Though the vast majority of all flowering plants depend on animals, and in particular insects, for pollination, and despite the global nature of ant-plant interactions, ants are poorly represented as pollinators and assumed to be primarily nectar thieves. Several characteristics of ants make them poor candidates for pollinator relationships: they are often too small to trigger floral mechanisms properly or carry pollen over significant distances; their aggressive nature may scare off other pollinators; they self-clean frequently, removing deposited pollen; and their antimicrobial secretions lower pollen viability on contact.
A threatened species of the Proteaceae, Conospermum undulatum, has been observed to have a high number of ants as floral visitors, leading to speculation that the ants may be fulfilling the role of pollinator in addition to their established role as seed dispersers. The plant occurs in the Mediterranean-type habitat of the sandplains of southwestern Australia, where it is possible that selection for antimicrobial secretions may be lower due to the dry climate.
In a recent article published in Annals of Botany, lead author Nicola Delnevo and colleagues set out to determine whether ants are an important pollinator of C. undulatum. The group tested pollen germination rates after exposure to ants to assess whether the pollen is resistant to damage and looked at whether ants can carry a sufficient pollen load to enable pollination. They also carried out exclusion experiments barring either ants or winged insects and quantifying resulting changes in pollination rates.
The researchers found that ants served as an important secondary pollinator for C. undulatum, after the native bee Leioproctus conospermi, which appears to be a highly specialized principal pollinator for the plant. The ants carried a great enough pollen load to get the job done, and when winged insects were excluded during flowering, ant-only pollination still produced almost 63% of the control group seed-set.
Conospermum undulatum plants seem to have adapted their pollen grains to withstand the ants’ secretions, as their germination rates were not significantly inhibited, nor were several others of their genus. Conversely, the germination rates of pollen grains from five other unrelated plant species flowering nearby at the same time were severely reduced after contact with the ants, indicating that the dry climate did not affect ant usage of antimicrobial compounds. Conospermum is known to have exceptionally fast pollen tube growth, which may provide a clue as to how the plants resist damage to their pollen by the ants’ secretions.
Conospermum undulatum lacks the features associated with the limited documented cases of ant pollination, such as small, open flowers, a small amount of pollen, and easily-accessible nectaries. Further study will be needed to better understand this system and how the plant may have co-evolved to allow pollination by ants despite their secretions. “This [result] highlights the complexity of ant–flower interactions and reinforces the fact that our understanding of these systems is still in its infancy,” write the authors. “In addition, we demonstrated that C. undulatum has a highly specialized pollination mutualism with a native Leioproctus bee. Identification of such specific pollination associations are important for management of threatened species to ensure maintenance of effective pollination services to ensure long-term population viability.”