Invasive European rats in Britain and New Zealand: same species, different outcomes

Abstract

Two species of European commensal murids, the Norway rat norvegicus and the ship rat Rattus rattus, have colonized two island archipelagos of comparable size and temperate climate but in opposite hemispheres and with opposite outcomes. Ship rats were common commensals in Britain until replaced by Norway rats; Norway rats were hugely abundant in native forests throughout New Zealand until widely replaced by ship rats. Interference explains the first case, as ship rats are smaller than Norway rats and are always vulnerable to aggressive competition from them, but some other explanation is needed for the second case. We used the marginal value theorem to investigate exploitation competition between these two species in arboreal habitats. We observed the climbing behaviour and ‘giving-up time’ of captive rats of both species searching for food at different heights above the ground. Our data confirmed that the smaller size and greater agility of R. rattus give it a competitive advantage in foraging for scattered small food items above ground. We propose that (1) the outcomes of the interactions between the two rat species in any given place depend on the distribution of food resources in structurally complex habitat, moderated by winter temperatures; (2) the different outcomes of invasions by the two species can be explained in Britain by interference competition, and in New Zealand by exploitation competition and by the absence of specialist arboreal rodents (squirrels).