Identifying hidden sinks in growing populations from individual fates and movements: The feral horses of Sable Island

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. © 2013 The Wildlife Society.

Local density and group size interacts with age and sex to determine direction and rate of social dispersal in a polygynous mammal

“…The broad‐scale treatments of horse movements on Sable Island by Contasti et al. (, ) and van Beest et al. () are more in line with the discrete nature in which density‐dependent dispersal has thus far been addressed by most researchers (e.g., Amarasekare ,b; Clobert et al.…”

“…'s () west, central, and east divisions of Sable Island, movements among these spatially distant zones are significantly linked to density (Contasti et al. , ; van Beest et al. ).…”

“…Contasti et al. (, ) and van Beest et al. () describe the study area, including the west–east gradient that underlies variation in individual exposures to local density (Fig.…”

Environmental and demographic drivers of male mating success vary across sequential reproductive episodes in a polygynous breeder

“…Sable Island National Park Preserve is an emergent 49‐km long by 1.5‐km wide sandbar located 275 km southeast of Halifax, Nova Scotia, Canada (43°55′N, 60°00′W). East–west resource gradients (i.e., in availability of freshwater and forage; Figure ) across the island's length underlie spatially heterogeneous demographic rates and density‐dependent habitat selection processes in this horse population (Contasti, van Beest, Vander Wal, & McLoughlin, ). The horses have been completely isolated from introgression since 1935 and are without confounding effects of predation, human interference, or interspecific competition (they are unmanaged and the island's only terrestrial mammal; Christie, ; Plante et al, ).…”

“…Mating success for each dominant male was defined as the number of females qualified to mate (≥2 years) within a band during each breeding season (Berger, ; Lucas, Raeside, & Betteridge, ; Shuster & Wade, ). We considered adult female abundance as a qualified index of mating success because pregnancy rates are typically high in this population (>75%; Welsh, ), leading to high rates of population growth (Contasti et al, ) and a correlation between the number of pregnancies and females. We assigned mating success only to dominant males, including those in bands where a tag male was present (<0.05% of our observations).…”