2.1 | Study area and population
Sable Island National Park Reserve, a crescent-shaped emergent sand bar, located 175 km off the east coast of Nova Scotia, Canada, spans ~49 km and is ~1.2 km at its widest point (Figure 1). The treeless island is dominated by marram grass (Ammophila breviligulata ), common species in early-successional grasslands, occurring both in pure swards and in mixed communities alongside other species such as red fescue (Festuca rubra ), beach pea (Lathyrus japonicus var. maritimus ), and forbs such as meadow rue (Thalictrum pubescens ) or pearly everlasting (Anaphalis margaritacea ). These grasslands comprise the most common vegetation association (Contasti, Tissier, Johnstone, & McLoughlin, 2012). Sheltered by 10–30-m high dunes, in the interior of the island grasslands give way to late-successional mixed heath communities characterized by shrubs (e.g. common juniper [Juniperus communis var. megistocarpa ], lowbush blueberry [Vaccinium angustifolium ], northern bayberry [Myrica pensylvanica ]), and the presence of an organic soil layer (Catiling, Lucas, & Freedman, 2009; Tissier, Mcloughlin, Sheard, & Johnstone, 2013). Dune height and vegetated landcover decrease as the island tapers towards its longitudinal extremes, where the semi-succulent forb sandwort (Honckenya peploides ) dominates at the edges of dunes, and beach pea and seaside goldenrod (Solidago sempervirens ) are co-dominant with marram grass (Catiling et al., 2009; Tissier et al., 2013).
Introduced to the island circa 1750 and studied intensively by our research group since 2007 (Contasti, Van Beest, Vander Wal, & McLoughlin, 2013; Gold et al., 2019), the feral horses are the only terrestrial mammal found on the island (Freedman, 2016). Since their introduction, the horses have remained unmanaged with very limited introgression from mainland domestic stock (most recently a single adult male in the 1930s; Welsh, 1975). The horse population, 550 individuals in 2014, declines sharply in density from west to east (Marjamäki, Contasti, Coulson, & Mcloughlin, 2013). A polygynous mating system exists among the island’s horses, characterized by mixed-sex social bands guarded by (often) a single dominant adult male (stallion) against mating attempts by other males. Females in the population invariably segregate across these mixed-sex social bands which are comprised of the dominant stallion, adult females (mares), and subadult (<3 years of age) offspring (Regan et al., 2019). Bands can therefore be as small as 2 (one adult male and one female), although bands as large as 16 horses have been observed (Manning & McLoughlin, 2017). Band memberships are stable across years but 67% of adult females were known to disperse to a different social band at least once during a 7-year period (Debeffe, Richard, Medill, Weisgerber, & McLoughlin, 2015). Outside of social dispersal events, social bands traverse the landscape together but very rarely stray farther than 4000 m in either direction from the centre of their home-range during the summer. Most bands constrain their movements to <2000 m from their home-range’s centre (Rozen-Rechels et al., 2015). Bacterial dispersal between horses is expected to occur primarily between members of the same—or interacting—social bands and be facilitated by grooming, coprophagy, interactions with faecal territorial markers (stud piles), or the use of shared resources (Figure 2). Social dispersal of horses might likewise facilitate bacterial transmission between social bands over longer distances.