Mitigation Project of the Endangered Brackish Water Damselfly, Mortonagrion Hirosei
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Mitigation project of the endangered brackish water damselfly, Mortonagrion hirosei Mamoru Watanabe Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan Abstract In 1998, a tiny habitat of the brackish water damselfly, Mortonagrion hirosei, which is an endan- gered species in Japan was discovered in Ise, Mie Pref. It was a dense reed community on the brack- ish water, from which had be reclaimed under the construction of sewage plant. The local govern- ment of Mie Prefecture decided to preserve the local population of the damselfly. Then the mitiga- tion project was started, because the habitat seemed to be too small to maintain the local population, and because surroundings of the habitat would become unavailable vegetation for the damselfly due to the sewage plant development. In those days, however, there were few reports on the biology and ecology of the species. Therefore, we had to begin to clarify quantitatively the population parameters of larvae and adults, life history, behaviour, flight characteristics, body colour change for adults with age after emergence, saline tolerance of larvae as well as to measure abiotic environment, such as saline concentration, water depth, water temperature and relative light intensity in the original habi- tat, a reed community. According to the accumulation of information on the quantitative environ- mental factors, the design for a newly established habitat next to the original habitat was proposed. Then, in early spring of 2003, huge number of reed rhizomes were collected from the abandoned rice paddy fields near the original habitat, and transplanted for establishing the new habitat. Artificial brackish water was continuously supplied throughout the year. The reed community has developed year after year and nearly completed to the dense community, overcoming a lot of problems ap- peared. Techniques for estimating the number of adults and larvae in the dense reed community, and for evaluating the habitat availability, were developed. Mark-and-recapture method and line census method were adopted. Saline concentration and water supply in the artificially established habitat were monitored every week. Consequently, the adult population has increased in both the original and the established habitat. The mitigation project has now proved successful. Introduction Loss and degradation of wetlands, particularly in estuarine areas, has been a serious issue in biodiversity conservation. In estuarine landscapes, the brackish water habitat is characterized by a subtle changing mixture of fresh water and sea tides and by mud flats of fine sedimentary material carried into the habitat from rivers and sea tides (McLusky & Elliott, 2004), and the estuarine land- scapes provide a unique habitat for Odonata species in Japan. Ponds, rice paddy fields, channels and sewage drains are the major components of these landscapes and contain varying degrees of saline water, though such water environment seems to be disadvantageous for the survival of odonate lar- vae (Corbet, 1999). Mortonagrion hirosei (Zygoptera: Coenagrionidae) inhabits the understory of dense reed communities. It is a solitary endemic species of Japan as a brackish water damselfly, which was just discovered at Hinuma, Ibaraki, Japan in 1971, and now it is classified as an endangered species by the IUCN, principally due to habitat loss caused by practices such as river improvement and the fill- ing of wetlands (Watanabe & Mimura, 2003). Larvae of M. hirosei have a relatively high survival rate in brackish water with saline concentration of 15‰ (Iwata & Watanabe, 2004). Corbet (1999) reviewed the dragonfly's tolerance of high conductivity environments, and a few species have been found in brackish water habitats, probably because of their high saline tolerance during larval stages. Around 30 local populations of M. hirosei have been discovered, mainly on Honshu Island, Japan, and all of them are located in estuaries (Matsuda et al. 2002), and now the habitats of M. hiro- sei are declining (Hirose, 1985; Hara, 2000). Intensive conservation projects are thus essential for preventing the local extinction of the species. In Mie Prefecture, Japan, M. hirosei populations are found in a few reed communities lo- cated far apart from each other, where they form closed local populations within the habitat (Watan- abe & Iwata, 2007). Most habitats are pure dense communities of the reed, Phragmites communis, which produces a shady understory that results in low adult flight activities and subsequently a local discrete population (Watanabe & Mimura, 2004). At initial phases of the conservation planning, de- tailed information on species abundance and the habitats is lacking. Therefore, the quantitative data that can predict and evaluate the population parameters of the species are important in impact as- sessments on the artificially established habitat. In 1998, an isolated small habitat of M. hirosei was discovered in the estuarine near Miya- gawa River, Mie Prefecture, Japan, in an area where