生物学的侵入と人間活動 ため池の管理形態が水棲外来動物の分布に及ぼす影響

Abstract

Biological invasion is one of the major threats to farm pond biodiversity. Farm ponds may serve as biodiversity hot spots because they provide refuge sites for a variety of animals and plants inhabiting floodplain marshes. Developing effective management methods with regard to the multifunctionality of farm ponds is one of the major concerns among environmental managers. Here, we investigated through a field survey and interviews the effects of pond management on the presence/absence of alien species in 64 farm ponds in Hyogo Prefecture, Japan. The result of the field survey indicated that bluegill sunfish (Lepomis macrochirus) and red swamp crayfish (Procambarus clarkii) are the most widespread and abundant aquatic invaders in the study region. Although it is generally believed that pond draining is effective in eradicating alien fish, pond draining showed no effect on the presence/absence of bluegill sunfish, but influenced the occurrence of red swamp crayfish, with crayfish generally found in drained ponds. The presence/absence of bluegill sunfish was generally high where dam water or agriculture draining is used as the major water supply. Thus, the alien bluegill sunfish probably immigrate into farm ponds from dams or irrigation systems even though they may temporarily disappear following pond draining. If the (mutually exclusive) distributions of bluegill sunfish and red swamp crayfish are determined at least in part by interspecific interactions between the two species, eradication of one species may lead to an increased density of the other.