In April this year (2023), as part of the Ardclough Biodiversity Action Plan, we were planting trees near Williams Grove.  We emptied a bag of mulch that came from a local garden and in the mulch, there was a shrew. 

My children were with us, and as it bumbled its way through the grass to find cover, I was excited to show them, what I assumed to be, a Pygmy Shrew.  The poignancy of the moment only struck me later that day.  From what I knew of shrews in Ireland, it seemed that, as adults, our children will not ever see a pygmy shrew unless they have reason to go looking for them in their last remaining refuges – possibly our islands – possibly nowhere.

Shrews in Ireland

For thousands of years, learning about Ireland’s shrews was easy.  Our only shrew species was the pygmy shrew.  It was most likely introduced by early human settlers.  In 2007, the first signs were found, of greater white-toothed shrews in Ireland.

The Pygmy Shrew

The pygmy shrew is Ireland’s smallest mammal species.  To those who aren’t inclined to examine a rodent in intricate detail (most normal people, I will admit), it looks like a small mouse with a long nose.  It is primarily an insectivore, mainly eating spiders, woodlice and various insects and has a super-fast metabolism which means that it must consume more than its own body weight of these each day.  In fact, it can’t go much longer than a couple of hours without eating, to avoid starvation and so, pygmy shrews are active day and night, sleeping only in short naps.

The pygmy shrew, for those who have taken time to notice them, seems more endearing than a mouse.  With poorer eyesight, they move slower and appear to snuffle their way about.  They are good at remaining hidden, but if you do stumble across one, they don’t have that same ability to dart out of the way and can seem a little helpless in comparison to our more vilified rodents.  Also, since the pygmy shrew has little interest in what we people eat, they are much more likely to remain out in our hedgerows and woodlands than to come in around our homes.  For most people, their experience of a pygmy shrew is courtesy of their cat bringing it to their attention (or to their doorstep).

New Shrews

In 2007, pellets regurgitated by barn owls were collected in Co. Tipperary as part of a study into the diet of the owls.  These pellets and further pellets from owls and kestrels in Co. Limerick, were found to contain the bones of another shrew species, the greater white-toothed shrew. 

In March 2008, live traps were used, and seven greater white-toothed shrews were caught in Tipperary and Limerick.  Given the range and number of greater white-toothed shrews at that time, it is estimated that they likely started to establish in Ireland about 2001. 

Since their discovery in Ireland, their range has continued to expand rapidly.

The Greater White-Toothed Shrew

The greater white-toothed shrew is native to mainland Europe and North Africa.  It grows to roughly 3 times the weight of a pygmy shrew.  This sounds dramatic, but they’re still significantly smaller than a mouse and so it can be hard to identify which species of shrew it is, without practice.  The clearest indicator is that the teeth of a pygmy shrew have rust-red tips and the teeth of the greater white-toothed shrew are, unsurprisingly, white.  That said, even most ecologists seem unlikely to pick up a live shrew and try to prise its lips apart. 

The greater white-toothed shrew, when you do get your eye in, has a heavier snout, more prominent ears and a relatively shorter tail when compared to its body length.

Greater white-toothed shrews share the same diet as the pygmy shrew and live in similar habitats, but they do seem a little more likely to be found around gardens and homes too.

Introduced Species

When new species get introduced to an ecosystem, it can be hard to predict what the outcomes will be.  We have all learned that introduced species can have very negative consequences.  A lot of species though, will fail to establish in another environment, some will establish but have little obvious effect on the species already present or in some cases may even seem somewhat beneficial.  The specific interactions that play out can be unendingly complex.

So, what of the greater white-toothed shrew?

The Good News

One of the first effects noticed, of the growing population of greater white-toothed shrew, is that they are popular with our owls and other birds of prey.  Barn owls and kestrels have been in serious decline and the new shrew population has proven itself to be a welcome boost to their diet.  Where greater white-toothed shrew has established, there is strong evidence that barn owl and kestrel populations have increased.

Around Ardclough, we have, in fact, a booming population of long-eared owls.  Barn owls, kestrels and sparrowhawks are seen daily in Ardclough too, and all appear to be breeding in the area or nearby. 

As an aside, Birdwatch Ireland members are currently building some barn owl and kestrel nest boxes for Ardclough Biodiversity Group that will be erected soon.  The most important things we can do to help that recovery are avoid the use of rodenticides (poisons for mice and rats, etc.) and avoid reducing our ever-diminishing hedgerows and tree cover – even dead trees are important for nest sites and to support insect populations.

