Variability and specialization of plant–pollinator systems in a northern maritime grassland

To detect plant specialization to pollinator insects and to further examine the ‘pollination syndrome’ hypotheses, flowering angiosperms and insect visitors were recorded in a northern maritime grassland community. Associations between plant species and insect groups were analyzed using binomial tests based on census data obtained from two sites over 3 years. ‘Preference to an insect group by a plant species’ was expressed as a significant deviation of the actual proportion (i.e. the proportion of the number of flower visits by the insect group to the plant species in the total number of visits to the plant species) of the expected proportion (i.e. the proportion of the number of visits by the insect group in the total visits during its flowering period). ‘Preference to a plant species by an insect group’ was similarly expressed using the number of flowers of the plant species visited by the insect group. Most significant preferences varied temporally or spatially. Variability in the preferences is suggested to have resulted from temporal and spatial variations in the abundance and species composition of both flowers and insects. However, in species showing variable preferences, significant specialization in the pollinator insect group (i.e. relatively constant, mutual preferences) were demonstrated for seven and five plant species at the two study sites, respectively. Most specializations were found in associations with bumblebees. ‘Bumblebee specialists’ were significantly well represented in the flower shape types ‘gullet’ and ‘flag’ and the flower color types ‘violet’, which supports the ‘pollination syndrome’ theory.