The first study to measure wild ungulate (axis deer, mouflon sheep, goats, pigs, etc.) grazing on Hawaiʻi rangelands finds forage losses at two-thirds of sites surveyed, with axis deer the clearest driver of decline across ranches on Hawaiʻi Island and Maui.
Led by researchers at the University of Hawaiʻi at 惭ā苍辞补’蝉 College of Tropical Agriculture and Human Resilience (CTAHR), the research provides the first evidence that grazing by wild ungulates, particularly invasive axis deer, is reducing the forage at Hawaiʻi’s working cattle ranches. Published in Rangeland Ecology & Management, .
“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 Professor Melissa Price, corresponding author and researcher at CTAHR’s Department of Natural Resources and Environmental Management (NREM).
Measuring forage loss
Non-native ungulates 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 2,150 meters.
“Axis deer came out as the biggest driver of declining rangeland condition, which matches what ranchers have been telling us for years,” said CTAHR Extension agent Mark Thorne, a study co-author and rangeland specialist.
The team compared vegetation growth with and without ungulates using motion-triggered trail cameras alongside fenced exclusion plots. Sites dominated by axis deer produced about 24% 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.
Slow recovery
Several Maui sites sat within areas burned by the August 2023 wildfires. Researchers warned that overgrazed rangelands recover more slowly after fire and stay vulnerable to fire-prone weeds such as fireweed, found at nearly 70% of 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 co-author on the study. “It’s good to finally see numbers behind something ranching families here have been living with for a long time.”
The study was funded by the United States Department of Agriculture (USDA) Western Sustainable Agriculture Research and Education (SARE) program and led by Lauren Katayama, who recently earned a master’s degree from NREM.