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.