title: Integrated population modelling reveals a perceived source to be a cryptic sink.
abstract: Demographic links among fragmented populations are commonly studied as source-sink dynamics, whereby source populations exhibit net recruitment and net emigration, while sinks suffer net mortality but enjoy net immigration. It is commonly assumed that large, persistent aggregations of individuals must be sources, but this ignores the possibility that they are sinks instead, buoyed demographically by immigration. We tested this assumption using Bayesian integrated population modelling of Greenland white-fronted geese (Anser albifrons flavirostris) at their largest wintering site (Wexford, Ireland), combining capture-mark-recapture, census and recruitment data collected from 1982 to 2010. Management for this subspecies occurs largely on wintering areas; thus, study of source-sink dynamics of discrete regular wintering units provides unprecedented insights into population regulation and enables identification of likely processes influencing population dynamics at Wexford and among 70 other Greenland white-fronted goose wintering subpopulations. Using results from integrated population modelling, we parameterized an age-structured population projection matrix to determine the contribution of movement rates (emigration and immigration), recruitment and mortality to the dynamics of the Wexford subpopulation. Survival estimates for juvenile and adult birds at Wexford and adult birds elsewhere fluctuated over the 29-year study period, but were not identifiably different. However, per capita recruitment rates at Wexford in later years (post-1995) were identifiably lower than in earlier years (pre-1995). The observed persistence of the Wexford subpopulation was only possible with high rates of immigration, which exceeded emigration in each year. Thus, despite its apparent stability, Wexford has functioned as a sink over the entire study period. These results demonstrate that even large subpopulations can potentially be sinks, and that movement dynamics (e.g. immigration) among winters can dramatically obscure key processes driving subpopulation size. Further, novel population models which integrate capture-mark-recapture, census and recruitment data are essential to correctly ascribing source-sink status and accurately informing development of site-safeguard networks.
authors.affiliation: Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK.|Wildfowl & Wetlands Trust, Slimbridge, Gloucester, GL2 7BT, UK.
authors.forename: Mitch D
authors.initials: MD
authors.lastname: Weegman
authors.affiliation: Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK.
authors.forename: Stuart
authors.initials: S
authors.lastname: Bearhop
authors.affiliation: Department of Bioscience, Aarhus University, Kalø, Grenåvej 14, Rønde, DK-8410, Denmark.
authors.forename: Anthony D
authors.initials: AD
authors.lastname: Fox
authors.affiliation: Wildfowl & Wetlands Trust, Slimbridge, Gloucester, GL2 7BT, UK.
authors.forename: Geoff M
authors.initials: GM
authors.lastname: Hilton
authors.affiliation: National Parks and Wildlife Service, Wexford Wildfowl Reserve, North Slob, Wexford, Ireland.
authors.forename: Alyn J
authors.initials: AJ
authors.lastname: Walsh
authors.affiliation: Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: McDonald
authors.affiliation: Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK.
authors.forename: David J
authors.initials: DJ
authors.lastname: Hodgson
mesh_terms: D025041:Animal Migration; D000818:Animals; D001499:Bayes Theorem; D005777:Geese / Q000502:physiology*; D007494:Ireland; D008954:Models, Biological; D011157:Population Dynamics; D012621:Seasons
keywords: capture-mark-recapture; demography; density dependence; dispersal; extinction risk; hierarchical model; long-distance migrant bird; metapopulation model