Habitat management of little terns in Japan’s highly developed landscape
Introduction
Japan is one of the most highly urbanized countries in the world. Adverse effects of urbanization have been particularly serious for wildlife in coastal habitats because these habitats are adjacent to urban areas (Torii et al., 2000, Ministry of the Environment, 2003). Although there is limited information regarding the impact that human activities have had on the wildlife in the coastal habitats of Japan, a number of species living in such habitats are thought to be at the risk of extinction.
The little tern (Sterna albifrons) is a migratory species that establishes breeding colonies on the bare ground of sandy beaches and along river banks with little vegetation cover (Higgins and Davies, 1996, Fasola et al., 2002). Because their population is declining (Amano and Yamaura, 2007), this species is listed as “vulnerable” in the Red List by the Ministry of the Environment of Japan (2008). Loss of habitat is a prime cause of this decline (Kanai et al., 1991). For instance, along Tokyo Bay, located adjacent to the highest human population in Japan, more than 90% of the coast has been reclaimed (Ministry of the Environment, 2003). This means that most of the natural breeding habitats for the terns along the bay have been degraded or destroyed. The only possible nesting sites that remain are temporary sites such as reclaimed land that has been cleared prior to construction work (Kanai et al., 1991, Hayashi and Okada, 1992). Conservation of the little tern requires either that permanent colony sites be established and managed, or that steps are taken to ensure a stable number of sites during each breeding season (Kanai et al., 1991, Fujita et al., 1994, Hayashi et al., 2005).
In the last decade, efforts to develop artificial colony sites were undertaken (Fujita et al., 1994, Hayashi et al., 2005). A number of reports about other tern species in North America and Europe suggested that terns might prefer artificial colony sites and could achieve higher reproductive success in the artificial sites than in natural sites (least terns, Sterna antillarum: Jackson and Jackson, 1985, Gore and Kinnison, 1991, Zambrano et al., 1997, Voigts, 1999, DeVires and Forms, 2004, Forys and Borboen-Abrams, 2006, Butcher et al., 2007; common terns, Sterna hirundo: Groen et al., 1995). These activities in Japan, however, have not always been successful (Hayashi et al., 2005). In some cases, the terns did not use the created sites, and in others, terns nested at the sites but experienced lower hatching success and eventually abandoned the sites (Hayakawa, 2001).
Several possible explanations for poor results may be offered. The artificial sites in Japan were relatively small in area (e.g., 0.01–1.2 ha; Fujita et al., 1994, Little Tern Group in Kanagawa chapter of WBSJ, 1999). In addition, predation by jungle crows (Corvus macrorhynchos) and carrion crows (Corvus corone) had a significant impact on the hatching success of terns using the artificial sites (Hayakawa, 2001, Hayashi et al., 2005). It is likely that if a created site is too small to sustain a large colony against high predation pressure, terns might abandon or avoid the site altogether. To ensure future success for artificial nesting sites, we must therefore examine environmental factors related to breeding success and nest-site selection of the terns in Japan.
The first case of colony settlement on artificial sites was observed in 2002 on the rooftop of a sewage treatment plant located on reclaimed land in Tokyo Bay (Hayashi et al., 2005; Table 1; Fig. 1, Fig. 2). In developed coastal areas of the Tokyo metropolitan area, there are many such plants along with a number of other rooftop sites with potential to be used for artificial colonies. Lack of knowledge regarding the factors that will lead to successful establishment of a viable artificial nesting site is the sole obstacle to developing many of these locations into potential colony sites for terns.
Merely protecting colony sites is not adequate for improving the breeding success of terns; suitable foraging habitats surrounding the colony site should also be conserved. In other tern species, food shortage results in decreased chick survival (Schaffner, 1986, Safina et al., 1988, Safina and Burger, 1989, Suddaby and Ratcliffe, 1997). In addition, conservation of a particular area is not enough, because even if the conditions of a colony site are kept stable, terns sometimes shift their colony sites for unknown reasons (Akai and Kuwabara, 1994, Ministry of the Environment, 2007). This suggests that it is better to conserve multiple breeding sites within a larger conservation unit. To design the conservation unit, information regarding the dispersal range of terns is essential.
In the present study, we sought to develop a strategy for management of breeding habitat for little terns. We analyzed factors related to nest-site selection and breeding success. These factors included colony size, type of ground substrate at the colony sites, and type of foraging habitat near the colonies. Such information is necessary to improve the design of each artificial colony site by including foraging habitat. We also attempted to infer the dispersal range of the terns, which may play an important role to design arrangements of reserves for the breeding terns within a region.
Section snippets
Study sites and management history of the main study site
Our main study area (site A) was an artificial colony site established to provide a permanent nesting habitat for little terns and was located along the northwest shoreline of Tokyo Bay, central Japan (Fig. 1, Fig. 2). Tokyo Bay is one of the major breeding areas of this species in Japan (Fig. 1, Ministry of Environment Japan, 2003).
In 2001, at site A, tern nests were found on the rooftop of a building used for sewage treatment, but most of the eggs failed to hatch because of predation, heavy
Effects of ground substrate on vegetation cover, nest density, and predation rate
In site A, vegetation cover in the area covered with only sludgerite was higher than in the area covered with sludgerite and crushed-concrete (Mann–Whitney U = 1103.1 and p < 0.01; Fig. 4a). Further, vegetation cover was higher in the area covered with combined sludgerites and crushed-concrete cover than in the area covered with crushed-concrete alone (U = 1042.7 and p < 0.01).
Where ground cover was lowest, nest density tended to be highest: we found higher nest densities in the crushed-concrete only
Discussion
In the artificial colony site on the building rooftop, there was a strong correlation between colony size and the hatching success of little terns. These results suggest that colony size is likely to affect the hatching success of the terns, which is similar to results from studies on relationships between colony size and reproductive success in other colonial seabirds (Götmark and Andersson, 1984, Becker, 1995, Krause and Ruxton, 2002, Hernández-Matías et al., 2003).
In 2003, the area of our
Acknowledgements
We thank T. Amano, K. Katoh, Y. Watanuki, and M. Hayakawa for their helpful comments our manuscript. We also thank S. Someya, T. Tanaka, M. Nitta, M. Ito, R. Tanaka, Y. Yagi, E. Hayashi, K. Nakase, and other members of Little Tern Project, and K. Kuwabara, Y. Minowa, and members of Field Assistant Network for help with fieldwork. Bureau of Sewerage, Tokyo Metropolitan Government provided field facilities. This research was funded by the University of Tokyo 21st Century COE Program “Biodiversity
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