Integrated population modelling reveals a perceived source to be a cryptic sink

Abstract

Summary1. 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. 2. 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 sourcesink 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. 3. 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. 4. 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.5. 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-markrecapture, census and recruitment data are essential to correctly ascribing source-sink status and accurately informing development of site-safeguard networks.

Source-Sink Dynamics: a Neglected Problem for Landscape-Scale Biodiversity Conservation in the Tropics

“…Typically, long time series are also necessary to identify likely sources and sinks with any confidence (e.g. [11]), and to discount the possibility that apparent sinks are actually 'pseudo-sinks', where population growth rates are temporarily suppressed due to density-dependent competition, rather than underlying habitat quality [21]. It seems likely that the difficulty and costliness of collecting the required data to detect a source-sink pattern is the primary reason for the paucity of research.…”

“…Failure to identify sinks can lead to erroneous or perverse outcomes from conservation decision-making. For example, Wexford Slobs, Ireland, has the largest and most stable subpopulation of the threatened Greenland White-fronted Goose Anser albifrons flavirostris and thus has been identified as the critical location for protection [11]. However, using 29-years of capture-mark-recapture, census and recruitment data to inform integrated population modelling, Weegman et al [11] showed that Wexford Slobs are actually a sink, with persistence of the subpopulation only possible via high rates of immigration that exceeded emigration in each year.…”

“…For example, Wexford Slobs, Ireland, has the largest and most stable subpopulation of the threatened Greenland White-fronted Goose Anser albifrons flavirostris and thus has been identified as the critical location for protection [11]. However, using 29-years of capture-mark-recapture, census and recruitment data to inform integrated population modelling, Weegman et al [11] showed that Wexford Slobs are actually a sink, with persistence of the subpopulation only possible via high rates of immigration that exceeded emigration in each year. As another example, Gulo gulo wolverines in Norway are hunted in high-density areas where they are assumed to have large productive populations, but analysis of survival and emigration patterns instead reveals that these populations are maintained by compensatory immigration from Sweden, where wolverines are totally protected [12].…”

Integrated population model reveals that kestrels breeding in nest boxes operate as a source population

“…Indeed large populations may be sinks maintained by small source populations (Pulliam 1988) and density could be decoupled from habitat quality (Van Horne 1983). However, estimating all these parameters for the same population is highly challenging and few studies have been able to investigate source-sink dynamics with this level of accuracy (Weegman et al 2016). The comparison between the realized population growth rate (λ real ) and the theoretical population growth rate (λ) calculated from survival and reproduction has often been used to diagnose whether a population is a sink or a source (Pulliam 1988, Runge et al 2006).…”

“…These models jointly analyze different demographic datasets on the same population (Besbeas et al 2002). IPMs are powerful tools to make inference about dispersal parameters (immigration, emigration) (Abadi et al 2010) and have been recently used to assess the source-sink status (Weegman et al 2016, Millsap 2017. IPMs are powerful tools to make inference about dispersal parameters (immigration, emigration) (Abadi et al 2010) and have been recently used to assess the source-sink status (Weegman et al 2016, Millsap 2017.…”