Resource competition drives an invasion-replacement event among shrew species on an island

Invasive 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 cause the decline/extinction of a native species on an island when
they normally coexist elsewhere in their overlapping mainland ranges. We
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. We used DNA metabarcoding of gut contents from >300
individuals of both species to determine each species’ diet and measured
the body size (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 body size were compared in these different stages. The
body size of the invasive C. russula and the composition of its diet
change between when it first invades an area and after it becomes
established. During the initial stages of the invasion, individual shrews
are larger and consume larger-sized invertebrate prey species. During the
later stages of the invasion, C. russula switch to consuming smaller prey
taxa that are more essential for the native species. 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. Here we show that an invasive species can quickly alter its
dietary niche in a new environment, ultimately causing the replacement of
a native species. In addition, the invasive shrew could also be
potentially exhausting the local resources of larger invertebrate species.
These subsequent changes in terrestrial invertebrate communities could
have severe impacts further downstream on ecosystem functioning and
services.