Download Sable Island

Sable Island
Natural History
Top: S. Coffen-Smout
Bottom: D. fenton
Perched on the edge of the Continental
Shelf clings a crescent shaped isle
alone in the Northwest Atlantic. This is
Sable Island, Nova Scotia; a vegetated
sand bar approximately 290 km from
Halifax, at 44˚N 60˚W. Roughly 42 km in
length, and no more than 1.3 km at its
widest, Sable Island spans a total area
of 3000 ha, approximately seven times
the size of McNabs Island.
Location & Geography
The island is flanked on the north and south
by two long parallel beaches separated by a
mature dune system. The interior topography
is characterized by low rolling dunes with a
number of freshwater ponds, especially in the
west. Both the eastern and western tips are
2-6 km long flat expanses of sand known as
the East and West Spits. In the centre of
the island, and running for approximately
two-thirds of the total length of the island,
is a line of high dunes which is one of Eastern
Canada’s largest dune systems.
A freshwater lens, maintained by precipitation,
underlies Sable Island and surface exposures
of this lens form the island’s freshwater ponds.
The freshwater lens floats on top of the salt
water due to the difference in specific gravity.
The ponds provide habitat for unique plant
and animal communities and fresh water for
the island’s inhabitants. Potential saltwater
contamination of the ponds is considered a
major threat, particularly if dunes erode or
if the rate of consumption becomes greater
than the ponds’ capacity to recharge. Lake
Wallace, a very shallow brackish-water lake in
the centre of the island to the south, is about
1 km in length, but was once much larger,
having mostly been filled in by blowing sand.
The climate of Sable Island is temperate;
however, during winter months, the moderating
influence of the Gulf Stream results in some of
the warmest temperatures in Canada. Given
its remote position in the Northwest Atlantic,
the island is often exposed to very strong
winds. There are frequent heavy fogs in the
area: on average there are 127 days out of the
year that have at least one hour of fog.
The ponds provide
habitat for unique
plant and animal
communities and
fresh water for the
island’s inhabitants.
Photos: Bottom left – D. Fenton;
Top left – Environment Canada; Right – D. Fenton
Flora & Fauna
Sable Island’s unique topography and
dune system owe their existence to the
sand-anchoring properties of vegetation.
Approximately one-third of the island is
vegetated1 and most of the native flora of Sable
Island is typical to similar dune environments
in Eastern North America. The island’s
interior is largely dominated by heath-type
vegetation, while the extremities are vegetated
mainly by salt-tolerant and sand-loving plants,
such as sandwort and marram grass. Over
190 plant species have been identified on
Sable and 20 of them are considered to have
a restricted distribution elsewhere.2
Since the 16th century, many domestic
animals, including horses, cattle, goats, and
rabbits, have been released on the island.
The ecology of the island prior to such
introductions is largely unknown. Currently,
the only introduced mammal on the island
is the feral horse. The horses are believed to
have been introduced sometime shortly after
1738. Their numbers have ranged between
150 and 400 individuals, with current estimates
placing them at a minimum of 375 animals.3
They are dependent upon the island’s
vegetation for their nutrition, but supplement
their diet with kelp and seaweed that wash
up on the beach.
Photos: Bottom left – D. austin; Top Right – D. Austin
The horses are believed
to have been introduced
sometime shortly after 1738.
The island was once home to a breeding
population of the Atlantic Walrus (listed as
Extirpated under the Species at Risk Act).
They were hunted heavily for their ivory and
were last seen on the island in the late 1800s.
Today evidence of walrus on the island can be
found when shifting sands reveal remaining
walrus tusks, having been buried for possibly
hundreds of years.
Sable Island is home to the world’s largest
breeding colony of grey seals, which pup
between late December and early February.
The population has experienced significant
growth in recent decades, but the rate of
growth has slowed more recently.4 During
the breeding season, grey seals occupy
the beaches and both vegetated and
non-vegetated inland habitats throughout
the island. For the remainder of the year, grey
seals generally haul out on the beaches in
groups of dozens to thousands of individuals,
with a particularly large number on the island
during the annual moult in May and June.
There is also a small resident population of
harbour seals which pup from mid-May to
mid-June. This population has decreased
in recent years, with shark predation and
competition from grey seals suggested as
reasons for the decline.5 Other visitors include
ringed, harp and hooded seals; however, they
do not breed on the island.
Flora & Fauna
Sable Island is part of a key migratory flyway,
with numerous bird species (over 350)
recorded. These include species similar to
those found on the mainland, but also a high
number of vagrants that are far from their normal
habitats. Sixteen bird species have been
confirmed to breed on the island during the
spring and summer months. The best-known
bird associated with the island is the Ipswich
Sparrow, a subspecies of the Savannah
Sparrow, which breeds only on Sable Island.
The island also has a significant gull population
that can be found there year-round. Both
the Ipswich Sparrow and Roseate Tern are
classified as Special Concern and Endangered,
respectively, under the Species at Risk Act.
Critical habitat for Roseate Terns has been
identified in several locations and must be
protected from disturbance during the
breeding season.
The freshwater ponds on the island (primarily
in the area between Main Station and BIO
House) support a number of fish species,
including mummichogs, sticklebacks and
American eel. Sable is home to at least six
invertebrate species found only on the island;
in particular, there are several species of
butterflies and moths that may be sufficiently
different from mainland populations that they
could be classified as subspecies.6 Noteworthy
among the stray species which have been
sighted on Sable are two tropical species of
moth/butterfly, recorded for the first time in
Nova Scotia.
The unique assemblage of the flora and fauna
of Sable Island combined with its unusual
topography results in a fascinating narrative
of geographical and ecological dynamics.
Sources
1
Freedman, B. 1996. Airphoto assessment of changes in plant cover
on Sable Island, Nova Scotia. Unpublished report, Canadian Wildlife
Service
2
Catling, P.M., Freedman, B. & Lucas, Z. 1984. The vegetation and
phytogeography of Sable Island, Nova Scotia. Proceedings of the
Nova Scotian Institute of Science 34: 181-245
3
Lucas, Z.L., McLoughlin, P.D., Coltman, D.W., & Barber, C. 2009
Multiscale analysis reveals restricted gene flow and a linear gradient
in heterozygosity for an island population of feral horses. Canadian
Journal of Zoology, 87: 310-316
4
DFO. 2009. Science Advice on Harvesting of Northwest Atlantic grey
seals (Halichoerus grypus) in 2009. DFO Can. Sci. Advis. Sec. Sci.
Advis. Rep. 2008/061
5
Bowen, W.D., Ellis, S.L., Iverson, S.J., and Boness, D.J. 2003.
Maternal and newborn life-history traits during periods of contrasting
population trends: implications for explaining the decline of harbour
seals (Phoca vitulina), on Sable Island. J. Zool. Lond. 261:155-163
6
Wright, B. 1989. “The Fauna of Sable Island.” Nova Scotia Museum
Curatorial Report Number 68
Photos: Bottom left – A. Boyne;
Top left – D. Austin; top right – Environment Canada