By Margaret Evans

Nowhere in Canada will you find a more unique, self-contained ecosystem than the one found on Sable Island. Nestled in the 42 km arc of sand, 300 kilometers southeast of Halifax, Nova Scotia, is an array of beaches, dunes, marram grass, wildflowers, shrub-heath, and ponds – which 300 bird species five species of seals, and 500 Sable Island horses call home. These horses came to the island during the dark days of deportation of the Acadian people 250 years ago. 

By 2016, to everyone’s delight, the horse population that had historically stabilized at 300 animals had increased to 500 and the ecological link in the food chain tied the horses to pregnant grey seals. In the 1960s seal numbers were less than 1,000 but today that has exploded to 400,000. Every year, pregnant females haul out on the beaches where they give birth to hundreds of pups. Their waste fertilizes the grasses, which in turn provide nutritional grazing for the horses.

These tough little horses face many challenges including exposure to harsh weather, an extremely high parasite load, the ingestion of large amounts of sand, and the abrasive action of sand on their teeth, which affects their ability to chew and extract needed nutrients. Photo: Shutterstock/Julie Marshall

But in 2017, the population took a hit when 50 horses died.

“In April 2017, there were almost equal numbers of emaciated yearlings as adults (about a dozen each) as well as more normal numbers of adult animals (5) with other causes of mortality (sand colics, bad teeth, bad feet),” says Dr. Emily Jenkins, Department of Veterinary Microbiology, University of Saskatchewan. “We did not examine all the mortalities on the island but estimate that there were probably about 50 in total (10 percent mortality). In 2018, there were only two younger animals and three adults, a more normal level of mortality (around one per cent).”

Despite the fact that the benefits of seal reproduction continued to provide fertilizer for vegetation, Jenkins says that seasonal growth has a big impact. It was clear from their examination of carcasses that starvation was a key factor in the animals’ deaths, especially the young ones. Given their lower status in the band, they would have had fewer options for grazing.

Every year, when the grey seals give birth on the beach, their waste fertilizes the grasses upon which the Sable Island horses graze. Photo: Wikimedia/Paul Gierszewski

Evidence for that is found in the horses’ bones. Healthy horses have fat reserves throughout their bodies, such as under the skin, around the organs, and in bone marrow. When food is scarce, fat in the marrow is the last to go.

“On average, the emaciated horses had less than or equal to 10 percent bone marrow fat,” says Jenkins. “In 2017, the green-up was late, likely due to severe weather conditions. There was snow on the ground and little green in sight on April 1, 2017. This and a stormy March likely hit the weaker animals particularly hard that year.”

What was especially telling was the amount of sand the horses had ingested. The research team found pounds and pounds of sand in the ingesta and suspected that all that sand in combination with bad teeth contributed to mortality in several cases, especially the adult horses.

Photo: Shutterstock/Julie Marshall

In addition, parasite levels are high for these horses. Live, healthy Sable Island horses have average fecal egg counts three times what is considered high in a domestic horse.

“[They have] 1,500 eggs per gram of feces compared to 500. Normal for them is ‘off the chart high’ for domestic horses,” says Jenkins.

The wild, remote conditions, their endless exposure to storms, and the changing climate bringing more frequent extreme weather events add greater stress to their existence. Their consumption of sand can clearly be lethal and their worn teeth from the abrasive sand content in grass means they can’t chew their food properly and extract the nutrients they need. Couple that with their own dynamics of reproduction and health, and the pattern of cyclic loss in any wild population, and it can be seen as a normal ebb and flow in the face of a perfect storm of conditions.

Photo: Shutterstock/Julie Marshall

“That is more likely given what we know about regulation of wild populations in general,” says Jenkins. “For Sable specifically, they have seasonally restricted nutrition and high densities, high levels of gastrointestinal parasites, a suite of respiratory and reproductive challenges (including diseases and selenium deficiency), inbreeding, and the occasional harsh winter to contend with. These periodic die-off events have been recorded previously. There was a similar die-off in 1972 according to the PhD student working on the island at that time, who also did about 30 necropsies.”

However, during an average year there are very few mortalities, even over the winter. Given the particularly harsh winter of 2017, the horses were hit hard. But in 2018, Jenkins says that it was noticeably warmer, snow-free, and more shoots were visible in early spring. The status of the horses this year, spring 2019, is also promising.

“It appears to be a normal year this spring – certainly nothing like the die-off event in 2017,” she says.

Satellite image of Sable Island, Nova Scotia, taken by NASA. 

Going forward, biologists with Parks Canada will be able to consult with Jenkins and her colleagues on their findings. Since 2010, when Sable Island was designated a national park, Parks Canada has been managing the wildlife and resources of the island.

“Parks Canada is openly consulting about this and our findings will hopefully provide evidence on which to base decisions about the sustainability and future of this incredible population.”

Main photo: Shutterstock/Julie Marshall