Invasive deer and other non-native animals are eating away Hawaiʻi’s ranchlands
A first-of-its kind study provides evidence of what ranchers have long observed: wild axis deer, mouflon sheep, goats and pigs are eating vegetation meant for cattle.
Axis deer are responsible for the greatest losses of grass and hay on ranches across Hawaiʻi Island and Maui, according to the first study to measure grazing on the state’s rangelands.
Researchers at the University of Hawaiʻi at Mānoa’s College of Tropical Agriculture and Human Resilience led the study, which provides the first evidence that grazing by wild ungulates – including axis deer, mouflon sheep, goats and pigs – is reducing the forage at Hawaiʻi’s working cattle ranches.
The study, published in Rangeland Ecology & Management and funded by the U.S. Department of Agriculture, documented forage losses at two-thirds of the 35 sites surveyed.
“This is the first study to actually measure how much forage wild ungulates are removing from working ranchlands in Hawaiʻi, and the losses are significant,” said study co-author, UH Professor Melissa Price, in a news release.
Non-native ungulates (hoofed animals) were largely introduced between the 1700s and 1960s. On Maui, axis deer were detected at 95% of surveyed sites, with roughly 47,000 deer ranging across a third of the island at elevations up to about 7,000 feet.
“Axis deer came out as the biggest driver of declining rangeland condition, which matches what ranchers have been telling us for years,” said extension agent Mark Thorne, study co-author and rangeland specialist.
Researchers used motion-triggered trail cameras to compare vegetation growth with and without ungulates. Sites dominated by axis deer produced nearly 25% less forage on average than ungulate-free sites. Feral goats showed similar impacts.
However, wild pigs and mouflon sheep frequented higher-elevation areas with generally faster forage production, illustrating how species-specific habits complicate land management.
Several Maui sites sat within areas burned by the August 2023 wildfires. Researchers warn that overgrazed rangelands recover more slowly after fire and remain vulnerable to fire-prone weeds such as fireweed, found at nearly 70% of the sites.
“We’ve had to shrink our herds some years because there just isn’t enough grass left once the deer get to it,” said Greg Friel, a Maui island cattle rancher and study co-author. “It’s good to finally see numbers behind something ranching families here have been living with for a long time.”
