Herbivory in invasive rats: criteria for food selection

Three species of rats (Rattus exulans, R. rattus, R. norvegicus) are widely invasive, having established populations in terrestrial habitats worldwide. These species exploit a wide variety of foods and can devastate native flora and fauna. Rats can consume a variety of plant parts, but may have the most dramatic effects on plant populations through consumption and destruction of seeds. The vulnerability of vegetation to rat consumption is influenced by many factors including size of plant part, and mechanical and chemical defenses. We reviewed the literature to find out what plant species and plant parts invasive rats are consuming and what characteristics these sources share that may influence selection by rats. Many of the studies we found were preformed in New Zealand and our analyses are, therefore, focused on this location. We also performed feeding trials in the laboratory with R. norvegicus to determine if seed hardness and palatability would influence rat consumptive choices. We found more reports of rat consumption of fruits and seeds versus vegetative plant parts, and smaller fruits and seeds versus larger. R. norvegicus are reported to consume proportionally more vegetative plant parts than either R. exulans or R. rattus, possibly due to their more ground dwelling habits. Large size and hard seed coats may deter rat feeding, but unpalatable chemicals may be even more effective deterrents to rats. Scientists and managers can better manage vegetation in rat invaded areas by understanding the criteria rats use to select food.