Divergence in floral trait preferences between nonflower‐specialized birds and insects on the Galápagos

Abstract

PREMISE OF THE STUDY:The characteristic scarcity of insects on remote oceanic islands has driven nonflower-specialized vertebrates to broaden their trophic niches and explore floral resources. From our previous studies in the Galápagos, we know that native insectivorous and frugivorous birds visit a wide range of entomophilous flowers and can also act as effective pollinators. Here, we tested whether opportunistic Galápagos birds show any preference for specific floral traits, and if so, this preference differs from that of insects. METHODS:Sixteen floral morphology and nectar traits of 26 native species were studied, as well as the frequency with which they are visited by birds and insects. Nonmetric multidimensional scaling (NMDS) was used to evaluate the distribution of flower traits values along two main dimensions and measure the similarity between the plants visited mostly by birds versus those by insects.KEY RESULTS: NMDS of floral traits resulted in two species groups: (1) bell-shaped, white flowers with wider corollas at nectary level and higher nectar volume, associated with high bird visitation rates; and (2) bowl and tubular-shaped flowers with narrower corollas at nectary level and lower nectar volume, associated with high insect visitation rates. CONCLUSIONS:Despite the divergence in floral trait preferences between opportunistic Galápagos birds and insects, bird-visited flowers display mixed traits not fitting the classical ornithophilous syndrome. This finding is compatible with the existence of a transitional or bet-hedging phenotype between insect and bird visitors and underscores the importance of coevolution and floral diversification in nonspecialized plant-visitor interactions.

Plants are visited by more pollinator species than pollination syndromes predicted in an oceanic island community

“…This may be a result of limited pollination in small, isolated and depauperated island systems such as Yongxing Island. Our results supported the viewpoint of Hervías-Parejo et al 24 that pollination syndrome thinking was based on a instable foundation, particularly on islands where decreasing resources can weaken specialization and blur the syndrome boundaries.…”

“…Similar results were also found in community-level studies in Mediterranean climates of the northern hemisphere 63,64 and in Tasmania 12 . On tropical islands, pollination by generalist birds is common 46 , and flowering plants are frequently visited by non-flower-specialized birds which do not fit a typical ornithophilous syndrome 24 . Floral traits in most plant species on oceanic island may have the potential for adapting to new conditions under changing pollination environments 65,66 .…”

“…To our knowledge, no other studies have examined the relationship between floral traits and pollinators, i.e. testing the pollination syndrome concept in the oceanic island community, even if such data are very valuable 10 , 24 . In our study, following previous studies 48 , 68 , we presumed the visitors carried the host plant pollen grains as the potential pollinators (i.e.…”

“…However, other studies still caution its utility and predictability 12 – 20 . Furthermore, plant-pollinator interactions have proved to be more generalized than was previously thought in nature, thus, the utility of pollination syndromes in predicting the specialization in pollination system and the association of particular floral traits with specific pollinators have been questioned 1 , 21 – 24 . Although the usefulness of pollination syndromes has been extensively studied in different plant groups and geographic regions across the world, the debate about the reliability of pollination syndromes is ever existing.…”

“…A few studies have focused on the bird pollination syndrome on islands, for example, Biddick and Burns 10 found that the phenotypic trait matching between birds and flowering plants supported the pollination syndromes. Whereas plants evolved generalist bird pollination as an adaptation to birds as a reliable pollinator group 46 , and bird-visited flowers display mixed traits not fitting the classical ornithophilous syndrome on islands 24 . Therefore, studies dealing with the relationship between floral traits and different pollinator functional groups in oceanic island communities are urgently needed.…”