Shiels, A.B. December
2010. Ecology and impacts of introduced rodents (Rattus spp.
and Mus musculus) in the Hawaiian islands. Ph.D.
Dissertation, University of Hawaii
Abstract: Introduced rats (Rattus
rattus, R. norvegicus, and R. exulans) and/or mice
(Mus musculus) occur on more than 80% of the world‘s island
groups, where they pose great threats to native species.
Understanding the interactions between these introduced rodents and
the environments which they have invaded can assist in land
management and conservation efforts on islands. In three mesic
forests in the Waianae Mountains, Oahu, Hawaii, rat and mouse
abundances were estimated using mark-and-recapture, microhabitat use
and den sites were determined using spool-and-line tracking, and rat
home-ranges were estimated using radio-tracking. The diets of three
of the rodents (R. norvegicus was absent from the three
sites) were assessed using stomach content and stable isotope
analyses. Additionally, field and captive-feeding trials were used
to assess fruit and seed removal and consumption, and seed predation
and dispersal, by R. rattus. Rattus rattus dominates
these forests in abundance (7.1 indiv./ha) relative to the two
smaller rodents, R. exulans (0.3 indiv./ha) and M.
musculus (3.7 indiv./ha). Home-range estimates for R. rattus
(N = 19) averaged 3.8 ha, and the single radio-tracked R.
exulans had a home-range of 1.8 ha. Except for one individual
M. musculus, all den sites of R. exulans and M.
musculus were belowground, whereas dens of R. rattus were
both above- and belowground. Most (> 88%) rodent activity occurred
in areas where vegetation closely (10-30 cm above individuals)
covered the rodent; 70% of the monitored movements of both M.
musculus and R. exulans were on the ground surface,
whereas R. rattus was mainly arboreal (32% ground, 64%
arboreal) and was typically observed at ca. 3 m height when
aboveground. Consistent with the evidence for (micro-) habitat
partitioning among these three rodents, the diets of the three
rodents may also provide evidence of niche partitioning. Rattus
exulans had an intermediate diet (stomachs
containing 60% plant and 38% arthropod; N = 12)
between the more carnivorous M. musculus (36% plant, 57%
arthropod; N = 47) and the more vegetarian R. rattus (81%
plant, 14% arthropod; N = 95); yet the lifetime average diet
determined by stable isotope analysis (δ15N and δ13C) of bone marrow
of R. exulans was indistinguishable from the lifetime diet of
The likelihood of seed predation and dispersal by R.
rattus was tested with field and laboratory experiments. In the
field, fruits of eight native and four non-native common woody plant
species were arranged individually on the forest floor in four
treatments that excluded vertebrates of various sizes. Eleven
species had a portion (3-100%) of their fruits removed from
vertebrate-accessible treatments, and automated cameras photographed
only R. rattus removing fruit. In the laboratory, R.
rattus were offered fruits of all 12 species used in the field
trials, as well as 21 of the most problematic non-native species in
Hawaii, to assess consumption and seed fate. Rats ate pericarps
(fruit tissue) and seeds of most species, and the impacts on these
plants ranged from potential dispersal of small-seeded (≤ 1.5 mm
length) species that survived gut passage (e.g., the native Kadua
affinis, and the non-natives Clidemia hirta, Buddleia
asiatica, Ficus microcarpa, Miconia calvescens, and Rubus
rosifolius) to predation where < 35% of the seeds survived. Many
species had some partly damaged or undamaged seeds that survived rat
exposure. Combining field and laboratory findings indicates that
many interactions between R. rattus and seeds of native and
non-native plants may result in seed dispersal. Therefore, rats are
likely to be affecting plant communities through both seed predation
and dispersal, and these findings should be applied to aid land
management efforts where introduced rodents have invaded.
Title pages, acknowledgements, abstract, table of contents, list of tables
and list of figures
Chapter 1: Introduction
Chapter 2: Population ecology of introduced rodents (Rattus
rattus, Rattus exulans, and Mus musculus) and
their habitat uses in Hawaiian mesic forest
Chapter 3: Niche partitioning based on diet analysis of three
introduced rodents in Hawaiian montane forest
Chapter 4: Are introduced rats (Rattus rattus) both seed
predators and dispersers in Hawaii?
Chapter 5: Frugivory by introduced black rats (Rattus rattus)
promotes dispersal of invasive plant seeds
Chapter 6: Conclusions