Whooping crane (Grus americana) demography and environmental factors in a population growth simulation model
Gil de Weir, Karine
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The Whooping Crane (Grus americana) is among North America??s most charismatic species. Between 1938 and 2004, the population that migrates between Aransas National Wildlife Refuge (ANWR) and Wood Buffalo National Park (WBNP), grew from 18 to 217 individuals. The recovery plan objective for this endangered species is to downlist the population in 2035, but this requires interpretive assessment of population responses to environmental factors over the long term. I analyzed 27 years of banding data, 37 years of nest monitoring data, and 20 years of winter reports to estimate age-specific mortality and fecundity rates. The resulting life table yielded an intrinsic rate of increase (r) of 0.14/y, a net reproductive rate (Ro) of 6.4/y, and a mean length of a generation (G) of 13y. Path analysis of environmental factors, demographic variables (natality and mortality), and the finite rate of population increase (lambda) showed that annual mortality, temperatures from the ANWR, WBNP and at a migration stop-over in Nebraska, and pond water depth were good predictors of lambda variability. However, other environmental factors were significantly correlated: at ANWR, October- March temperature (extreme minimum and maximum), December temperature (mean and extreme minimum), November-January precipitation, and September-March freshwater inflow; at WBNP, March-September precipitation, March-May temperature, and temperatures during the September - October fall migration. The Pacific Decadal Oscillation (PDO) affected lambda indirectly through environmental factors in Nebraska and ANWR. I graphically analyzed relevant data trends from 1967 to 2004 to identify the relation between phases of PDO and environmental and demographic variables. During PDO cold phases, a synchronization of ??extreme?? environmental values was observed from the different regions; during warm phases extreme environmental values were scattered. Most periods of Whooping Crane population decline happened during cold phases. I developed a compartment model to represent Whooping Crane population dynamics utilizing the new data on survivorship and fecundity from banded birds. The model was capable of simulating historical population trends with adjustments in brood success and egg mortality. The model will allow future studies to test population responses to various environmental scenarios at the WBNP, during fall and spring migrations, and at the ANWR.