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

ADRIENNE L. CONTASTI, FLORIS M. VAN BEEST, ERIC VANDER WAL and PHILIP D. MCLOUGHLIN
The Journal of Wildlife Management
Vol. 77, No. 8 (November 2013), pp. 1545-1552
Published by: Wiley on behalf of the Wildlife Society
Stable URL: http://www.jstor.org/stable/24365355
Page Count: 8
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Abstract

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 (λ > 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 (λ = 0.951) without immigration. Further, the eastern subunit would have declined in 2 intervals (λ = 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.

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