Invasive Deer Caused Significant Forage Loss in Hawaii

A University of Hawaiʻi study linked widespread native forage reduction to invasive ungulates across ranchlands.

Updated on Sept. 27, 2026 in Botany

Isometric editorial illustration showing stylized volcanic soil and sparse patches of dry grass with invasive fireweed clusters. — A University of Hawaiʻi study reveals that invasive deer have severely reduced essential forage availability across two-thirds of surveyed Hawaiian ranchlands. AI Illustration. Upload story photo >

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Researchers at the University of Hawaiʻi at Mānoa found that invasive ungulates, particularly axis deer, have significantly degraded forage availability on state ranchlands. The study documented these losses across two-thirds of the 35 surveyed sites.

Why it matters

The presence of invasive hoofed animals directly threatens the productivity of Hawaiian ranching operations by competing for critical plant resources. This ecological shift is further complicated by the spread of invasive fireweed, which appeared at 70% of the study locations.

Researchers utilized motion-triggered trail cameras across 35 sites to compare vegetation growth. Sites dominated by axis deer, which number roughly 47,000 on Maui, showed a 25% decrease in forage yield.

The players

University of Hawaiʻi at Mānoa

This is a public research university that serves as the flagship campus of the University of Hawaiʻi system.

U.S. Department of Agriculture

This federal executive department is responsible for developing and executing laws related to farming, forestry, and food.

The details

The study, published in Rangeland Ecology & Management and funded by the U.S. Department of Agriculture, highlights how ungulates introduced between the 1700s and 1960s have altered the landscape. Investigators observed that fireweed has encroached upon 70% of the areas monitored for vegetation health.

Non-native hoofed animals were introduced to Hawaiʻi between the 1700s and 1960s.
Wildfires occurred on Maui in August 2023.
The study was published in Rangeland Ecology & Management in September 2026.

The Big Picture

This study follows a pattern set by the University of Hawaiʻi at Mānoa botany research program in documenting the impact of non-native species on Pacific ecosystems. The findings provide a new baseline for understanding how invasive ungulate populations alter native forage availability.

The forage reduction identified by researchers could lead to increased operational costs for local ranchers who must secure supplemental feed for livestock. These findings underscore the ongoing challenges of managing invasive species in agricultural zones across the state.

The takeaway

Land managers and ranchers may need to intensify ungulate control measures to protect essential forage resources. Mitigating these losses is critical for sustaining the long-term viability of Hawaii ranching industries.