Influence of historic landscapes and contemporary species management on Chesapeake Bay bald eagles and osprey.

摘要

Co-distributed species with well documented demographic histories can provide good models for testing alternative hypotheses about the impact of evolutionary history, contemporary landscapes, and species management on current distribution and population structure. The Osprey (Pandion haliaetus ) and Bald Eagle (Haliaeetus leucocephalus) have been extensively studied, managed and monitored across their North American breeding range, particularly in the Chesapeake Bay. We used a combination of ecological niche modelling, diet reconstruction, and population genetic modeling to understand the role of historic events---both shallow and deep time---on contemporary species distribution. The first objective of this study was to develop contemporary and paleo-distributional models for North American Bald Eagles and Osprey in order to explore the geographic histories of the two species, including the identity of possible Pleistocene refugia. Potential distribution during past (e.g. Last Glacial Maximum, LGM) and possible future climate scenarios were developed with species occurrence records for Osprey (n = 3034) and Bald Eagles (n = 8859) combined with 19 bioclimatic variables representing current conditions using the maximum entropy model (MaxEnt). Paleoclimatic models predict multiple putative refugia that may explain differences in migratory behavior between the two currently co-distributed species, as well as geographically defined sub-populations within each species. We conducted bulk stable isotope analysis of feathers collected from museum specimens and contemporary nests to investigate the influence of historic declines in critical prey species on distribution of Bald Eagles (n = 41 ) and Osprey (n = 45) in the Chesapeake Bay over the past 140 y. Stable Isotope Analysis in R (SIAR) was used to estimate the relative contribution of potential prey items in order to test the hypothesis that migration of estuarine-dependent and anadromous clupeid fishes represents an historically important seasonal subsidy in the form of marine-derived organic matter (MDOM). SIAR results demonstrate that MDOM contributed approximately 50% of the carbon and nutrients to Bald Eagle and Osprey occupying the upper estuary historically but declined to less than 5% of contemporary diets. Declines in anadromous prey in the diet correspond with historic spatial shifts in distribution and population growth of avian predators over the same period. Finally, we tested the hypothesis that conservation efforts, specifically translocation or "hacking" programs, rather than biogeographical history, best explains the current pattern of genetic variation exhibited by Osprey across their North American breeding range. We genotyped 11 microsatellite loci and a 513 base pair sequence of the cyt b region from 433 Osprey samples in order to investigate current population substructure, the genetic consequences of historic demographic bottlenecks, and the influence of hacking programs on contemporary gene flow. We calculated genetic differentiation (Dest) and Isolation-By-Distance (IBD) among regional populations and spatially cohesive genetic clusters identified using the program STRUCTURE. Our results indicate that although Osprey nesting in North America are subdivided into multiple cohesive genetic clusters, genetic differentiation among groups is low and unrelated to geographic variation. The findings of this study are discussed in light of past and present management practices and broader issues salient to species management and conservation of genetic diversity and adaptive response to future environmental change.