How important are seabirds in the diet of black rats on islands with a superpredator?

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

Invasive rats (Rattus spp.) are known to have pervasive impacts on island birds, particularly on their nesting success. To conserve or restore bird populations, numerous invasive rat control or eradication projects are undertaken on islands worldwide. However, such projects represent a huge investment and the decision-making process requires proper assessment of rat impacts. Here, we assessed the influence of two sympatric invasive rats (Rattus rattus and R. exulans) on native bird eggs in a New Caledonian rainforest, using artificial bird-nest monitoring. A total of 178 artificial nests containing two eggs of three different sizes were placed either on the ground or 1.5 m high and monitored at the start of the birds' breeding season. Overall, 12.4% of the nests were depredated during the first 7 days. At site 1, where nests were monitored during 16 days, 41.8% of the nests were depredated. The main predator was the native crow Corvus moneduloides, responsible for 62.9% of the overall predation events. Rats were responsible for only 22.9% of the events, and ate only small and medium eggs at both heights. Our experiment suggests that in New Caledonia, predation pressure by rats strengthens overall bird-nest predation, adding to that by native predators. Experimental rat control operations may allow reduced predation pressure on nests as well as the recording of biodiversity responses after rat population reduction.

The impact of Great Cormorants on biogenic pollution of land ecosystems: Stable isotope signatures in small mammals

Studying the isotopic composition of the hair of two rodent species trapped in the territories of Great Cormorant colonies, we aimed to show that Great Cormorants transfer biogens from aquatic ecosystems to terrestrial ecosystems, and that these substances reach small mammals through the trophic cascade, thus influencing the nutrient balance in the terrestrial ecosystem. Analysis of δ(13)C and δ(15)N was performed on two dominant species of small mammals, Apodemus flavicollis and Myodes glareolus, inhabiting the territories of the colonies. For both species, the values of δ(13)C and δ(15)N were higher in the animals trapped in the territories of the colonies than those in control territories. In the hair of A. flavicollis and M. glareolus, the highest values of δ(15)N (16.31±3.01‰ and 17.86±2.76‰, respectively) were determined in those animals trapped in the biggest Great Cormorant colony. δ(15)N values were age dependent, highest in adult A. flavicollis and M. glareolus and lowest in juvenile animals. For δ(13)C values, age-dependent differences were not registered. δ(15)N values in both small mammal species from the biggest Great Cormorant colony show direct dependence on the intensity of influence. Biogenic pollution is at its strongest in the territories of the colonies with nests, significantly diminishing in the ecotones of the colonies and further in the control zones, where the influence of birds is negligible. Thus, Great Cormorant colonies alter ecosystem functioning by enrichment with biogens, with stable isotope values in small mammals significantly higher in the affected territories.

Feeding Habits of Introduced Black Rats, Rattus rattus, in Nesting Colonies of Galapagos Petrel on San Cristóbal Island, Galapagos

Introduced rodents are responsible for ecosystem changes in islands around the world. In the Galapagos archipelago, their effects on the native flora and fauna are adverse, including the extinction of endemic rodents in some islands and the reduction in the reproductive success of the Galapagos petrel (Pterodroma phaeopygia) in its nesting zones. Understanding the feeding behavior of introduced rodents and their trophic interactions with native and non-native species on islands, can assist in the design of management strategies and conservation plans of invasive and endemic species respectively. Four petrel nesting colonies were monitored during June 2013 on San Cristóbal Island (El Plátano, El Junco, San Joaquín, and La Comuna). The feeding habits of black rats were evaluated by analyzing stomach contents and stable isotopes in hair. Three species of introduced rodents were captured. R. rattus was the most abundant at all sites (n=43, capture success (CS) = 55.8%), followed by the house mouse, Mus musculus (n = 17, CS = 37.8%), and the Norwegian rat, R. norvegicus (n = 4, CS = 4.5%), captured only at La Comuna. The omnivorous black rat ate mostly plants (98%) and arthropods (2%). Intact seeds of Miconia robinsoniana were the main food at all sites (relative abundance=72.1%, present in 95% of the analyzed stomachs), showing the black rats’ possible role in the archipelago as endemic seed dispersers. There was no evidence of petrel’s intake; however, its possible consumption is not discarded at all. The δ¹⁵N and δ¹³C analysis corroborated the primarily herbivorous diet of black rats. The isotopic signatures of the three rodent species reflect the inter- and intra-specific differential use of food resources. Black rat showed a wider diet in La Comuna, which was related to a lower availability of its primary prey and its ability to adapt to the available resources in its habitat.

Assessment of invasive rodent impacts on island avifauna: methods, limitations and the way forward

Bird conservation is nowadays a strong driving force for prioritising rodent eradications, but robust quantitative estimates of impacts are needed to ensure cost-effectiveness of management operations. Here, we review the published literature to investigate on what methodological basis rodent effects on island bird communities have been evaluated for the past six decades. We then discuss the advantages and limitations of each category of methods for the detection and quantification of impacts, and end with some recommendations on how to strengthen current approaches and extend our knowledge on the mechanisms of impacts. Impact studies (152 studies considered) emphasised seabirds (67%), black rats (63%) and the Pacific Ocean (57%). Among the most commonly used methods to study rodent impacts on birds were the observation of dead eggs or empty nests while monitoring bird breeding success, and the analyses of rodent diets, which can both lead to misleading conclusions if the data are not supported by direct field evidence of rodent predation. Direct observations of rodent–bird interactions (19% of studies) are still poorly considered despite their potential to reveal cryptic behaviours and shed light on the mechanisms of impacts. Rodent effects on birds were most often measured as a change or difference in bird breeding parameters (74% of studies), while estimates of bird population growth rates (4%) are lacking. Based on the outcomes of this literature review, we highlight the need for collecting unbiased population-level estimates of rodent impacts, which are essential prerequisites for predicting bird population growth scenarios and prioritising their conservation needs. This could be achieved by a more systematic integration of long-term monitoring of bird populations into rodent management operations and modelling bird population dynamics. We also strongly recommend including various complementary methods in impact assessment strategies to unravel complex interactions between rodents and birds and avoid faulty evidence. Finally, more research should be devoted to a better understanding of the cases of non-impacts (i.e. long-term coexistence) and those impacts mediated by mechanisms other than predation and ecosystem-level processes.