Abstract
Visitation of plants by different pollinators depends on individual plant traits, spatial context, and other factors. A neglected aspect of small-scale variation of plant–pollinator interactions is the role of vertical position of flowers. We conducted a series of experiments to study vertical stratification of plant–pollinator interactions in a dry grassland. We observed flower visitors on cut inflorescences of Centaurea scabiosa and Inula salicina placed at different heights above ground in two types of surrounding vegetation: short and tall. Even at such a small-scale, we detected significant shift in total visitation rate of inflorescences in response to their vertical position. In short vegetation, inflorescences close to the ground were visited more frequently, while in tall vegetation, inflorescences placed higher received more visits. Moreover, we found major differences in the composition of the pollinator community on flowers at different heights. In a second experiment, we measured flower visitation rate in inflorescences of Salvia verticillata of variable height. Total flower visitation rate increased markedly with inflorescence height in this case. Data on seed set of individual plants provide evidence for a corresponding positive pollinator-mediated selection on increased inflorescence height. Overall, our results demonstrate strong vertical stratification of plant–pollinator interactions at the scale of mere decimetres. This may have important ecological as well as evolutionary implications.
Highlights
Interactions between plants and their pollinators play an important role in the evolution (Grant & Grant, 1965; Bronstein, Alarcon & Geber, 2006; Suchan & Alvarez, 2015) and maintenance of biodiversity (Bascompte et al, 2003; Bascompte, Jordano & Olesen, 2006; Bascompte & Jordano, 2007) in terrestrial ecosystems
We focused on the effect of inflorescence height for flower visitation rate and its consequences for seed set in S. verticillata at the same site
Analysis using generalized additive models (GAM) (Table 1) showed that the relationship was significant in C. scabiosa as well as in I. salicina in both short and tall vegetation (Table 1)
Summary
Interactions between plants and their pollinators play an important role in the evolution (Grant & Grant, 1965; Bronstein, Alarcon & Geber, 2006; Suchan & Alvarez, 2015) and maintenance of biodiversity (Bascompte et al, 2003; Bascompte, Jordano & Olesen, 2006; Bascompte & Jordano, 2007) in terrestrial ecosystems. Spatial variation in plant–pollinator interactions is observed from continental scales across the entire distributional range of a plant species (Espındola, Pellissier & Alvarez, 2011), down to small habitat patches and individual plants (Ohashi & Yahara, 1998; Dupont et al, 2014; Akter, Biella & Klecka, 2017). Inflorescence height is highly variable at the intraspecific level and may be important in driving visitation of individual plants. In a trait-based analysis of a plant-flower visitor network in a German grassland, Junker et al (2013) found that inflorescence height was the most important species trait after phenology to explain which plant species were visited by which insects. Studies investigating the effects of inflorescence height at the intraspecific level found that inflorescence height is under significant pollinator-mediated selection (Sletvold, Grindeland & Agren, 2010; Jiang & Li, 2017; Trunschke, Sletvold & Agren, 2017), these studies did not include direct observations of flower visitors
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