The Bad News

I’ve already mentioned the most obvious bad news.  When the greater white-toothed shrew gets established in an area, the pygmy shrew disappears within about, a year.  Initially, the greater white-toothed shrew can forage on larger insects than the pygmy shrew, but as those larger insects get depleted it turns to the same diet as the pygmy shrew and outcompetes it.

It seems likely that within a few years, the only pygmy shrews remaining in Ireland will be in isolated spots that have not yet been invaded by the greater white-toothed.  Our islands will perhaps, be the last strongholds.

On top of this, the greater white-toothed shrew is having a significant impact on our invertebrate populations and is significantly changing the balance there too.  Our invertebrates (insects, beetles, spiders, snails, worms, etc.) are declining at an alarming rate for many reasons and this is yet another pressure they now face.

Some people find it hard to feel upset about losing populations of insects, beetles, and other bugs but our invertebrates are key to the survival of so much of our other wildlife too.  Research is now emerging that shows, the effects of the greater white-toothed shrew on our insect populations, is having an impact on our hedgerow bird species.

Sentimental Idealism?

One perspective that often comes up, is that perhaps we’re just being sentimental about “our” shrew, the pygmy shrew.  After all, the pygmy shrew was likely introduced to Ireland by early human settlers and though we’re losing the pygmy shrew, we’re gaining the greater white-toothed shrew.  This also might be the boost that our barn owls need and at the invertebrate level, though it would appear negative so far, there may be species that benefit from the changes there too. 

Are we just being prejudiced towards the greater white-toothed shrew because it’s “not native”?  Are we just being weirdly nationalistic about species?  Shouldn’t a scientist avoid being sentimental about these things and value these species equally?

Biodiversity loss

The science of Ecology does value all species equally.  I too (not an ecologist), if I were in France or Germany would be just as excited to spot a greater white-toothed shrew, as to spot a pygmy shrew at home.

The pygmy shrew isn’t facing extinction right across their range in Europe either, so what’s the problem?

The issue is that each time a species gets introduced to an ecosystem and starts to establish itself, the effect is typically that some other species will suffer.  Some species may benefit but some are pushed closer to, or into, extinction.  New species don’t evolve at a rate that is even comparable to the rate at which we are losing species.  It’s not a zero-sum game.  This is Biodiversity Loss.

Invasive species disrupting ecosystems is far from the only cause of this biodiversity loss, but it is a big factor.

You may look out your window at a patch of flowers and see 4 different types of bees.  When you were young, if you looked at a similar patch of flowers, you may have seen 6 different types.  Perhaps you have hundreds of birds feeding in the field behind your house and your hedgerows are full of song?  When you were young there may or may not have been more birds feeding in that field but there were probably more different types.  It’s not easy to see that pace of change, but equally, it is alarmingly quick.

Shrews in Ardclough

The moment with the shrew has played on my mind a lot since April.  It was great to point one out to our boys, even if they may not actually remember it in the future.  I thought, at least I can tell them that they saw a pygmy shrew when they were young.

In fact, I’ve stumbled upon several shrews this year – at least half a dozen or more.  It seems like more than usual.  It got to the point where recently, I thought there was a mouse in the garage.  When I trapped it, regretfully, it too turned out to be a shrew.  I looked at it more closely than most people would and did a little more research about “our” shrew and the greater white-toothed shrew.

I discovered that the range of the greater white-toothed shrew has now expanded to fill more than the Southern half of the island – if you draw a line roughly from Mayo to Louth, it is found everywhere South of that.

I went back to photos from April.

It turns out my children have never seen a pygmy shrew.  

If you spot a shrew around Ardclough today or in the future, it’s probably not a pygmy shrew either.

You shouldn’t be sentimental about these things though.  Right?

The greater white-toothed shrew that we saw while planting trees in Ardclough in April 2023

Footnote:

I’m neither an ecologist, nor a scientist.  Just a software engineer who has always had an interest in the natural world around me.  To be sure about the ID, I sent the photo in this article to Dr. Allan McDevitt, who is likely Ireland’s most qualified shrew expert.  He very promptly and very kindly confirmed that it is indeed, the greater white-toothed shrew.  You can follow Allan’s twitter account at https://twitter.com/ShrewGod, but be warned that you may end up learning about much more than just shrews.  🙂

The paper published in 2008, on the discovery of greater white-toothed shrew in Ireland is available here: First record of greater white-toothed shrew Crocidura russula in Ireland