Impacts of streaked shearwater (Calonectris leucomelas) on tree seedling regeneration in a warm-temperate evergreen forest on Kanmurijima Island, Japan

Abstract

The ground vegetation of an evergreen broad-leaved Persea thunbergii-dominated forest on Kanmurijima Island has been heavily damaged by a ground-burrowing seabird, the streaked shearwater (Calonectris leucomelas). To clarify the effects of seabird trampling and burrowing on the recruitment of tree seedlings, 22 paired quadrats, protected and unprotected against seabirds, were laid out under various degrees of canopy coverage, ranging from heavy- to light-shade. Protection from seabird activities resulted in an increase in species richness of tree seedlings. Seabird activities had a significant effect on tree seedling diversity, while canopy coverage was shown to be important for herbaceous species diversity. Though seedling emergence of Persea thunbergii, the evergreen canopy dominant, was not affected by seabird activities and canopy coverage, that of Mallotus japonicus, a deciduous pioneer tree, was negatively affected by the both factors. Seabird activities and evergreen heavy-shade canopy negatively affected seedling survivorship of both species. Low survivorship in seedlings of the canopy species may doom the present-day warm-temperate evergreen forest of Persea thunbergii on the island. Mechanical impacts of trampling and burrowing and the resulting soil erosion may play an important role in forest regeneration and dynamics.