Identifying Hidden Sinks in Growing Populations From Individual Fates and Movements: The Feral Horses of Sable Island

Identifying the existence of population sinks is critical for conservation and management. However, because of density-dependent dispersal, sinks can sometimes be masked by immigration events, especially during phases of population growth. We present a large-scale, empirical demonstration of within-population source-sink dynamics using the feral horses (Equus ferus caballus) of Sable Island National Park Reserve, Nova Scotia, Canada, as a model. We tracked the fates and movements of 98.7% of the female population (n=190-237) across 3 demographic clusters (subunits) during a period of rapid population growth (2008-2010; 24.7% increase in density). All subunits experienced increases in population size each year (>1.0). Our individual-based analysis showed that western Sable Island, where water availability was greatest, behaved as a source and would have grown with or without immigration in all years. However, the central (and fastest growing subunit) would have declined from 2008-2009 (=0.951) without immigration. Further, the eastern subunit would have declined in 2 intervals (=0.932, 0.999) without immigration. Our study demonstrates that the propensity of habitat to act as a sink can be masked during a period of population growth because of density-dependent immigration from adjacent habitats. These findings present a caution to managers charged with conserving wide-ranging species with long population cycles for which effects of immigration on local population growth rate can be difficult to isolate using standard methods of enumeration. (c) 2013 The Wildlife Society.