Seabird recovery and vegetation dynamics after Norway rat eradication at Tromelin Island, western Indian Ocean

Abstract

Seabirds are notoriously sensitive to introduced mammalian predators and eradication programs have benefitted seabird populations and their habitats on numerous islands throughout the world. However, less evidence is available from the tropics as to the benefits of rat eradication. Here, we report the seabird recovery and vegetation dynamics on a small coralline island of the tropical western Indian Ocean, eight years after Norway rat (Rattus norvegicus) eradication. Two species of seabirds were breeding before rat eradication (red-footed and masked boobies, Sula sula and Sula, dactylatra) and, in both species, the number of breeding pairs had an apparent increase of 22-23% per year after rat eradication. Such a high annual growth rate cannot be achieved by auto-recruitment only and our data suggest that immigration from other source populations never occurred in at least one of these species. We suggest that it is rather due to a rapid increase in breeding success, which rapidly increased the observed number of breeders since birds remained in the available-for-counting-as-breeders group for much longer. Two other species, the white tern (Gygis alba) and the brown booby (Sula leucogaster) were recorded breeding in 2014. The former species has not bred on the island since 1856 and the latter has never bred on the island. Plant cover (monospecific formation of the ruderal herb Boerhavia diffusa) dramatically increased from less than 30% of surface coverage to more than 70%. Although the initial restoration project was to eradicate all introduced mammals of the island simultaneously, house mouse (Mus musculus) eradication failed. Mouse density was high 8 years after rat eradication (32 mice/ha in dry season and 52 mice/ha in rainy season) but not higher than at a comparable tropical island of the region (Juan de Nova) where mice coexist with introduced black rats (Rattus rattus) and feral cats (Felis catus). These results are discussed in terms of the direct positive effects of rat eradication on seabirds and plants and the indirect positive effects of post-eradication seabird increase on soil manuring and vegetation recovery. Overall, our results show that on tropical islands, seabird and habitat recovery can be very rapid after rat eradication and should be implemented as a restoration tool wherever possible.

Trophic roles of black rats and seabird impacts on tropical islands: Mesopredator release or hyperpredation?

“…However, mice impacts on seabirds remain negligible compared to those of rats. Removal of brown rats (R. norvegicus) on Tromelin over 8 years ago has been beneficial to seabird populations and native vegetation despite higher densities of mice after rat eradication (Le Corre et al, 2015). While eradicating both rodents and cats is a desirable management goal, our data suggest that hyperpredation threatens remaining seabirds and mesopredator release effect is unlikely.…”

Invasive rat eradication strongly impacts plant recruitment on a tropical atoll

“…norvegicus ) are omnivores with a range of direct and indirect impacts on island communities. As predators of animals, they have caused the extinction of numerous animal species in insular communities, including reptiles, seabirds, landbirds and invertebrates [ 2 – 4 ], with many additional indirect impacts, such as effects to nutrient subsidies supplied by these animals [ 5 – 7 ]. In addition, as omnivores, non-native, invasive rats also consume seeds, seedlings, and adult plants [ 8 – 12 ], leading to changes in the abundance, composition, and structure of plant communities [ 13 – 15 ], including the extinction of some endemic island plant species [ 16 ].…”

Invasive rats strengthen predation pressure on bird eggs in a South Pacific island rainforest

“…Rats strengthen the overall predation on bird eggs and impact other endemic and threatened taxa such as macro-insects (wetas, stick insects) or squamata (Duron et al unpublished data). Experimental rat control operations may allow the recording of biodiversity responses after rat population reduction ( VanderWerf 2001 ; Le Corre et al 2015 ), which would pave the way towards optimal adaptive management ( Westgate et al 2013 ). Scientists and managers need to work together to ensure that rat control operations will enhance our knowledge of rats, which should lead to more effective conservation measures ( Ruffino et al 2015 ).…”