The Impact of Wild Ungulate Grazing on Rangeland Forage Production in the Hawaiian Islands

Invasive wild ungulates pose significant global concerns due to their impact on ecosystem functions and competition with native species. Many island systems lack large native predators to control wild ungulate populations, which intensifies these adverse effects, particularly on agricultural lands. Across the Hawaiian Islands, approximately 404 686 ha are dedicated to livestock production, yet the overabundance of axis deer ( Axis axis ), mouflon sheep ( Ovis musimon ), feral pigs ( Sus scrofa ), and feral goats ( Capra hircus ) has led to overgrazing, increased sediment runoff, the proliferation of invasive plants, and declining ecosystem health, threatening the sustainability of generational ranching. This study examined the effects of wild ungulate grazing and site conditions on forage production on rangelands across the islands of Hawaiʻi and Maui and assessed the relationship between plant community composition and wild ungulate relative abundance. We employed game cameras, grazing exclusion cages, and line-transect sampling to measure wild ungulate detections, forage production, and plant community composition. Elevation emerged as the primary environmental driver of forage production ( β = 594.44, SE = 238.21, P = 0.013), with wild ungulate abundance showing a negative but nonsignificant effect ( β = −18.90, SE = 17.81, P = 0.289) across model-averaged results. Plant community composition, characterized by functional groups including grass, forb, shrub, and bare ground, was not a significant predictor of wild ungulate relative abundance across study sites. Although elevation was the dominant predictor of forage production, vegetation loss was recorded at 66% of study sites, aligning with recent reports from ranchers regarding substantial impacts from wild ungulates on ranch operations. These findings underscore the ecological impact of wild ungulates on forage resources, highlight their widespread expansion across rangelands, and emphasize the need for site-specific management strategies to mitigate their effects.