JARQ 44 (4), 405 – 413 (2010) http://www.jircas.affrc.go.jpFundamental Experiment of Eco-friendly Techniques for Frogs in Paddy Areas Fundamental Experiments to Develop Eco-friendly Techniques for Conserving Frog Habitat in Paddy Areas: Escape Countermeasures for Frogs Falling into Agricultural Concrete Canals Keiji WATABE*, Atsushi MORI, Noriyuki KOIZUMI and Takeshi TAKEMURA Department of Rural Environment, National Institute for Rural Engineering, National Agriculture and Food Research Organization (Tsukuba, Ibaraki 305–8609, Japan) Abstract Frogs often drown in agricultural canals with deep concrete walls that are installed commonly in paddy areas after land consolidation projects in Japan because they cannot escape after falling into the canal. We propose a partial concrete canal with gently sloped walls as a countermeasure for frogs to escape the canal and investigated the preferable angle of the sloped walls, water depth and flow ve- locity for Rana porosa porosa. Our experiments showed that only 13 individuals (2%) escaped by leaping out of the canal, indicating that climbing up is the main escape behavior of R. p. porosa. Walls with slopes of 30–45 degrees allowed 50–60% of frogs to escape from experimental canals, the frogs especially easily climbed up walls with a 30 degree slope. Adjusting water depth to 5 cm or more would assist the frogs in reaching the escape countermeasures because at such depths frogs are not able to stand on the canal bottom and to move freely about. Flow velocity should be slower around the countermeasures because R. p. porosa is not good at long-distance swimming and cannot remain under running water for a long time. The broadened inlets of this new canal design provide water of slower velocity near the shores of the sloped walls, which encourages frogs to escape. Discipline: Agricultural environment Additional key words: climbing capability, ecosystem conservation, Japan, Rana porosa porosa, Tokyo Daruma Pond Frog ently threatened with extinction19 because of increasing Introduction environmental stresses related to human activities such as deforestation, chemical pollution, acid precipitation, Frogs are important bioindicators, a sort of barome- and other unknown causes. Conservation countermea- ter of the environmental health because they are in high sures are being considered and implemented worldwide environmental demand and occupy an intermediary posi- to protect amphibians from extinction, and numerous vol- tion in food webs13. Both aquatic habitats and terrestrial unteers in Western Europe, North and Central America, habitats are necessary for many species of frogs: tadpoles and Australia are taking personal measures by managing live in the water and metamorphosed frogs live on the and creating frog habitats. Countries such as the United shore. On the other hand, they are consumers of small States, Denmark, France, and Germany have enacted leg- animals such as insects and spiders6, and are also impor- islation to protect amphibians, and the Ministry of Agri- tant diets of large predatory animals such as freshwater culture, Forestry and Fisheries in Japan provides extra fish, snakes, herons, and Japanese minks in both aquatic subsidies to local governments and farmers’ organiza- and terrestrial food webs4,9,10,13. They are particularly tions for the construction of amphibian-friendly agricul- sensitive to environmental stresses such as habitat loss, tural canals. pollutants, ultraviolet light, alien species, infections, and Twelve frog species inhabit paddy fields and agri- climate change13. Approximately 32% of some 6,000 am- cultural canals in Japan (Table 1)5,11. Hyla japonica, Rana phibian species including frogs and salamanders are pres- japonica, Rana limnocharis, and Rhacophorus schlegelii *Corresponding author: e-mail [email protected] Received 22 December 2008; accepted 9 March 2010. 405 K. Watabe et al. are common species across the islands of Japan, and Rana area of Tokyo. The habitat and nursery-site functions of porosa porosa, Rana porosa brevipoda and Rana nigro- the Kanto Plain have been degraded by rapid develop- maculata are regionally common species. Some species ment and agricultural modernization such as usage of of frogs such as H. japonica tree frogs (Fig. 1a) have ad- pesticides and land improvement projects, including land hesive discs, whereas R. p. porosa lack discs (Fig. 1b). consolidation and the replacement of shallow earthen ca- Rana p. porosa is a common species on the Kanto Plain, nals with concrete irrigation and drainage canals. The where a modern farming system of paddy and vegetable new irrigation canals are separated from drainage canals; fields has been developed around the large metropolitan water is typically pumped into paddy fields via under- Table 1. Frogs that live in Honshu, Shikoku and Kyushu Islands, Japan Common name Nomenclature Main spawning sites Eastern-Japanese Common Toad Bufo japonicus formosus Paddy fields, temporary puddles