Final Comprehensive Conservation Plan - U.S. Fish and Wildlife ...

Hakalau Forest National Wildlife Refuge
Comprehensive Conservation Plan
rats have a large distributional range and can be found from sea level to nearly 10,000 ft. Norway rats
are restricted to areas below 6,000 ft (Tomich 1986). Polynesian rats and Norway rats nest
exclusively in terrestrial habitats, while black rats are arboreal nesters. This nesting difference may
contribute to a larger population of black rats in Hawai„i due to the presence of nonarboreal
mongoose predators (Hays and Conant 2007).
Globally, introduced rats have caused the decline, extirpation, or extinction of insular bird species
(Moors and Atkinson 1984, Atkinson 1985). In the main Hawaiian Islands, Atkinson (1977)
suggested that black rats caused the accelerated decline or extinction of many native forest birds
between 1870-1930. Polynesian rats are speculated to have been a contributing factor in the largescale
extinction of Hawaiian bird species during prehistoric Polynesian occupation (Olson and James
1982). Rats continue to be a major threat to waterbirds, seabirds, and forest birds in the Hawaiian
Islands (Mitchell et al. 2005). All three species in Hawai„i are known predators of eggs, nestlings,
young, and occasionally adults of endangered waterbirds (ae„o, „alae ke„oke„o, „alae ula, koloa
maoli), seabirds („a„o or Newell‟s shearwater, „ua„u kani or wedge-tailed shearwaters, moli or
Laysan albatross), migratory shorebirds, and forest birds (Harrison et al. 1984, Brisbin et al. 2002,
Engilis et al. 2002, Mitchell et al. 2005, USFWS 2005a, USFWS 2005b). Ground and burrow-nesting
seabirds are particularly vulnerable to rat predation, even by the arboreal black rat (Smith et al.
2006).
Rats also consume plants, insects, mollusks, herpetofauna, and other invertebrates. As herbivores,
rats consume seeds and fruits and prevent the regeneration of rare and endangered plants. These
mammals have also been observed causing indirect damage to young koa by stripping the bark off
seedlings (Scowcroft and Sakai 1984). Because invertebrate and plant species are also eaten by birds,
a reduction in these populations may indirectly affect avian populations (Nelson et al. 2002).
In the early 1990s rats were known to consume leaves and fruit of the few known endangered
Cyanea shipanii at HFU. Rats were thought to have killed at least two of the four remaining plants.
At KFU, some of the last remaining Cyanea sticophylla were known to be girdled by rats (USFWS,
unpubl.). The USGS-BRD conducted a study at Hakalau Forest NWR in the mid-1990s eradicating
rats from a 0.02 m 2 area and compared it with an adjacent area where no rats were removed. During
the first year of the study, a 25-75 percent increase in nesting success, depending on bird species, was
seen in the rat-free area (Fancy, pers. comm.)
The use of diphacinone rodenticide has been shown to have a positive effect in native bird survival in
the main Hawaiian Islands (VanderWerf and Smith 2002, Nelson et al. 2002) but is not currently
being used on the HFU.
Cats (Felis catus)
Cats are found on all the main Hawaiian Islands from sea level to nearly 10,000 ft (Tomich 1986).
They occur in montane wet forest, subalpine dry forests, and lowland dry forests (Smucker et al.
2000). Cats can breed year-round in Hawai„i due to the climate, producing between 2-3 litters
annually and 4-6 kittens per litter (Winter 2003, Winter and Wallace 2006).
Food habits of cats in Hawai„i include grasses, plant seeds, insects, centipedes, marine crustaceans,
lizards, mice, rats, and „ōpe„ape„a. They are also known to consume young and adult birds and their
eggs. Bird prey consists of four endangered Hawaiian waterbirds, migratory shorebirds, nesting
Chapter 4. Refuge Biology and Habitats 4-89