Invasion of the loquat Eriobotrya japonica into urban areas of central Tokyo facilitated by crows.
Mutualistic interactions between plants and animals can play a critical role in invasion by alien plants. We investigated the expansion patterns of the loquat Eriobotrya japonica in urbanized areas of Tokyo, Japan, and the potential contribution of crows (Corvus spp.) to this process. The loquat produces large fleshy fruits, and preliminary observations suggest that the Jungle Crow C. macrorhynchos is the main vector for loquat seed dispersal in the area. We searched for loquat plants between the 29 stations of the Yamanote Line, a railroad that encircles central Tokyo. We identified the origin of each plant (invasive or planted), and recorded their growth and fruiting patterns. Then, we investigated the relationships between fruit/seed sizes of the plant and the gape sizes of two crow species occurring in the area, along with other possible seed-dispersers (four bird species), to assess their effectiveness as seed dispersers. We confirmed that a considerable number of loquat plants have invaded the area, and that some of them have grown sufficiently large to produce fruits. Both invading and planted loquats were concentrated in the northern parts of the Yamanote Line, and their densities in each section were correlated. Comparison of fruit/seed sizes with avian gape widths suggests that only the two crow species have gapes of sufficient size to be able to consume loquat fruit, and swallow the seeds; thus they are the most likely agents of loquat seed dispersal. We concluded that, although the loquat has a limited range of seed dispersers, crows play a critical role in the invasion and expansion of the species.