The fruits of success: Examining the contributions of avian seed dispersal to the spread of invasive, woody, fleshy-fruited plants

Although fleshy-fruited plants are an important component of the invasive flora of the United States, no comprehensive examination exists describing how seed-dispersing animals promote the spread of such plants. By combining lab and field techniques, I focus on the ability of an invasive frugivore, the European starling (Sturnus vulgaris), to disperse the seeds of several fleshy-fruited invasives common in northeastern Connecticut. Starlings have the potential to be effective seed dispersers due to their large populations, widespread range, and fall and winter diets that include substantial amounts of fruit. Four years of seed collection at a starling roost revealed that 84% of the seeds starlings moved were from Asiatic bittersweet (Celastrus orbiculatus). In addition, captive European starlings and American robins (Turdus migratorius) significantly preferred invasive fruits to similar native fruits in two out of three choice tests; when selecting among invasive fruits only, both starlings and robins required significantly less time to eat the fruits of autumn olive (Elaeagnus umbellata) than Asiatic bittersweet or multiflora rose ( Rosa multiflora). When comparing starling and robin responses to novel foods, both significantly preferred familiar foods to novel foods in choice tests, and demonstrated no significant differences in the length of time required before beginning to feed when offered a novel food only. To estimate the ability of starlings to move the seeds of invasives, passage rate data from captive birds was combined with two years of radio-tracking data describing starling movements. The resulting seed shadows for autumn olive, Asiatic bittersweet, and multiflora rose suggest approximately 50% of seeds are dispersed less than 250 m, while for each plant, 5-10% of seeds may be moved 1 km or more. Finally, I examined how ingestion by starlings affects germination of autumn olive and Asiatic bittersweet seeds. In both cases, ingestion by starlings significantly increased the percentage of seeds germinating, but decreased the number of days until germination only for Asiatic bittersweet. Together, this work demonstrates that starling frugivory benefits certain fleshy-fruited invasive plants. Further work is needed to determine the extent to which this interaction benefits starlings, as well how native plants and animals are affected.