Nectar palatability can selectively filter bird and insect visitors to coral tree flowers

Abstract

Secondary compounds in nectar may play a decisive role in determining the spectrum of floral visitors on plants. Flowers of the African coral tree Erythrina caffra are visited mainly by generalist passerine nectarivores, such as weavers and bulbuls. As the nectar of this species tastes very bitter to humans, it was hypothesized that secondary compounds may repel sunbirds and honeybees which are common in the same habitats yet seldom consume the nectar. We conducted choice tests using fresh nectar and both sucrose and hexose (glucose/fructose) solutions of the same concentration as the nectar. White-bellied Sunbirds (Cinnyris talatala) were repelled by nectar of both E. caffra and a related species Erythrina lysistemon, but Dark-capped Bulbuls (Pycnonotus tricolor) did not discriminate between the Erythrina nectar and control sugar solution in terms of amounts consumed. Honeybees (Apis mellifera scutellata) probed exposed droplets of E. caffra nectar and a control sugar solution at the same rate, suggesting that there is no volatile deterrent, but they immediately withdrew their proboscis far more often from the droplets of Erythrina nectar than they did from the sugar solution, suggesting that they find Ery-thrina nectar distasteful. These results contribute to a growing awareness that non-sugar components of nectar can play important functional roles in plant pollination systems.

Comparison of the ecology and evolution of plants with a generalist bird pollination system between continents and islands worldwide

“…These plant species in many cases have evolved mechanisms to deter or prevent ineffective pollinators such as bees from visiting their flowers since bees remain abundant year round in the tropics and subtropics (Abrahamczyk et al, ). Such adaptations include unfavourable flower positions (Toledo, ; Vicentini & Fischer, )**, closed flowers that require specific adaptations in order to open them (Etcheverry & Alemán, ) or bitter nectar (Johnson, Hargreaves & Brown, ; Nicolson et al, ).…”

Scent chemistry is key in the evolutionary transition between insect and mammal pollination in African pineapple lilies

“…Specialised pollination systems are mostly characterised by flowers with complex architecture, allowing only a specific group of flower visitors access to rewards (Johnson & Steiner, 2000). However, there is increasing evidence that the chemistry of scent and nectar can mediate specialisation in pollination systems by functioning as highly specific cues or filters (Johnson et al, 2006;Nicolson et al, 2015;Sch€ affler et al, 2015). These chemical traits appear to be particularly important in determining the visitor fauna in morphologically unspecialised flowers that potentially allow a broad range of animal visitors to reach nectar.…”

Oilseed rape (Brassica napus) as a resource for farmland insect pollinators: quantifying floral traits in conventional varieties and breeding systems

“…Lepidoptera (Jervis & Boggs, 2005;Lebeau et al, 2016). The composition of nectar has evolved to attract pollinators and plant defenders and to protect against nectar robbers and pathogens (Kessler et al, 2008;Heil, 2011;Nicolson et al, 2015), and although the precise composition of floral nectar varies within and between species (Burkle & Irwin, 2009;Baude et al, 2011), it generally comprises up to 80% w/w sugars (sucrose, glucose and fructose), with the remainder comprising amino acids and lipids, as well as complexes of secondary metabolites (Baker & Baker, 1983;Nicolson & Thornburg, 2007). While pollination ecology studies of the interactions between pollinators and nectar are well established, e.g.…”