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Background: Rodents of the genus Rattus are among the most pervasive and successful invasive species, causing major vicissitudes in native ecological communities. A broad and flexible generalist diet has been suggested as key to the invasion success of Rattus spp. Here, we use an indirect approach to better understand foraging niche width, plasticity, and overlap within and between introduced Rattus spp. in anthropogenic habitats and natural humid forests of Madagascar.
Results: Based on stable carbon and nitrogen isotope values measured in hair samples of 589 individual rodents, we found that Rattus rattus had an extremely wide foraging niche, encompassing the isotopic space covered by a complete endemic forest-dwelling Malagasy small mammal community. Comparisons of Bayesian standard ellipses, as well as (multivariate) mixed-modeling analyses, revealed that the stable isotope niche of R. rattus tended to change seasonally and differed between natural forests and anthropogenic habitats, indicating plasticity in feeding niches. In co-occurrence, R. rattus and Rattus norvegicus partitioned feeding niches. Isotopic mismatch of signatures of individual R. rattus and the habitat in which they were captured, indicate frequent dispersal movements for this species between natural forest and anthropogenic habitats.
Conclusions: Since R. rattus are known to transmit a number of zoonoses, potentially affecting communities of endemic small mammals, as well as humans, these movements presumably increase transmission potential. Our results suggest that due to their generalist diet and potential movement between natural forest and anthropogenic habitats, Rattus spp. might affect native forest-dependent Malagasy rodents as competitors, predators, and disease vectors. The combination of these effects helps explain the invasion success of Rattus spp. and the detrimental effects of this genus on the endemic Malagasy rodent fauna.
The Anthropocene is the era of urbanization. The accelerating expansion of cities occurs at the expense of natural reservoirs of biodiversity and presents animals with challenges for which their evolutionary past might not have prepared them. Cognitive and behavioral adjustments to novelty could promote animals’ persistence under these altered conditions. We investigated the structure of, and covariance between, different aspects of responses to novelty in rural and urban small mammals of two non-commensal rodent species. We ran replicated experiments testing responses to three novelty types (object, food, or space) of 47 individual common voles (Microtus arvalis) and 41 individual striped field mice (Apodemus agrarius). We found partial support for the hypothesis that responses to novelty are structured, clustering (i) speed of responses, (ii) intensity of responses, and (iii) responses to food into separate dimensions. Rural and urban small mammals did not differ in most responses to novelty, suggesting that urban habitats do not reduce neophobia in these species. Further studies investigating whether comparable response patters are found throughout different stages of colonization, and along synurbanization processes of different duration, will help illuminate the dynamics of animals’ cognitive adjustments to urban life.