Ecological integration of eucalypts in Europe: Interactions with flower-visiting birds

Abstract

The degree of ecological integration of an exotic species depends on the emergence of new interactions with the host community, either antagonistic or mutualistic, such as pollination. This will determine the impact of the new species on the host community (positive or negative, direct or indirect) as well as the capacity of the new species to sustain biodiversity.Eucalypts are the most widely planted trees in the world and depend on pollination by animals for seed production. Blue gum (Eucalyptus globulus), one of the most commonly used and a dominant tree in NW of Spain, where this study has been conducted, is pollinated mainly by birds in its native range. In Europe pollination is mainly insect mediated, there are no specialized nectar-feeding birds and ornithophily (plant adaptation for bird pollination) is extremely rare (only recognized for one species, a Tertiary relict). Nevertheless, opportunistic nectar-feeding has been occasionally observed for some European songbirds.In this study, we determine the prevalence and strength of the interaction between eucalypts and birds by analyzing the occurrence and abundance of pollen in birds captured in mist nets in mixed landscapes with eucalypt plantations.Eucalypts represented the most important flower resource for the flower-visiting bird community. Eucalypt pollen was present in 171 individuals belonging to 19 bird species (out of the 485 birds captured in 33 species) and represented 98.2% of all pollen recorded. The large pollen loads found are indicative of frequent eucalypt flower use by the bird community.This study provides a good example of the ecological integration of a widely planted forestry species. The frequency of eucalypt pollen in birds points to the widespread use of this novel resource by birds and to the high ecological relevance of this interaction. The establishment of this novel interaction may favor seed production and, with it, population regeneration and spread of this exotic tree.