Hydrological disturbances, usually floods and drying, govern the distribution and abundance of biota and ecological processes in freshwater ecosystems. Reducing hydrological variability should reduce biodiversity and affect ecological processes. Many of Australia’s rivers have reduced variability with river regulation, but some remain free flowing. We tested the variability hypothesis using waterbird communities on 12 floodplain lakes, paired into six systems distributed across half of the continent, over the period from 1983 to 2001. Half of the floodplain lakes were regulated (reservoirs) with stable water levels, while the remainder had unaltered hydrology. We analyzed in more detail the waterbird community within the Menindee system, using eight floodplain lakes, paired into four groups of flow regimes. Similar but less marked patterns occurred within the Menindee system. Overall, mean density (61 SE) on unregulated floodplain lakes was significantly higher (6.04 6 1.64 waterbirds/ha), compared with regulated floodplain lakes (0.66 6 0.22 waterbirds/ha). The mean number of species on floodplain lakes was significantly higher on unregulated lakes (17.21 6 0.95, n 5 19), compared with regulated floodplain lakes (9.32 6 0.56, n 5 19). Numbers of waterbird species were significantly higher on unregulated lakes compared with regulated lakes, reaching a higher mean accumulation value over time. High diversity was observed in three of the five functional feeding groups that feed on invertebrate populations and aquatic vegetation. The other two feeding groups (piscivores and large wading species) were similar in number and density between regulated and unregulated lakes, possibly because exotic fish species thrived in regulated lakes. Ordination analyses supported these observations, clearly separating unregulated lakes and flow-regulated lakes. Reduction of hydrological variability, particularly removal of natural drying periods, affected the ecology of regulated floodplain lakes. High waterbird biodiversity requires natural flooding and drying disturbance on lowland rivers, where extensive floods are punctuated by drying periods.
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