Conservation implications of flooding rice fields on winter waterbird communities

The effects of flooding harvested rice fields on waterbird communities were studied during winter. Variation in the number of waterbird species, overall densities of all waterbirds, wading birds, waterfowl, and shorebirds, and a measure of conservation value that weighted species according to their relative abundance and population trends were examined. Each variable was tested for differences among: (a) flooded and unflooded fields; (b) flooded fields that received different rice straw manipulations; and (c) fields with different water depths. Flooded fields were used by waterbirds more than unflooded fields according to most criteria, although wading bird densities did not differ between flooded and unflooded fields. In terms of conservation value, flooded fields contributed considerably more to waterbird conservation than unflooded fields. The number of waterbird species, total waterbird density, and the density of wading birds differed significantly among straw management treatments, though in different ways. Water depth significantly affected all measures, but in all cases most of the variation went unexplained. Bird densities were explained best by asymptotic relationships, with shorebird densities greatest in shallow water and waterfowl and wading bird densities greatest in deeper conditions. Waterbird richness and conservation value both were greatest at depths of 10-15 cm. Intentionally flooding fields during winter significantly affected numerous aspects of the waterbird community. The method of flooding also influenced the waterbird community, although these effects often were small.