Plant Invasions In Hawai‘i: Fire Risks And Grass Invasion Patterns
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2024
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Abstract
The Hawaiian islands have been deeply impacted by invasive species introduced over the past ~250 years since European colonization, with grasses playing an outsized role. Invasive grasses (Poaceae) now cover ~ ⅓ of the land of Hawai‘i, displacing native species, modifying habitat structures, and fueling fires. Concerningly, new grasses and other fire promoting species continue to be introduced to the Hawaiian islands leaving many data gaps.
I developed a screening system to identify introduced plant species that are likely to increase wildfire risk. This tool uses a machine learning model to predict fire risk scores from traits obtained from literature and databases. The model revealed that just four variables can identify species categorized as higher fire risk by experts with 90% accuracy, while low risk species were identified with 79% accuracy. I then used the predictive model to screen > 140 recently naturalized plants in Hawaiʻi and identified species (6% of naturalizations in the last ~ 10 years) that are likely to pose a high fire risk and can be targeted for eradication or containment to reduce future wildfire risks. Grasses were prominent among these species.
I then focused on studying grasses in Hawai‘i through a historic lens, aiming to see how the current grass flora came to be by examining the herbarium record, newspapers, agricultural literature, and floras going back to the 1800’s. I also conducted a taxonomic revision of the Hawaiian introduced grass flora looking for undocumented naturalized grasses. Significant grass imporations began slowly following the proliferation of cattle
ranches after the 1840s, in search of “improved” pasture grasses suitable for high intensity grazing. The importation of foreign forage grasses accelerated dramatically in the early 1900s with the establishment of the Hawai‘i Agriculture Experiment Station (HAES) on O‘ahu by the United States government. The HAES imported seeds, trialed grasses in introduction gardens, and distributed seeds to ranchers across the islands. In total, 577 grasses were introduced post-1778, 158 of which were likely accidental introductions whereas 419 were deliberately imported. While importation of new grass species has stalled in recent decades, new accidental introductions continue to naturalize, with 30 newly naturalized grass species recorded between 2000 and 2023; some of these records were first recorded through my studies.
Through this historical examination of grass naturalizations, I noticed that there is often a delay between when a species first appears in the herbarium record, and when it is identified and published. This is significant as herbarium specimens and published reports serve as primary sources for the study and management of biological invasions and delays in reporting can bias these data. The Hawaiian naturalized grasses were used as a case study to examine these delays among 269 species introduced after European colonization. There was an average delay of 27 years (median of 17 years) between the first naturalized herbarium specimen and its publication, and this delay has decreased among more recent naturalizations. Adding to this delay is the time needed for correct identification of specimens, which was an average of 18 years, but a median of 4 years, indicating most grasses are quickly identified but others take decades. Expanded investment in and support of taxonomic experts at biodiversity institutions is
needed to reduce reporting delays for new naturalizations as these delays decrease the likelihood that populations will grow beyond what can be eradicated.
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Botany, Ecology, checklist, delay period, fire, history, invasive species, poaceae
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307 pages
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