It is a bit of a mystery how free-roaming horses came to live on Assateague Island, since they don’t naturally belong there.
Local legend says that the horses survived a harrowing shipwreck in the Atlantic Ocean centuries ago and swam to safety on Assateague. The more likely story is that in the 1600s European settlers trying to avoid livestock taxes brought them to the island, which stretches along the coastlines of Virginia and Maryland in the United States.
No matter how the horses ended up there, they’ve become an essential part of the island’s character, drawing thousands of visitors each year who come to see them wandering free in their coastal habitat. There are festivals and events to celebrate the horses and opportunities to view them and experience their beauty firsthand.
But the horses are an introduced species to the barrier island ecosystem, which is also home to many threatened species and rare plant communities. Over time, the herd grew exponentially and began to negatively affect the island’s unique ecology.
Barrier islands are typically long, narrow spits of land that play an important role in protecting coastlines from the worst wind and weather coming off the ocean. Assateague’s rich salt marsh ecosystem provides habitat for diverse wildlife, and on the dune side facing the open sea, native plants help hold the sand in place, resisting erosion.
The grazing of the horse herd was reducing the abundance of species, such as American beachgrass; without sufficient grasses to stabilize the dunes, the island would be at risk of being washed away by the waves. Also, the feeding and movement patterns of the horses were altering plant and animal communities, particularly in the salt marshes, allowing invasive plants to take hold and disrupting natural ecological processes and the nesting of many bird species.
The National Park Service, which manages Assateague Island National Seashore where the horses live, was faced with a challenge. In 1994, it had begun using noninvasive contraception on mares to slow the growth of the herd, but it wanted to define a target population size and determine the most effective way to get there. The goal was to manage the expanding population of horses to minimize negative effects on the island, and to do so in a way that would respect the public’s love for the horses—which meant not removing them from the island—and keep the herd robust and genetically diverse.
To help resolve the inherent conflicts between protecting both the feral horse population and the ecological integrity of Assateague Island, the National Park Service commissioned CPSG to lead them, along with several other stakeholder groups, through evaluating different management strategies for reaching their specific—and complicated—goals.
In early 2006, a group of stakeholders worked with CPSG to produce a horse population model that reflected the current situation on the island. Later in the year, the group gathered again to use this model to examine how the horse population and the island’s plant life would be affected by different management activities, such as modifying contraception rates.
After much discussion about potential options and competing goals for managing the island, the group agreed on a population target of 80 to 100 horses as the sweet spot. Any larger and the island’s plant communities would continue to decline; any smaller and the herd’s genetic health would suffer. This target could be adjusted as the effects of changing the population size were monitored.
The workshop results contributed a scientific foundation to a larger decision-making process organized by the Park Service to assess management options for the island. The chosen strategy reflected the workshop discussions and population model results, but also incorporated public comments and further interpretation of the best way to balance maintaining a healthy horse herd and restoring plant communities on the island.
“There’s no doubt that we will be referring to the workshop report for years,” said Carl Zimmerman, former National Park Service superintendent of Assateague Island. “It provided a great base for developing an environmental assessment of management strategies.”
Wildlife managers implemented the new plan to strategically decrease reproduction rates and over time successfully reduced the number of horses on the shores of Assateague. As of 2019, the population sits at the low end of its ideal range, so managers are allowing mares to reproduce freely until the population reaches the upper end of the range again.
The positive effects on the island’s ecosystem are apparent: American beach grass has rebounded. Other plants, like smooth cordgrass, have grown denser again in the absence of intensive grazing. As predicted, this horse population size has proven to be the right range for balancing multiple objectives in order to maintain the overall health of Assateague Island.
“The CPSG workshop process worked remarkably well and successfully navigated some very difficult issues,” said Zimmerman. “It really was the cornerstone of a very successful plan.”
