A Community-level Study On Plant-pollinator Interactions In An Alpine Meadow

A traditional view of diverse floral traits is that they reflect specialization onto different pollinator groups. This adaptive explanation of floral diversity assumes different pollinators promote directional selection for different floral traits, which led to the formation of pollination syndrome hyposithsis, referring to suites of floral traits evolved in relation to the attraction and utilization of a specific group of animals as pollinators. However, the assumption of specialization and the role of pollinators in the evolution of floral traits have been questioned recently by broad community surveys concerning community context variation in plant-pollinator interactions, which suggest generalization may be as important as specialization in pollination systems. In this study, we mainly focused on the pattern of plant-pollinator interactions, as well as its cause and significance by a community study on plant-pollinator interactions at an alpine meadow in Hengduan Mountains, China over three consecutive years.First, to illustrate pollinator roles in floral traits, it is necessary to quantify temporal variation in the interactional frequency. Here, we investigated the relationships between 14 floral traits of 29 species and 10 pollinator functional groups at a meadow in Shangri-La Alpine Botanical Garden during summer 2006 to 2008. Simposn’s diversity index was used to estimate pollinator generalization level of each plant species by considering both pollinator components and their relative visitation frequency. Multivariate analyses showed that our trait combinations had significantly predictive power on the composition of pollinator groups consistently. Furthermore, 8 of the 10 pollinator groups showed constant preferences for at least two floral traits, leading to a relatively stable level of ecological generalization for most floral traits (2/3 categories), in spite of the generalization level of the entire community varied across years. These results suggest that the temporal stabilized preference of diverse pollinators may cotribute to the specialized or generalized floral traits respectively, providing initial evidence for the role of pollinators in the divergent evolution of floral traits.Second, pollinator-mediated stabilizing selection (PMSS) has been proposed as the driver of the evolutionary shift from radial to bilateral symmetry of flowers. Studies have shown that variation in flower size is lower in bilateral than in radial species, but whether bilateral flowers experience more stabilizing selection pressures by employing fewer, more specialized pollinators than radial flowers remains unclear. To test the PMSS hypothesis, we examine:(1) variance in flower size and level of ecological generalization (pollinator diversity calculated using functional groups)in 14 bilateral and 13 radial species and (2) the role pollinator diversity played in explaining the difference of variance in flower size between bilateral and radial species. Our data showed that bilateral species had less variance in flower size and were visited by fewer pollinator groups. Pollinator diversity accounted for up to 40 percent of the difference in variance in flower size between bilateral and radial species. The mediator effect of pollinator diversity on the relationship between floral symmetry and variance in flower size in the community is consistent with the PMSS hypothesis.Third, it is not uncommon only floral traits were used to deduce the nature of pollination systems of a plant. However, it is well known that the pollinator assemblage of particular plant species varies temporally and spatially, therefore the plant-pollinator interactions may be influenced by community context. We examined how ten variables of community context influenced the ecological generalization level and the composition of pollinators of seven plant species differing in generalization levels and main pollinators. Our community survey demonstrated that the composition of pollinators of all plant species, as well as the generalization level of six plant species were significantly affected by at least one variable of community context, despite variables related to them differed among plant species. Most variables of community context (e.g. number of flowers of the neighbouring species) were positively related to generalization level of the focal species by influencing visitations of the uncommon pollinators. Our results suggest that studies on pollinator-mediated selection on floral traits should thus be approached from a community context, considering the multi-specific interactions in which it is embedded.