Abstract
Wetland loss and degradation have been extensive across the world, especially in California’s Central Valley where over 90% of the natural wetlands have been converted to agricultural and urban uses. In the Central Valley today, a much smaller network of managed wetlands and flooded agricultural fields supports almost five million waterfowl and half a million shorebirds. Over 50% of waterbird habitat in the Central Valley is provided by flooded agricultural land, primarily rice (Oryza sativa). Each year non-breeding waterbird habitat decreases in the late winter as flooded agricultural fields are drained after waterfowl hunting season in late-January to prepare for the next crop. This study evaluated a practice called ‘variable drawdown’ that involves delaying the removal of water from rice fields by 1, 2, and 3 weeks to extend the availability of flooded habitat later into February and March. We studied waterbird response to variable drawdown in 2012 and 2013 at twenty rice farms throughout the northern half of the Central Valley. The staggered drawdown created a mosaic of water depths throughout the six-week study period. The 3-week delay in drawdown supported more dabbling ducks than earlier drawdowns in the first half of the study and more shorebirds and long-legged wading birds during the second half of the study. The timing of highest use of each drawdown treatment differed for each waterbird guild; dabbling ducks, geese and swans benefited at the beginning, then long-legged wading birds, followed by shorebirds. Despite the presence of appropriate water depths for shorebirds across the treatments during the entire study period, shorebird densities were highest near the end of the study when the 3-week-delayed drawdown was providing the majority of the habitat on the landscape. This suggests that shorebirds may have concentrated in our study fields due to decreasing availability of shallow water habitat elsewhere. The practice of variable drawdown successfully extended the availability of waterbird habitat provided by post-harvest flooded rice fields later into winter.
Highlights
IntroductionWetland loss and degradation has been extensive across the world [1,2] and the Central Valley of California, USA is no exception where over 90% of the estimated two million hectares of natural wetlands have been converted to agricultural and urban uses [3,4]
This creates a challenge for waterbirds since large numbers are still using the Sacramento Valley; waterfowl generally begin their northbound migration out of the Sacramento Valley in early March [9] and many shorebird species are still wintering in March as peak migration out of and through the region is in April [7]
Staggering the drawdown of winter-flooded rice fields, as this study has shown, could add value to any rice fields that are flooded for rice residue decomposition and/or hunting opportunities in other parts of the world [58] and could potentially be added to agri-environmental schemes
Summary
Wetland loss and degradation has been extensive across the world [1,2] and the Central Valley of California, USA is no exception where over 90% of the estimated two million hectares of natural wetlands have been converted to agricultural and urban uses [3,4]. Precipitation may extend flooding of rice into March [14,15], rainfall is highly variable from year to year (30-year precipitation [cm]: February minimum = 0.2, maximum = 30.7, mean = 8.2; March minimum = 0, maximum = 20.1, mean = 6.0) This creates a challenge for waterbirds since large numbers are still using the Sacramento Valley; waterfowl generally begin their northbound migration out of the Sacramento Valley in early March [9] and many shorebird species are still wintering in March as peak migration out of and through the region is in April [7]. Shorebirds presumably take advantage of the short period of shallow water created from the traditional drawdown practice in rice, but must move to other habitats, such as managed wetlands, or leave the Sacramento Valley [16,17]
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