These data do not reflect wetland loss or change associated with the conversion of the tall grass prairies of southwestern Louisiana to agriculture. Essentially all tall grass prairie that existed north of the coastal marsh has been converted to agriculture. Further, much of the area has been land-leveled, which eliminated many depressional wetlands and upland ridges that were important habitat for mottled ducks and a number of other species (Hobaugh et al. 1989). The value of this habitat to waterfowl prior to conversion is not well known, but this grassland area had a moderate density of shallow depressional wetlands that probably provided fair waterfowl habitat. With conversion to agriculture dominated by rice production, the region increased dramatically in importance to ducks, geese, and shorebirds. Since 1974, rice production in the region has declined 47% as a result of world competition, escalating costs of water for irrigation, and the reduction or removal of government subsidies. Declines in production have been more severe in the Texas Mid-Coast (50%) and Texas Chenier Plain (65%) than in the Louisiana Chenier Plain (40%) (USDA National Agricultural Statistics Service). Land taken out of rice production in some cases is converted to other crops of less (milo) or no (sugar cane) value to waterfowl, dry of wet pasture of moderate value to some species of waterfowl, or is lost to invasive, exotic stands of Chinese tallow tree.
Causes of wetland loss along the Gulf coast are complex, and both natural and man-induced in origin. Two related factors are at the root of much wetland loss in the region: (1) sea level rise; and (2) subsidence. The average rate of sea level rise currently is about 0.12 cm/yr. Until recently, rate of sea level rise has been slow relative to the rate of subsidence of coastal lands (LCWCRTF 1998). Subsidence is the combined effect of compaction of loosely consolidated, highly organic sediment, and geological movement along fault lines (LCWCRTF 1998). The rate of subsidence across the coast varies from 0.4 to 1.3 cm/yr (Penland et al. 1989; Gagliano and Van Beek 1993, Kuecher 1994). In combination, however, the two contribute greatly to overall rates of marsh loss. Other factors interact with subsidence and sea level rise and affect marsh loss rates and waterfowl habitat carrying capacity. These include alterations to hydrology (and salinity levels), reduced sedimentation rates, storms, erosion, herbivory by exotic nutria, and losses to exotic vegetation including water hyacinth, giant and common salvinia, and Chinese tallow trees (LCWCRTF 1998).