Increasing Seal Pup Numbers Influence Feral-Horse Feeding Habits

Researchers reveal how seals affect vegetation patterns and influence the movement of feral horse populations on Sable Island in Canada.

Catherine Offord
Jul 31, 2016

SEA TO LAND: Feral horses on Sable Island, Canada, munch on marram grass that has been enriched with nitrogen from local seal populations. SARAH MEDILL


The paper
P.D. McLoughlin et al., “Density-dependent resource selection by a terrestrial herbivore in response to sea-to-land nutrient transfer by seals,” Ecology, doi:10.1002/ecy.1451, 2016.

Island living
Working on Sable Island, Nova Scotia, population ecologist Philip McLoughlin noticed that many of the local feral horses visited a small spit of land on the island’s west coast to eat marram grass and other vegetation. But flicking back through photos of the area, the University of Saskatchewan researcher found that 50 years ago, the spit had been just a strip of sand. “Something had happened since the 1960s to make this an important area for the horses,” he says.

Seal explosion
One thing the team knew had changed was the number of pupping gray seals in the area. In the 1960s, the population was probably under 1,000, McLoughlin says; now nearly 400,000 seals use the island. The animals could be transferring nutrients from the sea via defecation or their decaying carcasses, McLoughlin reasoned—promoting vegetation growth, and consequently influencing the behavior of the island’s largest herbivores.

Picky eaters
McLoughlin and colleagues used stable isotope measurements in marram grass to show that seals do indeed enrich vegetation with nitrogen. They then used modeling to demonstrate that horses preferentially selected those areas of nitrogen-enriched vegetation to eat. “It was really neat,” says Douglas McCauley of the University of California, Santa Barbara. “It takes the stage with a handful of studies that are wonderful examples of how intimately connected living systems actually are.”

The circle of life
McLoughlin now wants to look at how the seals’ impact perpetuates. “The next step is to ask what this means for how horses move around, their population dynamics, and how they are distributing these nutrients across the island.”