Abstract
The number of plants pollinated by ants is surprisingly low given the abundance of ants and the fact that they are common visitors of angiosperms. Generally ants are considered as nectar robbers that do not provide pollination service. We studied the pollination system of the endangered dry grassland forb Euphorbia seguieriana and found two ant species to be the most frequent visitors of its flowers. Workers of Formica cunicularia carried five times more pollen than smaller Tapinoma erraticum individuals, but significantly more viable pollen was recovered from the latter. Overall, the viability of pollen on ant cuticles was significantly lower (p < 0.001)—presumably an antibiotic effect of the metapleural gland secretion. A marking experiment suggested that ants were unlikely to facilitate outcrossing as workers repeatedly returned to the same individual plant. In open pollinated plants and when access was given exclusively to flying insects, fruit set was nearly 100%. In plants visited by ants only, roughly one third of flowers set fruit, and almost none set fruit when all insects were excluded. The germination rate of seeds from flowers pollinated by flying insects was 31 ± 7% in contrast to 1 ± 1% resulting from ant pollination. We conclude that inbreeding depression may be responsible for the very low germination rate in ant pollinated flowers and that ants, although the most frequent visitors, play a negligible or even deleterious role in the reproduction of E. seguieriana. Our study reiterates the need to investigate plant fitness effects beyond seed set in order to confirm ant-plant mutualisms.
Highlights
Ants are frequent visitors of angiosperms, and they may be involved in mutualistic interactions that benefit the plant such as seed dispersal or the removal of herbivorous insects, ants have traditionally been regarded as poor pollinators (Beattie, 2006; Rico-Gray & Oliveira, 2007; Rostás & Tautz, 2011)
The two ant species that foraged on E. seguieriana accounted for 65%
We found that two ant species were the most frequent visitors of E. seguieriana flowers and accounted for two thirds of all recorded insects
Summary
Ants are frequent visitors of angiosperms, and they may be involved in mutualistic interactions that benefit the plant such as seed dispersal or the removal of herbivorous insects, ants have traditionally been regarded as poor pollinators (Beattie, 2006; Rico-Gray & Oliveira, 2007; Rostás & Tautz, 2011). The main arguments against ants as vectors as facilitators of pollination in general are their foraging patterns and the presence of metapleural gland secretion on their integument. This secretion protects the ants from infection by microorganisms but may inadvertently have negative effects on the viability of pollen, except in some plants where ants are the primary pollinators (Beattie et al, 1984; Beattie et al, 1986; De Vega et al, 2009). Dutton & Frederickson (2012) compared several ant and plant species from temperate and tropical habitats and found reduced pollen germination in all cases with more pronounced effects in tropical ants, where microbes are expected to impose stronger selection for antibiotic defences This secretion protects the ants from infection by microorganisms but may inadvertently have negative effects on the viability of pollen, except in some plants where ants are the primary pollinators (Beattie et al, 1984; Beattie et al, 1986; De Vega et al, 2009). Dutton & Frederickson (2012) compared several ant and plant species from temperate and tropical habitats and found reduced pollen germination in all cases with more pronounced effects in tropical ants, where microbes are expected to impose stronger selection for antibiotic defences
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