Functional flower traits and their diversity drive pollinator visitation
Abstract
Recent studies have shown that the diversity of flowering plants can enhance pollinator richness and visitation frequency and thereby increase the resilience of pollination. It is assumed that flower traits explain these effects, but it is still unclear which flower traits are responsible, and knowing that, if pollinator richness and visitation frequency are more driven by mass‐ratio effects (mean trait values) or by trait diversity. Here, we analyse a three‐year data set of pollinator observations collected in a European grassland plant diversity experiment (The Jena experiment). The data entail comprehensive flower trait measurements, including reward traits (nectar and pollen amount), morphological traits (height, symmetry, area, colour spectra) and chemical traits (nectar‐amino acid and nectar‐sugar concentration). We test if pollinator species richness and visitation frequency of flower communities depend on overall functional diversity combining all flower traits within a community, single trait diversities (within trait variation) and community‐weighted means of the single traits, using Bayesian inference. Overall functional diversity did not affect pollinator species richness, but reduced visitation frequency. When looking at individual flower traits separately, we found that single trait diversity of flower reflectance and flower morphology were important predictors of pollinator visitation frequency. Moreover, independent of total flower abundance, community‐weighted means of flower height, area, reflectance, nectar‐sugar concentration and nectar‐amino acid concentration strongly affected both pollinator species richness and visitation frequency. Our results, challenge the idea that functional diversity always positively affects ecosystem functions. Nonetheless, we demonstrate that both single trait diversity and mass‐ratio effects of flower traits play an important role for diverse and frequent flower visits, which underlines the functionality of flower traits for pollination services.
What the paper shows and why it matters
(AI-generated)
Diverse plant communities can attract more pollinators, and varied flower traits are assumed to explain this, as many kinds of flowers should let more pollinator species coexist. Functional diversity, the spread of trait values in a community, is therefore expected to raise ecosystem functions. A rival view, the mass-ratio hypothesis, holds that the most abundant trait values, summarised as community-weighted means, drive a function instead. The paper argues that both should be tested in one analysis, which few studies do. It questions the assumption that higher functional diversity always benefits an ecosystem function, here pollinator richness and visitation.
The study finds that functional diversity of flowers does not always raise pollinator visitation: in a community of mostly generalist visitors, flower communities with more varied traits received fewer visits and no more pollinator species. It combined three years of pollinator observations in the Jena grassland experiment with 13 measured flower traits. Bayesian models separated the diversity of all traits combined, the diversity of single traits and the community-weighted mean of each trait, with flower cover as a control. Overall trait diversity lowered visitation frequency, though with wide uncertainty, and did not affect pollinator richness. Visits and richness instead followed dominant trait values: tall or large flowers went with more visitors, high amino acid concentration and strong green reflectance with fewer. Single-trait diversity affected visits only: diversity in reflectance raised them, diversity in flower morphology lowered them.
The paper reads this as a caution against assuming that functional diversity always benefits an ecosystem function. It suggests that generalist pollinators lose efficiency when flowers differ in traits such as height and shape, and that many bees stay constant to one flower type, so varied communities are less attractive. It expects the patterns could reverse where specialised pollinators are more common, as a simulation by Junker et al. (2015), in which functional diversity raised pollinator richness, suggests. The negative diversity effect contrasts with the positive effect of plant species richness found earlier in the same experiment (Ebeling et al. 2008), which the paper attributes to that analysis pooling surveys over time. Consequences for crop monocultures and diverse grasslands are left to future studies. As the paper notes, nectar was not sampled alongside the observations, so the amino acid result could be biased.
References
- Junker et al. (2015). Functional and phylogenetic diversity of plant communities differently affect the structure of flower-visitor interactions and reveal convergences in floral traits. Evolutionary Ecology 29, 437–450. doi:10.1007/s10682-014-9747-2
- Ebeling et al. (2008). How does plant richness affect pollinator richness and temporal stability of flower visits? Oikos 117, 1808–1815. doi:10.1111/j.1600-0706.2008.16819.x