Do alpine plants facilitate each other’s pollination? Experiments at a small spatial scale

Abstract

Short growing seasons, low temperatures, and frequent strong wind classify high mountains as adverse
environments, in which pollinator abundance and activity are reduced. In such environments, plants
growing in dense stands comprising several species and thus exhibiting larger and more diverse flower
displays may profit by attracting more visits from scarce alpine pollinators than do plants that grow
alone or in patches only composed of conspecifics. To study whether aggregation of plants increases
(facilitation) or decreases (competition) the attraction of pollinators, we measured the rate and numbers
with which insects entered experimental plots in the Swiss Alps, and their behaviour at flowers in plots
that they entered. The plots contained individuals of the blue-flowering cushion plant Eritrichium nanum,
either alone or mixed with white- to yellowish-flowering Saxifraga species. Pollinators were generally
rare: in 55% of 236 observation periods, no pollinators were observed. Over 95% of the pollinators were
Diptera. The average probability of observing any insect at all was higher in plots that contained some
Saxifraga flowers, including mixed plots, than in those containing only E. nanum flowers. However,
although insects tended to choose Saxifraga as the first flower visited in mixed plots, in all other regards
their visitation of Saxifraga and E. nanum flowers in such plots was statistically indistinguishable.We also
detected no effect of floral neighbourhood on the frequencies of potentially geitonogamous visits or of
transitions among individual plants of the same or different species. Thus, our study suggests that the
presence of Saxifraga may facilitate visitation to E. nanum at larger spatial scales, but gives no evidence
for either competition or facilitation at small scales within floral neighbourhoods.