Effects of material inputs by the Grey Heron Ardea cinerea on forest-floor necrophagous insects and understory plants in the breeding colony

Abstract

Piscivorous birds affect terrestrial ecosystems by transporting and introducing organic material and nutrients from aquatic systems. While most of these effects have been evaluated by simple comparisons of the abundance of terrestrial organisms within and outside colonies, little is known about the effects of nest density of piscivorous birds on rates of supply of material inputs within colonies, or on the abundance of terrestrial organisms consuming the materials. To clarify the effects of material inputs by the Grey Heron Ardea cinerea on necrophagous insects and understory plants in a forest, we evaluated the effects of nest density of herons on the spatial pattern of rates of supply of aquatic materials to the forest floor, and the response of necrophagous insects and understory plants to those supply. The herons transported aquatic secondary production in the form of chick carcasses and feces to the forest floor beneath their breeding colonies, and the supply rates were well explained by the nest density. Carcasses and feces increased the densities of necrophagous insects, but feces decreased the biomass of understory plants as supply rates increased. These findings suggest that to evaluate the effects of allochthonous inputs by piscivorous birds on terrestrial communities, it may be necessary to examine not only the presence or absence of bird colonies, but also to examine the relationship between nest density and terrestrial organisms.