Water is a vital element in the natural history of Bali and one is seldom out of earshot of the sound of water in motion, whether it be the roar of a rushing mountain stream or the pleasant chuckle of the man—made watercourses and conduits which irrigate the rice fields.The ultimate source of all freshwater is rainfall and this is distributed via three main pathways: overland, below the surface of the soil but above the water table, and in the groundwater It eventually collects in a depression, be it a puddle, a pond or a lake, or alternatively flows, via streams and rivers, to the sea. Mind you, plants take up quite a lot of the rain water from the soil which is subsequently returned to the atmosphere via pores in their leaves in the course of the natural cycle of transpiration. At the same time, a lot of surface water is also ‘lost’ to the atmosphere through evaporation, so that ultimately, only some 60-80 percent of the rain that falls on Bali actually ends up in the island’s rivers and lakes.
The fact that none of Bali’s rivers have long stretches of slow-moving water means that major freshwater fish species such as carp, minnows and catfish are almost entirely absent from the island. Norare there any native fish species found above an altitude of 500 metres on account of the cold, though at this altitude Bali’s rivers and streams are teeming with invertebrate life—dragonfly and damselfly nymphs, water cockroaches, waterboatmen, whirlygig beetles and countless other small aquatic beasts. Whirlygig beetles, which belong to the Gyrinidae family of Coleoptera, are singular creatures in that each eye is divided in two, creating the impression that they possess four eyes. This arrangement in fact allows the whirlygig beetle to simultaneously scan both the surface of the water and its depths in search of a tasty morsel to prey upon. Invertebrates are also prolific in the lower reaches of Balinese rivers which are generally poor in fish species. Indeed, the most common freshwater fish in Bali is probably the ubiquitous, but exotic, delta-tailed guppy,Poeciliareticulata, which was accidentally introduced from aquaria and now is found in almost every drop of water bigger than a puddle on the island.
Waterboatmen (Corixidae), have long oar-like hind legs equipped with hairs which they use to propel themselves across the surface ot ponds and slower moving water. They prey upon mosquito larvae and augment their diet with plant matter. When they dive, short hairs growing around the abdomen trap a ‘blanket’ of air which they use for breathing under water. Waterboatmen also have wings and can readily take to the air to colonise new domains should a particular location not be to their liking.
Dragonfly and damselfly nymphs are common in standing and flowing water where they dine voraciously upon tadpoles and small fish. Although they are slow movers, they possess a very effective hooked ‘lip’, or Iabium, which is equipped with claws and grapples for seizing their prey. When a nymph reaches maturity, which can take up to a year, it crawls out of the water onto the stem or leaf of a plant, whereupon a small rent appears in the skin of its back from which the adult emerges. Dragonflies spend most of their lives as nymphs, metamorphosing into the winged adult form as a prelude to mating. Death follows shortly afterthe completion of the reproductive cycle.