Battle of the Shrews

January 2020 Student of the Month - Sam Browett

PhD Research at University of Salford

I’m currently in the 4th year of my PhD at the University of Salford, of which I work within the Molecular Ecology Group. This group works on many projects including species monitoring using eDNA, phylogeography, disease and evolution of animals and plants around the world. Our website has details of the fantastic people I have had the chance to work with.

Battle of the shrews

My PhD focuses on the recent invasion of the greater white-toothed shrew (GWTS; Crocidura russula) into Ireland. They originate from Northern Africa but managed to cross the strait of Gibraltar around the time of the last glacial maximum. This remarkable species has since expanded its range through Europe. In the early 2000’s the GWTS arrived in Ireland, likely by ferry. The GWTS is now expanding its range by an estimated rate of 5.5 km per year and already covers a large proportion of Ireland. Ireland’s smallest resident mammal, the pygmy shrew (Sorex minutus), is now rapidly being out-competed and completely replaced by the invasive GWTS. If this current situation continues, there will be no pygmy shrews left in Ireland in the relatively near future. Considering these two species co-inhabit other regions of Europe, my goal is to determine why they cannot coexist in Ireland.

Both shrew species are insectivorous, and previous studies led by my supervisor, Allan McDevitt, suspect that the GWTS is out-competing the pygmy shrews for food resources (McDevitt et al 2014). The first question of my PhD asks: are both the shrews eating the same prey species in Ireland? This is not an easy task. Considering the tiny size of these shrews (pygmies are 4 g, while GWTS are ~12 g), there is not enough patience in the world to sit in hedgerows hoping to see hundreds of shrews hunt their prey.

Thankfully, there is a genetic technique referred to as DNA metabarcoding that can identify multiple prey species from the DNA found in a predator’s stomach contents or faeces. The technique essentially isolates a short section of DNA that acts like a ‘genetic barcode’. By cross-referencing our barcode sequences to an ever-growing online database, this genetic barcode can differentiate between prey species consumed. This technique was used to determine the diet of hundreds of individual shrews around Ireland. We also compared Irish shrews’ diets to shrews in Belle Île (France), where they both co-exist in high abundances.

Preliminary results suggest that there may be some key prey groups that the GWTS is consuming in Ireland that have a downstream effect on pygmy shrews. To support any conclusions about this, I have generated data on the shrews’ gut microbiome (microbial species community) which can act as a proxy to the status of the animal’s health during the invasion. Linking these dietary and microbiome changes has not been done before in an invasive mammal system and can highlight the competitive advantage of the GWTS over Ireland’s pygmy shrews. I’m finalising results for this project now and will announce final findings on my twitter (@ShrewlockHolmes) - the results will be submitted for publication soon.

Fieldwork

The fieldwork was an eye-opener for me. Until my PhD I never considered how marvellous the small mammal world could be. The hedgerow systems in Ireland are flush with diverse life that I took for granted growing up. I now cannot pass hedgerows without considering what creatures dwell within and what they’re doing. You are bound to look strange sticking out of hedgerows with bags of traps, so inevitably people stop to ask questions. I have to say that there was phenomenal enthusiasm from the locals to learn about my work, the shrews and our entire small mammal community. People of all ages got excited when I was able to show them a pygmy shrew for the first time in their lives, even though they live just outside our doorsteps. The excitement of people learning or discovering something new in nature is why I got into science and a big influence to continue in this line of work.

I benefited from enthusiastic volunteers to help me with fieldwork (even in the rain and snow) and I am ever grateful for them.