Effects of inhabitation of the common cormorant (Phalacrocorax carbo Kuroda) on forest ecosystems : a basic study for conservation and management of the habitat

The common cormorant (Phalacrocorax carbo Kuroda) is a large water-bird which breeds in colonies at waterside forests. Colonies and roosts of this species are often established in isolated forests such as islands or urban green tracts, and in such areas trees used for nesting and roosts have been declining because of the cormorants' various activities.In Japan there has been an increasing number of new colonics as a result of a recent increase in the number of cormorants and at some sites the decline or death of trees has become a serious problem in the conservation of forest environments. This study aimed to establish a practical system for conservation and management of forests colonized by avian populations which utilize trees as nests and roost in colonies. Some field surveys and experiments were performed to clarify the mechanisms of tree decline and the effects of cormorant colonization on the forest ecosystems. An outline of this study and major findings are summarized as follows. (1)Vegetational changes in forests colonized by the common cormorant 1.The species composition and structure of forest vegetation colonized by cormorants were surveyed from March to November 1992 in two colonies (Chikubu Island and Isaki Cape sites) at Lake Biwa, central Japan. 2.The status of vegetation in each colony was classified arbitrarily into three types according to the external appearance of tree,shrub and herb layers: dense tree layer (Type 1), sparse tree layer with sparse herb layer (Type 2) and sparse tree layer with dense herb layer (Type3). 3.Densities of cormorant nesting were mostly similar among the three types, except for part of Type2. 4.In particular, large trees on which the cormorants nested were heavily damaged, Chamaecyparis obtusa and Cryptomeria japonica being damaged more seriously than other evergreen or deciduous species. 5.The forest floor of the colony had few seedlings and saplings of woody plants, whereas a few herbaceous species such as Reynoutria japonica and Phytolacca americana were dominant. (2)Effects of cormorant activities on trees in the forests 1.Litterfall accumulation was surveyed in some forest stands both inside and outside the colony from December 1993 to November 1994 at the Unoyama site on the Chita Peninsula, central Japan. Litterfall in the colony was derived from leaves and stems of herbaceous plants,various organs of woody plants, dead insect bodies, detritus of fish and crustaceans (prawns and crabs) from the cormorants' food and cormoralnt feces. 2.Comparison of plots with a similar species composition of woody plants showed that the annual leaf litterfall was larger outside than inside the cololy, and larger in nesting sites with low than high cormorant density. The annual litterfall of twigs and branches showed a tendency similar to leaf litterfall between inside and outside the colony, but comparison between the inside and the outside of the colony with different densities of cormorant nesting showed an inverse relationship to that for leaf litterfall. The annual litterfall of other items was larger inside than outside the colony, which was reflected in the supply of cormorant feces. 3.Comparison between plots with a similar species composition of woody plants also demonstrated that there was no difference in the seasonal changes in leaf and twig litterfall, probably due to a difference in the status of the cormorant nesting habits inside and outside the colony and between different nesting densities. 4.The litterfall of twigs with leaves, which seems to reflect the intensity of twig snapping by the cormorants, was larger inside than outside the colony, and larger at nesting sites with high cormorant density than at those with low density. The litterfall inside the colony and high-density nesting sites increased during the period from May to June, which corresponded to the brooding period, and in November when the number of cormorants increased. (3) Changes in soil propeties in th