Pattern of Twig Cutting by Introduced Rats in Insular Cloud Forests

We examined seasonal patterns of twig cutting by the introduced black rat, Rattus rattus, on Haha-jima Island, an island in the Ogasawara (Bonin) group of Japan. Censuses were conducted along seven routes to count the number of trees damaged by twig cutting in each month. Overall, 42.6% (23/54 species) of woody species were damaged. Twig cutting was greatest in spring (March-May). Probability of damage by twig cutting was not correlated with species frequency in the vegetation. This suggests that twig cutting is associated with particular characteristics of target species. Endemic plants experienced a significantly higher probability of twig cutting than alien plants. This may be due to an evolutionary loss of plant defense mechanisms in the absence of herbivorous mammals. Because the overall proportion of individuals damaged by twig cutting was not high, the behavior is unlikely to influence the population dynamics of trees and cause vegetation change. But intense twig cutting was also found on critically endangered plants, so twig cutting by black rats could be a threat to those species.