinators have different activity time. For example, flowers of honeysuckle open at dusk and prioritize visits by hawkmoths, while postponing visits by pollen-wasteful bees to the next day.
Any strategy for trade-off mitigation occurs as novel combinations of floral traits. This might mean pollination syndromes, or sets of floral traits shared among distantly related angiosperm taxa, can arise not only from adaptive specialization for particular visitors, but also from adaptive generalization for particular visitor communities. Thus, our work offers a novel perspective on floral diversity in which diffuse interactions with various organisms are considered to enhance, rather than hinder, diversification.
The perspective of this study is also important in conservation, in that it emphasizes the importance of protecting the entire visitor community on a flower species, instead of an assumed specific pollinator group, to maintain its reproduction and phenotypic uniqueness. On a broader note, it could spur efforts to protect organisms under multi-species interactions.
Journal: Biological Reviews
Title: Trade-off mitigation: a conceptual framework for understanding floral adaptation in multispecies interactions
Authors: Kazuharu Ohashi, Andreas Jürgens, and James D. Thomson