Distribution, structure and simulation modelling of the Wattled Crane population in the Marromeu Complex of the Zambezi Delta, Mozambique

The status of the Vulnerable Wattled Crane (Grus carunculatus) in Mozambique is poorly known, but historical accounts indicate that the species was previously more abundant and widespread than today. Annual surveys during 1995–2002 suggest a core population of about 120 breeding pairs remains in the Zambezi Delta region. Wattled Cranes in the delta are exclusively associated with sedges of the genus Eleocharis, the tubers of which provide the adult cranes' main food supply. The main Eleocharis areas in the delta, and those supporting the highest density of Wattled Cranes, occur below the adjacent Cheringoma escarpment, where unregulated streams flow onto the floodplain. These wetlands experience some seasonal inundation in all years — conditions essential for the production of underground tubers — and high soil penetrability to enable the cranes to extract tubers. Eleocharis tuber production and soil penetrability is extremely low in the remaining vast areas of the delta that no longer receive regular annual flooding due to regulation of the Zambezi River. Significant differences in crane density between the Eleocharis beds of the Cheringoma and Zambezi floodplains suggest that the carrying capacity of the delta for cranes has been reduced. Simulation modelling suggests that the present population of Wattled Cranes in the Zambezi Delta is viable, despite the long-term, severe hydrological degradation of large parts of the floodplain. Restoration of the hydrological conditions in the delta may have global implications for the species, however. In 1990, an estimated 2 570 Wattled Cranes (more than 30% of the global population) were observed in the delta. This was likely an occasional flock from elsewhere in southern Africa, as prolonged regional drought resulted in failed floods, low tuber productivity and relatively impermeable soils in the region.