Ecology and impacts of introduced rodents (Rattus spp. and Mus musculus) in the Hawaiian Islands

Ph.D. University of Hawaii at Manoa 2010.Includes bibliographical references.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 M. musculus. 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