Potential Impacts of Black Rats (Rattus rattus) on Seeds and Seedlings of Woody Species in a Regenerating Island Rainforest Plant Community

Invasive rodents are among the world's most damaging invasive species linked to declines and extinctions of many vertebrate and invertebrate species, especially in island ecosystems. However, despite a mainly herbivorous diet, their impacts on plant communities are less well known, with few studies in Australian ecosystems. We studied the potential impacts of high-density populations of invasive black rats (Rattus rattus) on the restoration of an endangered ecological community of sub-tropical rainforest on two small islands in southeastern Australia. We used two experiments to investigate rat impacts on two woody rainforest species, Hibiscus heterophyllus and Acacia maidenii, that are frequently used for successional planting in restoration. We hypothesised that, as black rats generally obtain a large amount of their diet from plants, they would rapidly damage Hibiscus and Acacia seedlings and consume Hibiscus seeds. We found that using cages to exclude black rats for 11–14 days had significant effects on herbivory of Hibiscus seedlings but not for Acacia seedlings. Hibiscus seedlings with no exclusion cage lost an average of 17% more leaves (1.83 of initial average 10.6 leaves per plant) when compared to caged seedlings. We also found that excluding rats reduced the number of Hibiscus seedlings damaged, although 50% of the damage was suspected to be caused by insects. Although only 4.5% of total Hibiscus seeds were removed during the 3-day experimental period, the night-time removal rate (mostly attributed to black rats) was 10 times more than that of other diurnal foragers. This rate of seed removal by black rats has the potential to contribute to poor restoration in rainforest but was lower than black rat consumption of seeds in other systems. We conclude that, in our system, black rats appear to have minimal potential impacts on these rainforest species, despite high rat densities, confirming the successful use of these species in restoration. However, being novel to the system, the additive effects of rats on seed loss over time warrant further investigation.