Habitat use and seed removal by invasive rats (Rattus rattus) in disturbed and undisturbed rainforest, Puerto Rico

Despite frequent occurrences of invasive rats (Rattus spp.) on islands, their known effects on forests are limited. Where invasive rats have been studied, they generally have significant negative impacts on native plants, birds, and other animals. This study aimed to determine invasive rat distribution and effects on native plant populations via short-term seed removal trials in tropical rain forest habitats in the Luquillo Experimental Forest, Puerto Rico. To address the first objective, we used tracking tunnels (inked and baited cards inside tunnels enabling animal visitors’ foot prints to be identified) placed on the ground and in the lower canopy within disturbed (treefall gaps, hurricane plots, stream edges) and undisturbed (continuous forest) habitats. We found that rats are present in all habitats tested. Secondly, we compared seed removal of four native tree species (Guarea guidonia, Buchenavia capitata, Tetragastris balsamifera, and Prestoea acuminata) between vertebrate-excluded and free-access treatments in the same disturbed and undisturbed habitats. Trail cameras were used to identify animals responsible for seed contact and removal. Black rats (R. rattus) were responsible for 65.1% of the interactions with seeds, of which 28.6% were confirmed seed removals. Two plant species had significantly more seeds removed in disturbed (gaps) than undisturbed forest. Prestoea acuminata had the lowest seed removal (9% in 10 d), whereas all other species had >30% removal. Black rats are likely influencing fates of seeds on the forest floor, and possibly forest community composition, through dispersal or predation. Further understanding of rat-plant interactions may be useful for formulating conservation strategies.