Spatial variations in chironomid larvae and dragonfly predation in pools on a Japanese high mountain moor

Mountain moors, a typical landscape component of alpine and subalpine areas in Japan, frequently feature hundreds of pools ofvarious sizes. In these pools, larvae ofchironomidae (Diptera) are predominant, along with other aquatic insects and zooplankton (KURASAWA et al. 1982, IWAKUMA 1995). However, the abundance of such larvae and the species composition vary highly among pools even within the same moor (KURASAWA et al. 1982, UENO & IWAKUMA 1996). These spatial variations provide a unique opportunity to examine the factors that determine abundance of chironomid larvae and compositions o f their communities. Several studies have shown that community structures of chironomid larvae in fresh water are affected by various factors such as pH (WALKER et al. 1985, KRATZ et al. 1994), abundance and composition of macrophytes (CARDINALE et al. 1997, BRoDERSEN et al. 2001), productivity (SAETHER 1979, PERRIN & RrcHARDSON 1997), oviposition and colonization rates (ToKESHI & TowNSEND 1987), and predation (ThoRP & CoTHRAN 1984, WALDE & DAVIES 1984, JoHNSON et al. 1987, GoYKE & HERSHEY 1992, DIEHL 1995). However, little is known about the factors that determine community structures of chironornid larvae in mountain moors. In freshwater habitats, predation is crucial in determining abundance and composition of aquatic organisms (DIEHL 1995, WELLBORN et al. 1996). In the Shibakusa-Daira moors of the Zao Mountains in northeastem Japan, the abundance and composition of chironomidae larvae differ from pool to pool as they do in other moors (KuRASAWA et al. 1982, UENO & IWAKUMA 1996). These poo1s contain no fish but have several predatory animals, including the larvae of salamanders (Hynobious nigrescens) and alderflies (Sialis tohokuensis), and the nymphs of damselflies (Lestes sponsa) and dragonflies (Aeshna juncea andA. nigrojlava). Salamander larvae appear in these pools only in late spring to early summer, however, and the populations o f alderflies and damselflies are highly limited throughout the year. Thus, dragonfly nymphs are the most abundant predators in the pools at the Shibakusa-Daira. Aeshnajuncea are widely distributed from Eurasia to North America, but A. nigrojlava is endernic to Japan. Larvae o f this g en us are known to prey o n Diptera, including chironornid larvae (JOHNSON 1985, JOHANSSON 1993); therefore, we hypothesized that predation by Aeshna is one of the factors differentiating the abundance and composition of chironornid 1arvae among the Shibakusa-Daira pools. We tested this hypothesis by manipulating Aeshna abundance in a field experirnent.