Sympatric Invasive Rats Show Different Diets in a Tropical Rainforest of an Island Biodiversity Hotspot1

Abstract

Invasive rats (Rattus rattus, R. norvegicus, R. exulans) are recognized as a major threat to native island ecosystems and biodiversity. On many islands, two or three invasive rat species co-occur, often sharing the same habitat; however few studies have focused on the effects of coexisting invasive rat species on native biodiversity. We investigated rat population ecology and diet in a New-Caledonian rainforest where black (Rattus rattus) and Pacific rats (R. exulans) coexist. Black rats dominated Pacific rats in relative abundance with a proportion varying between 80.9 and 88.9%. A total of 374 black rats and 87 Pacific rats were sampled for diet assessment through stomach and caecum analysis. Rat diet was mainly composed of plants, invertebrates and to a lesser extent Squamata, with black rats being more frugivorous and Pacific rats being more omnivorous. Ten of 15 endemic skink and gecko species were consumed, nine species by black rats and six species by Pacific rats. Thus, the presence of both rat species may strengthen the overall predation rate on each native prey species, and/or broaden the total number of native prey species impacted in the New-Caledonian rainforest. These results highlight the importance of preventing new rat species introduction on islands to avoid the strengthening and/or the broadening of negative effects on native biodiversity, and the importance of following the proportion of each rat species during rat control operations. Research to assess the threats generated by various assortments of rodent species on native biodiversity could improve priority setting in conservation actions.

The ratting of North America: A 350-year retrospective on Rattus species compositions and competition

“…What is perhaps most surprising is not that the two taxa differ on average in their trophic position, but that this appears to occur regardless of whether they are living sympatrically or not. One pattern in the isotopic literature on contemporary rat communities is that sympatric populations tend to have interspecific differences in their isotopic niches [e.g., (53)(54)(55)(56)], reflecting the influence of competition between members of each species. Here, there appear to be more fundamental drivers at play, suggesting that, while there is overlap, each species tends toward a different suite of food resources even when not competing with the other.…”

Combining live and lethal trapping to inform the management of alien invasive rodent populations in a tropical montane forest

“…150 years ago (Beauvais et al 2006). Both rat species are omnivorous, eating mainly plants, invertebrates and, to a lesser extent, Squamata, with R. rattus being more frugivorous and R. exulans more omnivorous (Duron et al 2019).…”

“…In the South Pacific archipelago of New Caledonia, Black and Pacific rats (Rattus rattus and R. exulans) are among the main predators of endemic and micro-endemic species in the dense evergreen rainforests (Thibault et al 2017;Duron et al 2019). As eradication of the 15 971 km² main island, which is inhabited and composed by complex ecosystems, is not currently practical, rat population control in targeted areas over prolonged periods represents a candidate option to restore native ecosystems.…”