The interaction between signal diversity and pollinator behaviour along a land use gradient
Final Report Abstract
We have found that land use intensity can be negatively associated with flower colour diversity and with shifts from grasslands being either dominated by blue (Schwäbische Alb) or yellow (Hainich-Dün) flower species to those being dominated by white flower species. This is the first time that such an effect of land use intensity has been documented. These results suggest that increasing land use intensity can have previously unknown effects on the visual communication between plants and their insect visitors. However, we documented that the effects on flower colour diversity are spatially and temporarily variable; they were found only found at Schwäbische Alb and only prior to mowing. At Schwäbische Alb, where land use affected floral colour diversity, we found furthermore that flower colours matched the spectral sensitivities of insect visitors. This result was consistent across insect visitor groups and documents the importance of visual communication between plants and insects. Together with the spatio-temporal overlap between the phenologies of insects and plants, visual matching contributed to explain the degree of generalisation in interactions with insects. At the two other Exploratories, flower colours did not match the spectral sensitivities of insects and did therefore not contribute to explain plant generalisation. These analyses show the spatial variability in the factors influencing plant-insect interactions. We found that flower colour rather than the quantity or quality of reward did explain the degree of specialisation of insect visitors. Thus, our data suggest that flower phenology, floral reflectance and morphology are more important in shaping flower–visitor interactions and thus also network structure than resource quantity. Based on theoretical considerations we posited that mimicry is an unlikely route to explain deception in plants in anthropogenically influenced habitats. Consistent with this conjecture, experiments in the Exploratories showed that both yellow and white flowers of 19 species received more insect visits and hence probably benefitted from common white flowers in the background (relative to common yellow flowers in the background). This result suggests that the observed change from yellow to white floral colour with increasing land use intensity in Hainich-Dün does not necessarily affect the pollination of grassland species negatively. It could lead to a facilitation effect of increased flower visitation. Overall, we have found that land use can affect floral colour diversity in complex and previously unknown ways. Two possible mechanisms could explain these effects proximally: 1) an overall decline in pollinator species and in particular in pollinator specialists could explain the observed prevalence of less specialised, white flowers in intensively used grasslands. 2) Pleiotropic interactions with white flower colours could explain why these dominate in intensively used grasslands. How such changes affect pollination, and thereby plant fitness, needs to be determined in the future. Particular care has to be taken as any effects of land use on flower colours are spatially and temporarily variable and generalisations are not yet warranted.
Publications
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Increasing land-use intensity decreases floral colour diversity of plant communities in temperate grasslands. Oecologia
Binkenstein J., Renoult J. P., Schaefer H. M.
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2009: Deception in plants: mimicry or perceptual exploitation? Trends in Ecology & Evolution 24: 676-685
Schaefer, H. M. & G. Ruxton
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(2010): Which flower traits explain visitor composition? A multivariate approach. Annual meeting of the Scandianvian Association of Pollination Ecology Oct. 21.-24., Tovetorp, Schweden
Junker, R., Brehm, T., Binkenstein, J., Blüthgen, N., Paulus, J. & Stang, M.
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(2011). Does floral colour determine the structure of plant communities in temperate grasslands? 41st Annual Conference of the Ecological Society of Germany, Austria and Switzerland Sep. 5.-9., Oldenburg, Germany
Binkenstein, J., Renoult, J. & Schaefer, H. M.
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(2011): Flower colours do not structure plant communities in temperate grasslands. 25th Annual meeting of the Scandianvian Association of Pollination Ecology Oct. 27.-30.2011, Vingstedcentret, Denmark
Binkenstein, J. & Schaefer, H. M.
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2013: Specialisation on traits as structuring mechanisms of flower-visitor networks and promoter of diversity. Functional Ecology 27: 329-341
Junker, R., Blüthgen, N., Brehm, T., Binkenstein, J., Paulus, J., Schaefer, H. M. & M. Stang
