Impacts of Invasive Coqui Frogs on Hawaiian Forests—In Hawaii, invasive plants and animals may alter food webs because invasive plants have traits that differ from native plant species and abundant invasive predators can selectively reduce prey populations. The relative importance of these two processes on the litter invertebrate community in Hawaii is important and could affect the large number of endemic and endangered invertebrates.
In a recent study, National Wildlife Research Center (NWRC) researchers and colleagues at Utah State University determined the relative importance of litter resources, represented by leaf litter of an invasive nitrogen-fixer, Falcataria moluccana and a native tree, Metrosideros polymorpha, and predation by the invasive coqui frog (Eleutherodactylus coqui), on the abundance and composition leaf litter invertebrate community. The researchers found that thelitter of the invasive F. moluccana has an associated invertebrate community that differs greatly from that found in the native M. polymorpha litter. The invasive tree was also associated with a 400% increase in the abundance of nonnative crustaceans (Orders Amphipoda and Isopoda) that fragment plant material during feeding and a 200% increase in the abundance of nonnative predatory ants. The coqui frog had less effect on the litter invertebrate community, reducing microbivores by 40% in F. moluccana and non-native ants by 30% across litter types. Coqui frog stomach contents were similar in abundance and composition in both litter communities, despite dramatic differences in the invertebrate community.
These results suggest that invertebrate community differences between the litter types did not cascade to influence the growth or survivorship of coqui frogs. The researchers conclude that the invasive tree species has a greater influence on litter invertebrate community abundance and composition than the invasive coqui frog does.
Efficacy of Rodenticide Baits for Invasive Polynesian Rats, Roof Rats and House Mice—For several centuries, invasive rodents have decimated native flora and fauna in Hawaii and other areas of the Pacific Basin, reduced agriculture production, and threatened human health. Although many rodenticides are commercially available nationally, few are available for use in Hawaii or have been tested with wild rodents commonly found in the Pacific. To help address this issue, researchers at the NWRC field station in Hilo, HI, tested the efficacy and palatability of nine commercial rodenticide bait formulations (0.005% diphacinone; 0.0025% brodifacoum; 0.005% bromadialone; 0.005% chlorophacinone; 0.0025% difethialone; 0.01% bromethalin; 2% zinc phosphide in two different formulations; and 0.025% warfarin) on Polynesian rats ( Rattus exulans), roof rats ( R. rattus), and house mice ( Mus musculus).
Efficacy varied by rodenticide tested and rodent species. Generally, rodenticides were more effective against mice than they were against either of the rat species, and mice tended to consume more rodenticide bait than the laboratory chow alternative food. Efficacy was generally highest for the second-generation anticoagulants tested; however, this result varied across products, and one first-generation rodenticide had a similar level effectiveness. The only rodenticide formulation that had 80% or higher mortality for all three species was a difethialone-based rodenticide. Rodenticide baits containing either bromadialone or brodifacoum had 70% or higher mortality rates for all three rodent species.
Bait acceptance (palatability) also varied both by rodenticide and by rodent species. Acceptance was the lowest for the acute rodenticides. The chlorophacinone bait product was notable in having high acceptance by all three rodent species, although mortality percentages were below 80 percent for R. rattus and M. musculus. The warfarin bait, zinc phosphide pellets, zinc phosphide oats, and bromethlin-based product were generally not well-accepted by the two rat species but were eaten by the mice. Bait acceptance appeared to substantially affect the efficacy of rodenticides; materials that were not well-accepted produced lower mortality rates.
Rodenticide products currently registered for use in Hawaii performed less effectively in this study than other available products not yet registered. Although markets for rodent control products for use on islands are limited, the results of this study show that there are advantages to having additional products registered for island use in agriculture, conservation, and public health.