Flower visitation by European birds offers the first evidence of interaction release in continents

Aim All species are imbedded in a network of interactions with other species, which define an important component of their ecological niche. These interactions are dynamic and can change the emergence of vacant niches in the environment. Niche adjustments have been predicted to be particularly common on insular communities as a response to the poor and disharmonic biota of oceanic island – the interaction release hypothesis, however, the phenomena has not yet been reported on continents. Taxonomic groups of specialized nectarivorous birds are present on all continents except in Europe where they became extinct in the Oligocene, likely leaving behind underexplored flower resources. We performed the first community-level assessment of flower visitation by European birds to evaluate if insectivorous and granivorous birds show an interaction release towards consuming flower resources in Europe. Location Larca – Coimbra, Portugal, Europe. Methods During one year, we collected pollen loads from the forehead of 634 birds. Pollen loads were prepared by acetolysis and all pollen grains were identified under a microscope. All interactions were compiled into a quantitative interaction matrix describing the first pollen-transport network by European birds. Results One-fifth of the bird individuals, corresponding to two thirds of the bird species sampled carried pollen from 45 different types. The vast majority of the plant species found were native but the alien Eucalyptus globulus was by far the species most commonly found in the birds’ pollen loads. Overall, the structure of the pollen-transport network from Europe shared many attributes of other networks that include taxonomic groups of specialized nectarivorous birds. Main conclusions We show that the interaction release hypothesis is not exclusive to insular communities but can also be observed in continents, greatly increasing the potential geographical distribution of this phenomenon. However, it seems considerably less pronounced in Europe than in the Galapagos, where it was first described, probably due to the much stronger selective pressures on the simplified ecosystems of oceanic islands.