Effects of eco-friendly canal improvements on the distribution and abundance of dragonfly larvae: A citizen-science-based survey in Tama City, Tokyo, Japan

Abstract

The Ichinomiya Canal is an irrigation and drainage channel, lined with two- and three-sided concrete revetments, that flows through paddy fields alongside the Tama River in Tokyo, Japan. The progressive collapse of these revetments in various sections of the canal prompted a construction project in early 2014. To enhance biological habitats associated with the canal, various improvements were undertaken, including a timber piling revetment constructed on one bank (section I-3), and lowering of the canal bottom by 30 cm beneath a bridge that spans the canal (section I-2). A preliminary larval survey was conducted in November 2013 (immediately before construction began). Between 2014 and 2021, with the aid of public participation, four canal sections were surveyed to determine the effects of the improvements on dragonfly populations. The target area extended from upstream (section I-4) to a downstream section where Iris pseudacorus L. had been planted along one bank (section I-1); sections I-1 and I-4 had been lined with two-sided concrete revetments during previous repair works. In all sections, we recorded the numbers of each species of dragonfly larvae, as well as various environmental parameters. The surveys identified a total of nine dragonfly species belonging to six families in all sections combined. Comparisons of each section after construction revealed that overhanging plants thrived better in section I-3 than in section I-4, and that Orthetrum albistylum speciosum and Shaogomphus postocularis were more abundant in section I-3 than in the other sections. In sections I-3 and I-4, sand and mud covered the canal bottom and submerged plants thrived; in section I-2, mud and leaf litter covering the bottom maintained a gentle, stable aquatic environment that led to greater abundance of Sieboldius albardae and Macromia amphigena amphigena than in the other sections. Anax parthenope julius, Ischnura asiatica, and Atrocalopteryx atrata were more abundant in section I-1, where Iris pseudacorus L. was planted on one bank, than in the other sections. In section I-3, the larvae of only one species, Orthetrum albistylum speciosum, were observed in November 2013, prior to canal improvement, whereas surveys conducted between 2014 and 2021 recorded six species of dragonfly larvae belonging to five families, following the construction of timber piling revetments. Water flow velocity and depth, litter and mud depth, and vegetation cover differed significantly among the surveyed sections. This environmental diversity along the canal may have a complex relationship with dragonfly species diversity. Our findings demonstrate the effectiveness of continuous monitoring with the aid of public participation.