Walnut seed dispersal: mixed effects of tree squirrels and field mice with different hoarding ability.

Abstract

AbstractA study was conducted in suburban forests at Hachioji City, Western Tokyo, Central Honshu Island, Japan, to compare food selection by two sympatric species of rodents: tree squirrels and field mice. One of the most preferred foods, walnuts (Juglans ailanthifolia), was counted in the field to investigate predation rates by the two species. The hoarding rates, hoarding sites and retrieval rates of cache by both rodent species were evaluated. Results showed that the walnut seeds were preferentially used by tree squirrels, but not field mice. The number of seeds with evidence of mouse feeding was three times that of squirrels. The number of scatterhoarded walnuts versus eaten walnuts by the field mice was 1:1.58 on the average of 1996 and 2000. Scatterhoarding was never observed in the winter of 1996 and only 17% in the winter of 2000. The transport distance for hoarding was not different between years and was 6.2 m on the average. Differences in body size seem to affect the differences in transport distance between the two species. The field in the study transported the acorns of Lithocarpus edulis farther than walnut seeds. The seed size may affect the transport distance by the field mice. There were two walnut trees producing seeds at both the northern and western edges (79 m apart from each other). Farther transportation from the parent trees seems to be advantageous for seedlings to increase their survival.