USGS PIERC - Mammalian Predators in Hawaii wet forests

Mammalian Predators in Hawaii wet forests

Depredation of eggs, nestlings and adult birds by introduced predators has been widely postulated as a leading cause of the accelerated decline and extirpation of endemic Hawaiian avian species and a major factor limiting present populations of endangered birds. In Hawaii, four species of introduced rodents, the black rat (Rattus rattus). The Polynesian rat (R. exulans), the Norway rat (R. norvegicus), and the house mouse (Mus musculus) are found in a variety of habitats from sea level to 3050 m elevations. These rodents, together with the introduced Indian mongoose (Herpestes auropunctatus) and the feral cat (Felis catus) inhabit forest habitat in varying degrees of sympathy with endangered Hawaiian forest birds. Of the four rodent species, the black rat, because of its size, arboreal behavior and nocturnal habits, is considered the greatest threat to native forest birds. Mongooses are not normally arboreal and have poor climbing abilities. The role of feral cats as predators to Hawaiian forest birds is poorly known, but results of several studies suggest that they may be a major predator in Hawaiian forest habitats.

Black rat

Polynesian rat

feral cat

Here are three common predators in Hawaiian montane wet forest

Population structure and movements of introduced rats in an Hawaiian rainforest

1. Lindsey, G. D., S. M. Mosher, S. G. Fancy and T. D. Smucker. 1999. Population structure and movements of introduced rats in an Hawaiian rainforest. Pacific Conservation Biology. IN PRESS.

Abstract: We studied populations of introduced rats in three areas in Hawaiian rainforests as part of a larger study to determine the factors limiting endangered bird populations. Species composition among 1,264 rats live trapped was Black Rats Rattus rattus 60.8%, Polynesian Rats R. exulans 38.1%, and Norway Rats R. norvegicus 1.0%. Capture rates did not differ among the three study areas or four seasonal periods for any of the species. Males comprised 53.0% of 921 Black Rats and 55.3% of 783 Polynesian Rats that were sexed. Juvenile rats comprised 26.2% of Black Rats and 31.7% of Polynesian Rats captured and weighed. Black and Polynesian Rats reproduced throughout the year, but the highest proportion of juveniles in the population was captured in September-November and the lowest proportion in April-June. Mean home range size was 3.6 ha for four Black Rats (three males and one female) and 3.0 ha for three Polynesian Rats (two males and one female). All daytime nests of Black Rats found were above ground, generally in cavities within tree canopies. Mean annual rat activity in tree canopies as measured with tracking tunnels was 43% (range 21 to 80%). Of 44 rats captured in trees, 43 were Black Rats and one was a Polynesian rat.

View of montane wet forest within the Hakalau Forest National Wildlife Refuge on the island of Hawaii.

Picture of Polynesian rat w/eartag

Live captured rats were identified to species, weighted, sexed, eartagged, and released at the capture site. Mean capture rate (number rats/100 trap nights) was 19.9 " 1.2 for black rats, 10.2 " 1.6 for Polynesian rats, and 0.3 " 0,1 for Norway rats.

Picture of black rat with 3.6 g transmitter

Black rat carrying a 3.6g transmitter. Black and Polynesian rats exhibited overlapping home ranges 1.6 to 4.4 ha in size. Both species spent the day in nests (black rats in cavities of large koa and ohia trees; Polynesian rats in ground nests) and foraged, primarily on the ground, throughout the night. All rats became active immediately after dark and remained active until just before dawn.

tracking tunnel in tree

Tracking tunnels placed a mean 7.8 inches .37 m above ground in trees were used to determine relative canopy use by rats. Rats leave tracks on treated paper when they walk through the tunnels baited with coconut chunks.

2. Studies in Progress

a. Abundance, home range and diet of feral cats in Hawaii
b. Testing small mammal toxicants and application methods in Hawaiian mesic and wet forests.