Western-Japanese Common Toad B. j. japonicus Paddy fields, temporary puddles Japanese Stream Toad B. torrenticola Pools of mountain streams Japanese Tree Frog Hyla japonica Paddy fields, temporary puddles Japanese Brown Frog Rana japonica Paddy fields Montane Brown Frog R. ornativentris Paddy fields, temporary puddles Black-spotted Pond Frog R. nigromaculata Paddy fields Tokyo Daruma Pond Frog R. porosa porosa Paddy fields Daruma Pond Frog R. p. brevipoda Paddy fields Wrinkled Frog R. rugosa Paddy fields, small streams, mountain streams Indian Rice Frog R. limnocharis Paddy fields Tago’s Brown Frog R. tagoi tagoi Mountain streams Stream Brown Frog R. sakuraii Pools of mountain streams Forest Green Tree Frog Rhacophorus arboreus Ponds, paddy fields Schlegel’s Green Tree Frog Rh. schlegelii Paddy fields Kajika Frog Buergeria buergeri Shallows of mountain streams Made by Hasegawa (2003) 5 Fig. 1. Frogs that live in paddy areas a): Hyla japonica the Japanese Tree Frog that has adhesive discs. b): Rana porosa porosa the Tokyo Daruma Pond Frog that lacks adhesive discs. 406 JARQ 44 (4) 2010 Fundamental Experiment of Eco-friendly Techniques for Frogs in Paddy Areas ground pipes and drained from fields into deep, U- in the fields, but frogs cannot easily reach the rough sur- shaped, concrete canals in which the water level may be faces. Neither type A nor type B canals have components 70 cm or more below that of the rice fields3. The typical that reduce water flow and allow pools or inlets to form. U-shaped canal consists of a concrete channel (width 50– Therefore, we propose a design for type C canals, which 80 cm) with vertical sides (height 60–90 cm)3, and water have some wider areas with deeply sloped walls (Fig. 3). depth and velocity in the canals change seasonally from 0 The design of type C canals allows reduced water flow in to 20 cm and from around 0 to 10 cm/s, respectively. The these wider areas, which form artificial inlets with shal- changes in habitats caused by the projects, especially the low water depth, and slow water flow along each sloped replacement of earthen canals with concrete canals, may wall should make it easier for frogs to reach the sloped be one of the factors causing frog population declines in wall and escape. Japan. Fujioka & Lane3 reported that only few frogs lack- A successful design for type C canals must take into ing adhesive discs live in the paddy fields after land im- consideration the behavior and physical characteristics of provement projects. Matsuzawa et al.12 reported that Rh. frogs. However, this information has hardly been studied schlegelii individuals, which have adhesive discs, often with only a few reports on climbing ability and jumping drown in concrete agricultural canals with a width of 30 ability1,2,16. Of this information, the hydraulic condition cm. These observations indicate that frogs falling into of the canal, as well as the angle of sloped walls as a coun- the concrete canals cannot escape and are washed down- stream and drown in the canals. Countermeasures to compensate for changing ca- nals or as part of environmental improvement activities by farmers are necessary to enable frogs to move freely across agricultural canals and escape easily once they en- ter the canals. Mizutani et al.14 reported the effectiveness of placing a lid on canals to prevent frogs from falling into canals. Existing canal designs that enable frogs to escape are broadly classified into types A and B. Type A canals contain a ramp sloped toward the opposite direc- tion of flow in the canals (Fig. 2a). This type cannot help frogs to escape if they are washed out through the main flow. The nearly vertical walls of type B canals have a Fig. 3. Proposed type C escape countermeasure with rough surface to assist frogs in escaping (Fig. 2b). Such placement of partially concrete sloped walls toward nearly vertical walls mean type B canals take less space the vertical direction of flow a) b) Fig. 2. Example of the escape countermeasures for frogs a): Placement of partially sloped walls in the canal, type A. b): Placement of rough surfaces on sloped walls of the canal, type B. 407 K. Watabe et al. termeasure that enables frogs to escape easily, was fo- ly sloped concrete wall of a U-shaped flume, as in actual cused on because water depth and flow velocity of canals canals, with regard to the ease of climbing by frogs. may affect frog behavior until getting on the countermea- sure. 3. Experimental conditions In this study, R. p. porosa the Tokyo Daruma Pond In this study, (1) the preferable angle of slope for Frog (Fig. 1b) was targeted because this frog is now des- walls that frogs can easily climb (experiment 1) and (2) ignated as a “Near Threatened” species by the Ministry the preferable water depth and average flow velocity that of the Environment. Even though they were common urge the frogs to reach the sloped walls (experiment 2) species decades ago, they are now threatened because of were investigated.