This blog post is provided by Allan McDevitt and Samuel Browett and tells the #StoryBehindthePaper for the paper “Resource competition drives an invasion-replacement event among shrew species on an island”, which was recently published in Journal of Animal Ecology. In their paper they determine how a recently invading shrew is outcompeting a native shrew for prey resources, ultimately leading to the rapid disappearance of the native species.

Invasive species are one of the leading causes of native species extinctions globally, and nowhere is this more evident than on islands. Invasive species are responsible for >80% of species extinctions on islands, and it is invasive mammals in particular that are responsible for the vast majority of these. 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.

An invading shrew

In 2007, the greater white-toothed shrew (Crocidura russula) was first discovered on the island of Ireland in the pellets of birds of prey. The species was normally distributed in northern Africa and western Europe but was absent from Ireland and Great Britain. Given that Ireland had relatively few small mammal species, it was initially thought that the species might provide a beneficial additional prey resource for predators on the island. However, subsequent surveys to show the extent of the new mammal’s distribution range in Ireland revealed a worrying finding: the native pygmy shrew (Sorex minutus) had completely disappeared where the new shrew had become established. This was unexpected as although the pygmy shrew was Ireland’s only shrew species prior to the new shrew’s arrival, the two species normally overlap in western Europe. So, what is going on in Ireland that is different from areas they normally co-exist together?

One of the original ideas proposed for the rapid disappearance of the pygmy shrew in Ireland was that it has experienced a competitive release on the island in the absence of other shrew species. This means that it is now not able to adapt quickly enough to a new invasive shrew competing for the same invertebrate food resources. In order to establish if this was the case, Browett and colleagues used DNA metabarcoding (i.e. the simultaneous identification of multiple species using a standardised region of DNA) on their gut contents to determine which invertebrates were being consumed by the invasive and native shrew and how much dietary overlap there was between them in Ireland.

Figure 1. The invasive greater white-toothed shrew (left) and the native pygmy shrew (right). Photo credits: Ruth Carden and Samuel Browett.

A disappearing native shrew

By comparing their diet before, during and after the invasion in a real-time setting, we revealed that the invasive shrew was quickly altering its diet from when it first invaded an area to after it became established. During the initial stages of invasion, the invasive shrew was consuming larger invertebrate prey species not generally consumed by the smaller native shrew. These invasive shrews during the initial invasion were also around 20% larger in terms of their body weight. However, once the invasive shrew became established, it had switched to consuming smaller invertebrate species that were more essential to the diet of the native pygmy shrew. As a result, the level of dietary overlap increased from between 11–14% when the two species of shrew first come into contact with each other, to between 39–46% after the invasive shrew became established. This is how the invasive shrew out-competes the native shrew for small prey resources that are key for its survival. This is why there is only a brief area of physical overlap, which eventually means that they cannot coexist, and the native pygmy shrew disappears rapidly in as little as one year.

Figure 2. Non-metric multidimensional scaling (NMDS) plot showing less overlap in diet composition ‘during’ the invasion (i.e. when the species first come into contact with each other) and then showing more similarity/overlap in diet ‘after’ the invasion (i.e. when the invasive greater white-toothed shrew becomes established).

Long-term impacts

Given that the eradication of an invasive greater white-toothed shrew on an island of Ireland’s size would not be logistically feasible, this is obviously a concerning scenario for the future of the pygmy shrew on the island. At present, the pygmy shrew is present on many small offshore islands around Ireland and these may be vital in preserving the unique heritage of the Irish population. As an additional concern, the greater white-toothed shrew was recently discovered in Great Britain and it remains to be seen what impacts this invasive shrew may have on this island. It also needs to be taken into consideration that these impacts may go beyond the small mammal communities. Greater white-toothed shrews can potentially exhaust local resources of larger invertebrate species and subsequent changes in terrestrial invertebrate communities can of course have severe impacts further downstream on ecosystem functioning and services. Therefore, it is vital to determine if this invasive shrew is altering the composition of invertebrate communities as its invasion rapidly progresses, and what potential impacts this may have on the wider ecosystem on the island.

About the authors

Allan McDevitt is a lecturer in the Atlantic Technological University in Galway, Ireland. Allan is primarily a molecular ecologist and uses genetic techniques to address conservation issues in a variety of species and ecosystems. He has worked on shrews in Ireland for 20 years, from studying their origins on the island to determining the negative impacts of the recently invading greater white-toothed shrew.

Twitter: @ShrewGod

Samuel Browett is a lecturer in the South East Technological University in Waterford, Ireland. Samuel conducted his PhD research on using genetic techniques to study the interactions between the native pygmy shrew and invasive greater white-toothed shrew in Ireland.

Twitter: @ShrewlockHolmes

Read the paper

Read the full paper here: Browett, S. S., Synnott, R., O’Meara, D. B., Antwis, R. E., Browett, S. S., Bown, K. J., Wangensteen, O. S., Dawson, D. A., Searle, J. B., Yearsley, J. M., & McDevitt, A. D. (2022). Resource competition drives an invasion-replacement event among shrew species on an island. Journal of Animal Ecology, 00, 1– 12. https://doi.org/10.1111/1365-2656.13855