Are introduced rats (Rattus rattus) both seed predators and dispersers in Hawaii?

Abstract

Invasive rodents are among the most ubiquitous and problematic species introduced to islands; more than 80% of the world’s island groups have been invaded. Introduced rats (black rat, Rattus rattus; Norway rat, R. norvegicus; Pacific rat, R. exulans) are well known as seed predators but are often overlooked as potential seed dispersers despite their common habit of transporting fruits and seeds prior to consumption. The relative likelihood of seed predation and dispersal by the black rat, which is the most common rat in Hawaiian forest, 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 different sizes. Eleven species had a portion (3–100%) of their fruits removed from vertebrate-accessible treatments, and automated cameras photographed only black rats removing fruit. In the laboratory, black rats were offered fruits of all 12 species to assess consumption and seed fate. Seeds of two species (non-native Clidemia hirta and native Kadua affinis) passed intact through the digestive tracts of rats. Most of the remaining larger-seeded species had their seeds chewed and destroyed, but for several of these, some partly damaged or undamaged seeds survived rat exposure. The combined field and laboratory findings indicate that many interactions between black rats and seeds of native and non-native plants may result in dispersal. Rats are likely to be affecting plant communities through both seed predation and dispersal.