Resource competition drives an invasion-replacement event among shrew species on an island
O_LIInvasive mammals are responsible for the majority of native species extinctions on islands. While most of these extinction events will be due to novel interactions between species (e.g. exotic predators and naive prey), it is more unusual to find incidences where a newly invasive species causes the decline/extinction of a native species on an island when they normally coexist elsewhere in their overlapping mainland ranges. C_LIO_LIWe investigated if resource competition between two insectivorous small mammals was playing a significant role in the rapid replacement of the native pygmy shrew (Sorex minutus) in the presence of the recently invading greater white-toothed shrew (Crocidura russula) on the island of Ireland. C_LIO_LIWe used DNA metabarcoding of gut contents from >300 individuals of both species to determine each species diet and measured the size of individuals (weight and length) during different stages of the invasion in Ireland (before, during and after the species come into contact with one another) and on a French island where both species have long coexisted (acting as a natural control site). Dietary composition, niche width and overlap and size were compared in these different stages. C_LIO_LIThe size of the invasive C. russula and composition of its diet changes between when it first invades an area and after it becomes established. Individuals are larger and they consume larger invertebrates at the invasion front, before switching towards the smaller prey taxa that are more essential for the survival of the native species after establishment. As a result, the level of interspecific dietary overlap increases from between 11-14% when they first come into contact with each other to between 39-46% after the invasion. C_LIO_LIHere we show that an invasive species can quickly alter its dietary niche in a new environment, leading to negative impacts that were not previously predicted based on the coexistence of these species in other parts of their mainland ranges. As well as causing the replacement of a native small mammal, the invasive shrew may be rapidly exhausting local resources of larger invertebrate species. These subsequent changes in terrestrial invertebrate communities could have severe impacts further downstream on ecosystem functioning and services. C_LI