Frequency-dependent foraging by pollinators
Pollinators that forage in a frequency-dependent manner will exhibit flower constancy[2] for a certain morph, but the preferred floral type will be dependent on its frequency.
[8][9] Interest in frequency-dependent selection dates back to the time of Charles Darwin, who predicted that insects should demonstrate flower constancy[10] and puzzled over the occurrence of deceptive orchid species.
[12] Much less research has been conducted on frequency-dependent foraging on rewarding species, but experiments using bumblebees have illustrated that frequency likely plays a role in reproductive success of flowering plants.
[13] Researchers studying frequency-dependent visitation behavior seek to understand if pollinator preference is strong enough to induce fixation of traits or to maintain floral polymorphisms observed in natural populations.
[12] Even though these experiments demonstrate that bumblebees forage in a frequency-dependent manner, the strength of this response can be asymmetric for different colors.
Instead, morph frequency may need to be calculated over large spatial ranges to determine the extent to which pollinators are foraging in a frequency-dependent manner.
However, if rare morphs have similar nectar quality, skipping over these equally rewarding flowers appears to be inconsistent with optimal foraging theory.
Studies have demonstrated that the degree of frequency-dependence increases with the number of flowers visited, which suggests this is a learned response that develops gradually.
[4] Studies on other organisms have provided evidence that foraging can occur in long runs, but this preference develops after only visiting a few morphs.
[13] Pollinators appear to forage in a negative frequency-dependent manner when flowers do not provide nectar rewards, likely to avoid unrewarding morphs.
[29] As pollinators do not appear to be able to distinguish between rewarding and unrewarding flowers prior to landing,[30] they need to make test visits so they can learn to avoid particular morph types.
In other words, if deceptive species were to occur at a low enough frequency that pollinators do not encounter them very often, it is unlikely they will have the opportunity to relocate this information to their long-term memory.
[14] For example, many species of obligately animal-pollinated, deceptive orchids that co-occur with rewarding flowers are only reproductively successful when they occur at low frequencies.
Regardless of the mechanism, pollinators foraging in a frequency-dependent manner on common morphs will lead to assortative mating between similar phenotypes.
[33] Additionally, rare morphs may be at a disadvantage if reproductive success is correlated with number of pollinator visits, and this may lead to higher rates of selfing and ultimately inbreeding depression, in self-compatible plants.
In response to climate change, plants may begin to flower earlier in the season due to regional aridification and a rise in mean global temperature.