a construction project for a sewage plant was going to be undertaken (Watanabe & Mimura, 2004). Brackish water conditions in the community had been maintained by the supply of freshwater from upstream and seawater from downstream. When the sewage plant is functional, it will interfere with the supply of fresh water, resulting in the extinction of this damselfly population due to increased salinity and/or increased sediment and litter deposition in the community. Therefore, the local government of Mie Prefecture decided not only to preserve the original habitat of M. hirosei, but also to establish a new habitat to aid in the conserva- tion of this population. In order to preserve the local population of M. hirosei, a reed community was artificially established on the abandoned rice paddy fields adjacent to the original habitat by trans- planting reed rhizomes in January 2003 (Watanabe & Matsu'ura, 2006). Mitigation plans have recently become more common for conserving species in developed areas. However, an analysis of 43 habitat conservation plans in the United States found that most included little information about habitat quantity and quality (Harding et al., 2001). In the present project, we investigated the growth and microenvironment of the newly established community to evaluate its development relative to the original community, as well as the population parameter of M. hirosei. This allowed us to evaluate the mitigation project for the newly established community as M. hirosei habitat. Species and Habitat The endangered damselfly, M. hirosei does not perform a maiden flight (Watanabe & Mimura, 2003). Throughout their life span, adults stay among the reeds, where they show reproduc- tive behavior when sexually mature (Hirose & Kosuge, 1973; Someya, 1998; Fukui & Kato, 1999). A number of odonate studies have revealed that the sexually immature period of adults is normally the principal period of odonate dispersal (e.g. Michiels & Dhondt, 1991), and many odonate species fly a considerable distance away from water (Corbet, 1999). The immature adults of M. hirosei, however, do not leave their emergence sites. Each M. hirosei local population seems to be a relatively closed population, since its suit- able habitats are isolated from each other (Someya, 1998). The absence of detectable dispersal in immature M. hirosei makes the population structure of this species ideal for intensive study. How- ever, little attention has been paid to the population ecology of M. hirosei in Japan, probably in part due to their cryptic habits. In 1998, the discovered tiny habitat (50 m long, 10 m wide) in the city of Ise, Mie Prefec- ture, Japan, consists mainly of very slow-flowing streams with muddy bottoms flowing into fresh water from sewage and saline water from the sea, as shown in Fig. 1. The water depth was less than 10 cm in pure dense communities of the reed which produces a shady understory for the community, a relative light intensity of about 10% (Watanabe et al., 2002). The mean density of 440 live and withered reed stems per square metre corresponds to a mean distance of approximately 5 cm be- tween reed stems. No reeds were trimmed until 2001, and the community was about 2-2.5 m tall. No pesticide was used on the paddy fields during the flight season. To the east, the stream continues to Ise-Bay and the sea. Adults of M. hirosei perch mainly on live and dead shoots of reeds, preferring the tips of broken stems or the dead blades of reeds 20 cm above the water surface. Further, they show little flight activity (Watanabe & Mimura, 2004). In the area for mitigation project, four damselfly species, M. hirosei, M. selenion, Ischmura senegalensis and I. asiatica, are common in estuaries in which reed communities are dominant (Mat- su'ura & Watanabe, 2004). Iwata & Watanabe (2004) showed that young larvae of I. senegalensis have saline tolerance similar to that of M. hirosei. The two Ischnura species are regarded as being predators of M. hirosei (Hirose & Kosuge, 1973), though the two Ischnura species and M. selenion prefer open habitats, that is, along the margins of the dense reed community inhabited by M. hirosei. Therefore, as a strategy for conserving M. hirosei, these three coexisting species should be excluded. Population Parameters in the Original Habitat The mark-and-recapture method was used to study the population dynamics of M. hirosei adults in the original habitat from late May to early August. Adults were captured with a net and were anaesthetised by CO2. Then, each adult was marked with its own number on the undersurface of the left hind wing using a black felt-tip pen. They were released at the same site immediately after recording the date, sex and age. The marking was considered to have only minor effects on flight activities (and the probability of mating), since most of them began to fly normally and then to perch soon after recovering from the anaesthetic. Individuals injured during marking were treated as dead in the calculations. On the basis of the recapture data, seven classes were identified, mainly by wing condition and body colours, as in the case of other zygopteran species (e.g.