Invasive rodents (rats and mice) commonly occur on islands and often damage natural resources largely by predation of native species. Suppressing invasive rodent populations and their damages is therefore a common practice in many parts of the Hawaiian Islands, and land managers such as the Army Natural Resources Program on Oahu often control rodent populations by using large-scale rat snap-trapping and Goodnature A24 automated rat traps (henceforth A24s). While rat traps can be effective at suppressing rat populations, mouse populations are not generally suppressed and may expand greatly. In an effort to reduce rodent populations to levels below that accomplished with rat traps alone at a 5-ha mesic forest on Oahu (Ohikilolo), we assessed the effectiveness of a one-time (two application) hand-broadcast of anticoagulant (Diphacinone-50) bait pellets applied at 13.8 kg/ha per application while A24s and rat snap-traps were active. We monitored rat and mouse activity during trapping and before, during, and after the bait applications using tracking tunnels, which are baited ink cards placed in tunnels so that foot prints of animal visitors can be identified. We found that rat trapping alone was effective at reducing rat populations but not the mouse population, and that the one-time hand-broadcast of diphacinone bait reduced both rat and mouse activity to 0% tracking for about 1 month. However, rat and mouse populations rebounded 2 months later to 15% rat tracking and 41% mouse tracking, which were roughly pre-treatment levels. Rat suppression using A24s at Ohikilolo appeared much more effective year-round than at a nearby 26 ha site (Kahanahaiki), though mouse suppression was poor at Ohikilolo relative to Kahanahaiki. The hand-broadcast of diphacinone bait at both Ohikilolo and Kahanahaiki was effective but short-lived, so repeated baiting during the seasonal peaks in rodent abundance and increasing the size of the buffer area would more likely protect target natural resources from rats and mice.

Invasive rats are among the most damaging animals to native species on many island ecosystems including those in Hawaii. On Oahu Island, U.S. Army Garrison Natural Resources Program currently manages invasive rat populations to protect natural resources by using grids of A24 automated traps, and previously snap-trap grids and rodenticide bait stations. Despite these control efforts generally suppressing rats, some lands with natural resources that are at risk to rat predation are not easily accessible for implementing these traditional rat control methods. In a 430-ha mesic forest on Oahu where ungulates are excluded and site access is limited due to military training and presence of live ordnance, we tested the efficacy of aerial application of anticoagulant rodenticide bait pellets (Diphacinone-50 Conservation), applied in two applications at a rate of 12.82 kg/ha per application. We measured the effectiveness of the rodenticide bait application by deploying tracking tunnels (inked and baited cards to identify rat presence) before, during, and after applications within treated and nearby untreated areas. Due to restricted access, we failed to estimate nest success of an endangered bird; yet previous research showed rat control increases this bird’s population. We also measured diphacinone residues in stream water at the treatment site to determine this method’s risk level to the aquatic ecosystem. The aerial application resulted in immediate and sustained reduction in the rat population, as evidenced by rat activity decreasing from ~44% to 3.8% during the first three months after bait application and maintained <20% rat activity for 10 months. Trail cameras and recovered rat carcasses also highlighted effectiveness. One of 34 stream samples analyzed had detectable diphacinone residues and this single sample was taken one week after application and it had very low levels of diphacinone (below levels quantifiable). Aerial application of diphacinone appears to be an efficient and effective rat suppression technique for natural resource protection in complex landscapes.

The Hawaiian Islands have been identified as a global biodiversity hotspot. We examine the Normalized Difference Vegetation Index (NDVI) using Climate Data Records products (0.05 × 0.05°) to identify significant differences in NDVI between neutral El Niño-Southern Oscillation years (1984, 2019) and significant long-term changes over the entire time series (1982-2019) for the Hawaiian Islands and six land cover classes. Overall, there has been a significant decline in NDVI (i.e., browning) across the Hawaiian Islands from 1982 to 2019 with the islands of Lānai and Hawaii experiencing the greatest decreases in NDVI (≥44%). All land cover classes significantly decreased in NDVI for most months, especially during the wet season month of March. Native vegetation cover across all islands also experienced significant declines in NDVI, with the leeward, southwestern side of the island of Hawaii experiencing the greatest declines. The long-term trends in the annual total precipitation and annual mean Palmer Drought Severity Index (PDSI) for 1982-2019 on the Hawaiian Islands show significant concurrent declines. Primarily positive correlations between the native ecosystem NDVI and precipitation imply that significant decreases in precipitation may exacerbate the decrease in NDVI of native ecosystems. NDVI-PDSI correlations were primarily negative on the windward side of the islands and positive on the leeward sides, suggesting a higher sensitivity to drought for leeward native ecosystems. Multi-decadal time series and spatially explicit data for native landscapes provide natural resource managers with long-term trends and monthly changes associated with vegetation health and stability.