Temporal and spatial variability in seasonal breeding success of the Florida wood stork (Mycteria americana).

摘要

Since the wood stork (Mycteria americana, 1758) was first listed as endangered by federal authorities in 1984 (USFWS 1984), scientists have sought to identify what factors were responsible for driving down population numbers, particularly in Florida. Effectively managing these populations requires a thorough understanding of their habitat requirements and physiological tolerances at different life-history stages. Many scientists have focused on early-stage survival, which is most critical for population sustainability. For this study, surveys were conducted at four east-central Florida breeding bird colonies to determine which habitat requirements and environmental factors were most influential in determining the number of nestlings that survived to fledge. Deseret Ranch, North Fork, Bird Island, and Pelican Island were all surveyed weekly during the 2004, 2005, and 2006 breeding season to determine (1) the number of fledges produced per nest, (2) the survival time of nestlings, (3) the timing of nestling mortality, (4) the number of nests per colony, and (5) number of failures per colony. The mean number of fledges produced per nest was compared among colonies and among years using the Generalized Linear Model with an ordinal multinomial distribution and a logit function. Kaplan Meier survival curves were derived and compared for both pre-hatch and post-hatch nests for each site over time. Relationships with environmental variables were examined by regressing the productivity data against wetland habitat availability, monthly average maximum temperature, and monthly total precipitation. Wetland availability was quantified using a weighted proportionality index (WPI), calculated by the product of weighting factor and the number of hectares of wetland within a 5, 10, 15, and 20 kilometer radius of each colony. A logit regression was used to determine whether the failure of nests was related to the timing of nest initiation relative to the onset of the wet season.;Deseret Ranch and North Fork had the highest mean fledging success during each of the three years colonies were surveyed. The lowest productivity occurred during 2005, while the highest occurred during 2006. Among colony comparisons showed that the two inland colonies, Deseret Ranch and North Fork, differed from the coastal colonies during active years. All survival curves for post-hatch nests significantly differed among years, except for the curves generated for the 2004 and 2005 breeding season at North Fork and Bird Island. The only year where pre-hatch survival curves differed among colonies was 2005. Pelican Island failed to hatch any nestlings in 2005, and failed to start in 2006.;Regression analysis of productivity data generated by this study and 6 other Florida sites, showed that fledging success increased with wetland habitat availability (r2 = 0.63 and p = 0.006). There was also a significant relationship between the difference in yearly mean fledging success and (1) the monthly average maximum temperature in April and May (R = 0.81 and 0.71, respectively)) and (2) the monthly total precipitation for March (R = -0.89).;The availability of wetland habitat may contribute to colony productivity, which could ultimately impact future population growth. Modeling projections of future population numbers, based on specified gains or losses of wetland habitat, demonstrate that continuing annual losses of 5000, 10,000, and 15,000 acres of wetland habitat could result in population declines of the U.S. wood stork population within the next 50 years. Maintaining and protecting existing wetland habitats may be the best insurance to protect the U.S. wood stork population from future declines.